Why is cannibalism not an evolutionary stable situation?

Why is cannibalism not an evolutionary stable situation?

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In the 'the selfish gene' Dawkins writes (page 109): "The reason lions do not hunt lions is that it would not be an ESS (evolutionary stable situation) for them to do so. A cannibal strategy would be unstable for the same reason as the hawk strategy in the earlier example."

To quickly summarise this: we consider an interaction between two animals where one possibly stands something to gain. Dawkins then defines 'hawk' behaviour as attacking behaviour, whilst 'dove' is described as peaceful and retreating behaviour. He then argues that a population of purely hawk or purely dove is unstable, since in either of the situations the other kind of behaviour is more favourable. As it turns out, the stable situation is 7:5 for hawk:dove.

Turning to cannibalism, we can regard the hawk behaviour as 'engaging in cannibalism' and dove to be the refraining party. Why, then, don't we see similar behaviour when considering cannibalism, i.e. why don't we see a reasonably large amount of lions engaging in cannibalism whilst the others refrain from it? If I am not mistaken we see in fact the situation more closely described by completely dove? Any insights are appreciated!

Cannibalism is incredibly common in nature. There are more species that eat their own kind than do not. Chimps eat other chimps. Lions eat other lions. Wolves eat other wolves. Prairie dogs go into the burrows of other prairie dogs and eat their babies to get the protein-rich meals needed to produce milk for their own litter. The only animals that don't regularly engage in cannibalism are some birds, and that is more because they physically cannot fit other members of their species down their throat than any moral qualms.

However, cannibalism is typically a bad evolutionary strategy, long term. For several reasons…

  1. in a dioecious species (i.e., one with two sexes), cannibalism means a 50% chance of eating a potential mate and losing the opportunity to pass on your genes
  2. diseases are more likely to transfer between species the more closely related they are. If you are a cannibal you are eating a species with a 99% similarity to you and are more likely to get diseases or parasites. Like kuru/mad cow/chronic wasting disease.
  3. it precludes social behavior and all its benefits. Why engage in mutually profitable behavior when you can just eat those around you? This is thought to be one reason why pack-hunting is rare in some groups of vertebrates like monitor lizards.
  4. on a broad-scale evolutionary perspective, cannibalism means removing members of your own population from the gene pool. It may be good for the individual but it's bad for the species as whole
  5. cannibalism means you are feeding on an animal that is potentially your own size. That's a bad strategy in general. Most cannibalism in nature tends to be adults eating infants or adults scavenging carrion of their own species. By contrast predators are normally larger than their prey, with a few exceptions like pack-hunting wolves and lions.

Attack of the inner-cannibal mega-shark

Massive megalodons ruled the seas some 23 million to 2.5 million years ago. Why these giants got so big isn’t known. New research suggests it may be due to warm-bloodedness, lots of food — and cannibalization in the womb.

Warpaintcobra/iStock / Getty Images Plus

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November 10, 2020 at 6:30 am

Otodus megalodon is a giant among sharks. The 14-meter (45-foot) carnivore with razor-sharp teeth cruised the seas some 23 million to 2.5 million years ago. No other predatory shark has matched its size and ferocity. How did this creature get so ginormous? A group of scientists now suspects part of the reason may be that megalodon ate its siblings while still in its mother’s womb.

They call it intrauterine (In-trah-YU-tur-in) cannibalism. It means the strongest embryo in the womb devours its weaker siblings. Some of megalodon’s living relatives do this, such as great white sharks. Scientists think megalodon and some other ancient sharks did it too.

What ‘The Meg’ doesn’t quite get right about megalodon sharks

Kenshu Shimada works at DePaul University in Chicago, Ill. As a paleobiologist, he studies ancient animals. Along with some colleagues, he came up with this out-of-the-box theory: The adults’ size may reflect the appetites of megalodon babies in the womb. Their constant chowing down allowed these ancient beasts to become the largest shark ever to hunt the Earth’s oceans. They were twice the size of a double-decker bus and half as large as today’s whale sharks, filter feeders that can grow to 30 meters (100 feet).

Shimada’s team came up with its cannibalism idea after analyzing modern and ancient shark teeth. Scientists used these teeth to estimate the body sizes of fish. The researchers focused on lamniforms, also known as mackeral shark. About 15 of these species exist today. They include the great white, mako and basking sharks. But throughout history, there have been more than 200 lamniform shark species. Megalodon was the largest of all time.

It wasn’t easy for scientists to figure out its body size. That’s because shark skeletons are made of cartilage. Unlike bone, cartilage doesn’t last long once an animal dies. Luckily, sharks have teeth. Lots and lots of teeth. These remain behind when a shark goes belly up. And those choppers can tell a story of extinct species.

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Scientists concluded megalodon was at least 14 meters long. Four other extinct species of lamniforms grew to more than 6 meters (20 feet). That means they also exhibited “gigantism.” While that’s not as titanic as megalodon, it’s still pretty big. Gigantism also occurs in several modern lamniform species.

Exactly why megalodon and its cousins became so big is still a bit murky. Warm-bloodedness — or endothermy — may be one key. Extinct and modern lamniforms are more warm-blooded than other sharks. By regulating their body temperatures, they can swim faster to catch large, speedy prey in cold waters. Warm-blooded sharks also need more food than slower moving, colder-blooded ones. More and bigger food may have allowed these sharks to get bigger.

Megalodon tooth (left) is far larger than that of a great white shark (right). Both sharks belong to a group with a unique reproductive strategy. The first pup to hatch inside mom eats up the other eggs. This lets it grow big and strong before leaving the womb and being “born.” Scientists now suspect this could have put such species on the path to warm-bloodedness — and ultimately gigantic body sizes. Mark Kostich/iStock / Getty Images Plus

But that theory doesn’t explain why these sharks were warm blooded in the first place. Shimada and his group say something else must have been at play. Their novel idea: Competition for food in the womb. It could have affected megalodon’s size and body temperature.

Sharks don’t breed like other fish. Most female fish lay eggs outside their bodies. Thousands of them. Males then come by and fertilize those eggs. Sharks embryos instead develop inside the eggs. But the mother squirrels them away in her womb until they are ready to hatch.

This takes a gnarly turn in living lamniform sharks. Unlike other sharks, lamniforms practice intrauterine cannibalism. All living lamniform sharks do this. So Shimada and his colleagues reasoned that ancient ones like megalodon did too.

How does it work? Once the first shark pup hatches inside its mom, it begins gobbling up the remaining eggs. It’s like yet-to-be-born Pacman. As the pup eats, it grows. Its constant feeding also forces its mom to eat more. As she eats more, her waiting-to-be-born pup grows bigger. As that baby packs on the pounds, its body temperature rises. Finally, a large, fattened shark emerges from its mother and swims away.

This study seeks to link tooth, jaw and body size in fossils of extinct lamniform sharks, says Humberto Férron. He is a paleobiologist at the University of Bristol in England. The idea that in-the-womb cannibalism may have contributed to megalodon’s size, he says, “is novel.”

Stephen Godfrey agrees. He’s a paleontologist at Calvert Marine Museum in Solomons, Md. It’s possible that in-the-womb cannibalism helped some lamniforms become warm-blooded in the first place, he says. It could have helped them grow big enough to take on bigger prey. That could have driven a need for more energy and evolutionary change, such as warm-bloodedness.

But it still doesn’t fully explain the unique super-gigantism of megalodon, Godfrey adds. “If there had been no large prey, I very much doubt that there would have been macro-predatory giant sharks,” he says.

Clearly, this is no ordinary fish tale.

Power Words

blue whale: A species of baleen whale (Balaenoptera musculus) that is the largest animal ever known to have existed. They can grow to lengths of 30 meters (almost 100 feet) and weigh up to 170 metric tons.

breed: (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.

carnivore: An animal that either exclusively or primarily eats other animals.

cartilage: (adj. cartilaginous) A type of strong connective tissue often found in joints, the nose and ear. In certain primitive fishes, such as sharks and rays, cartilage provides an internal structure — or skeleton — for their bodies.

colleague: Someone who works with another a co-worker or team member.

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size, mental maturity, size or sometimes even shape.

egg: The unfertilized reproductive cell made by females.

embryo: The early stages of a developing organism, or animal with a backbone, consisting only one or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.

endothermic: (in biology) A term for an animal’s generation of heat to keep its body temperature comfortable when the environment turns cold. (in chemistry) A reaction that absorbs energy — usually heat — from its environment.

evolutionary: An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

extinct: An adjective that describes a species for which there are no living members.

fertilize: (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

fossil: Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

link: A connection between two people or things.

marine: Having to do with the ocean world or environment.

megalodon: An extinct shark species, Carcharocles megalodon, that lived between the early Miocene (an epoch which started some 23 million years ago) and the end of the Pliocene (roughly 2.6 million years ago). Most scientists believe it was the largest fish to ever live. Its name comes from the Greek and means gigantic tooth. The average adult member of this species could have spanned more than 10 meters (33 feet) and weighed 30 metric tons (66,000 pounds) or more.

novel: Something that is clever or unusual and new, as in never seen before.

order: (in biology) It is that place on the tree of life directly above species, genus and family.

paleobiologist: A scientist who studies organisms that lived in ancient times — especially geologically ancient periods, such as the dinosaur era.

paleontologist: A scientist who specializes in studying fossils, the remains of ancient organisms.

prey: (n.) Animal species eaten by others. (v.) To attack and eat another species.

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sharks: A family of primitive fishes that rely on skeletons formed of cartilage, not bone. Like skates and rays, they belong to a group known as elasmobranchs. Then tend to grow and mature slowly and have few young. Some lay eggs, others give birth to live young.

sibling: An offspring that shares the same parents (with its brother or sister).

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

unique: Something that is unlike anything else the only one of its kind.

warm-blooded: Adjective for animals (chiefly mammals and birds) that maintain a constant body temperature, typically above that of their surroundings. Scientists generally prefer the term endothermic to describe animals that generate heat to control their body’s temperature.

whale: A common, but fairly imprecise, term for a class of large mammals that lives in the ocean. This group includes dolphins and porpoises.

womb: Another name for the uterus, the organ in mammals in which a fetus grows and matures in preparation for birth.


About Carolyn Gramling

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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Using evolutionary dynamics and game theory to understand personal relations

When we ask friends if we can stay at their place, we prefer them to say yes without asking details such as for how long. Yet, if the answer is going to be no, then we often prefer them to seek more information from us first. At first glance, this situation seems very different from how we react when we are in an exclusive relationship and our partner flirts with someone else. However, a probability-based analysis with Bayesian game theory shows that each involves differing degrees of manipulation and preferential interaction, MIT researchers report in a recent paper.

MIT postdoc Alfonso Pérez-Escudero and colleagues analyzed how these manipulation and preferential interaction mechanisms play out in “the envelope game,” a framework developed by Harvard University researchers Martin Nowak, Erez Yoeli, and Moshe Hoffman. “These are two situations that, in principle, I wouldn’t put together, and thanks to the framework that these researchers developed, we realized that they belong to the same family of situations,” Pérez-Escudero says. The original framework contained the manipulative mechanism but not the preferential interaction mechanism. “Our contribution is to realize that this family has two sub-families that can be mixed. We developed a generalization, creating a model that can describe both of these mechanisms at the same time and that contains the original model as a particular case,” he says.

The paper, co-authored by Pérez-Escudero, postdoc Jonathan Friedman, and MIT Latham Family Career Development Associate Professor of Physics Jeff Gore, was published in the Proceedings of the National Academy of Sciences on Dec. 6, 2016. The Gore Laboratory, in the Physics of Living Systems group at MIT, more often uses game theory to explain evolutionary dynamics such as cooperation among microbes. “Here we use the same math that you can use to describe evolution in biology to describe human behavior and human psychology, building a unifying framework between biological problems and human ones,” Gore says.

An unequal game

In the envelope game, each player has a potential payoff, but the first player’s actions determine the second player’s payoff, so their roles are unequal, or asymmetric. The envelope contains a bonus prize, which is sometimes of low value and sometimes of high value, and Player 1 can choose whether to look in the envelope. After either looking in the envelope or choosing not to look, Player 1 will decide next whether to “cooperate,” in which case both players get a payoff, or to “defect,” in which case only Player 1 gets a payoff and Player 2 takes a loss. Player 2’s only choice is whether to continue the game or to quit.

“The player who can cooperate or defect has power over payoffs in the current round, while the other player has the power to continue the game for more rounds or stop,” Pérez-Escudero explains. “It’s unsurprising that Player 2 ends the game in retaliation if Player 1 defects. The surprising part is that Player 2 may also end the game if Player 1 cooperates — just because he looked in the envelope — even though looking has no effect on Player 2’s payoffs.”

Strategic choice, open signal

Each player is assigned a strategy profile, which is a set of rules that tells each player what to do in each situation. Since both players stand to win more by continuing the game, Player 2’s choice whether to continue or to end the game will influence player 1’s strategy. Players who always cooperate are said to be “reliable,” whereas players who sometimes, or always, defect are “unreliable.” When a set of strategies for Player 1 and Player 2 reaches a balanced state where neither player will benefit by following a different strategy, it is said to be in “equilibrium,” in mathematical terms, either a Nash equilibrium or a sequential equilibrium.

The player who looks in the envelope is sending a signal to the other player. “By opening the envelope, she is telling us something,” Pérez-Escudero says. “This means that her decision wasn’t very clear. She wasn’t 100 percent sure she was going to cooperate, nor 100 percent sure she was going to defect. She needed this piece of information to make her decision. So the price of learning the contents of the envelope is giving away some information about herself.” The intuition, then, is that the other player will prefer not to interact with someone who was not sure from the beginning, because she might change her mind if the conditions change slightly. “But this is what piqued our curiosity,” he says. “In the original paper, both players know each other perfectly — the only uncertainty is about the content of the envelope. And if I already know that you are an unreliable cooperator, I will not learn anything when I see that you look in the envelope looking should play no role. But the results were there: The envelope game does have a Nash equilibrium in which looking plays a fundamental role.” Something was missing.

By analyzing the game more closely, the MIT researchers realized that a different mechanism was driving the results: By threatening to end the game if Player 1 looks, Player 2 can force her to make a blind decision. This manipulative mechanism makes looking a key element of the game’s Nash equilibrium, even if both players know each other perfectly. “But this is not what happens in real life. Uncertainty is everywhere, and even if we know a person, we are never sure of their true feelings and thoughts. So we implemented this uncertainty into the model, turning it into a Bayesian game,” Pérez-Escudero says. [In everyday life, applying Bayesian rules is how email programs filter out spam.]

Computer simulations

Using computer simulations, Pérez-Escudero modeled how different strategies play out over several thousand rounds, which yields data for about 100,000 to 1 million possible combinations. The model runs a mathematical formula to simulate the repeated games and differing strategies. What he found is that when Player 1 always acts in the same way, only manipulation can make looking matter in the game. Player 2 accomplishes this manipulation by ending the game if Player 1 looks, which effectively punishes Player 1 by denying her any future gains and also protects Player 2 against any further losses. But where the game introduces two varieties of Player 1 with different payoffs and strategies, Player 2 will see “favorable” and “unfavorable” types, and pay attention to looking as a cue to tell them apart. In this scenario, no punishment is required.

The mathematical formula, which is called a replicator equation, comes from evolutionary biology. “Imagine you have a population with 1,000 people that are playing slightly different strategies those with more successful strategies are going to have more children, and they are going in the end to take over the population. The replicator equation was designed to describe this kind of situation, but it was found later that it can also describe cultural evolution, where a given idea (or behavior) can be learned and copied, making it a powerful tool to analyze human behavior. But to use it properly, one needs to enumerate all the possible strategies that can exist in the game. If I enumerate all these strategies, then the replicator equation can tell me who wins,” Pérez-Escudero explains.

But simulations alone would not be enough. The envelope game has infinite possible strategies, so it’s just not possible to enumerate them all. Simulations were therefore complemented by a different tool from game theory called the one-shot deviation principle, which acts to put a limit on what otherwise would be infinite calculations in order to draw meaningful conclusions. “Thanks to this principle, we can prove that a strategy is optimal even if we don’t know what other strategies are out there. You start from your strategy and test every decision you make, one by one. If you cannot benefit by deviating from the strategy in any single decision, then it is a best response and potentially part of a Nash equilibrium, or in our game, a refinement called sequential equilibrium,” he says. “Simulations, even if they cannot prove the equilibrium, were still useful to check that the equilibriums we were finding were also stable.”

Manipulation versus preference

These mathematical models neatly simulate personal interactions, where both manipulation and preferential interactions play a role — often together. “Take for example an exclusive couple relationship. If I have an exclusive relationship with my girlfriend and I flirt with other people, I can expect my girlfriend to punish me. She can get very angry she can leave me. In this case, there is true leverage from one person to the other, and then it’s very likely that the manipulative mechanism is playing a role.” The preferential interaction mechanism can also play a role here because one partner’s decision to flirt also informs the other that her partner is perhaps not very invested in the relationship. “Maybe she would prefer another person who is more invested in the relationship,” he says. “Here there are these two mechanisms. On the one hand, she is learning something about me and maybe she prefers not to interact more with me because of what she learns. On the other hand, she has the power to punish me if I do something she doesn’t want me to do,” Pérez-Escudero says.

Another key finding of the study is that the preferential interactions mechanism can give rise to the opposite effect: preference for looking. The defining characteristic is whether Player 1 ends up cooperating or defecting. “If Player 1 cooperates, I prefer her to cooperate without looking, because she’s a reliable cooperator. If Player 1 defects, I prefer her to look, because then she could be an unreliable defector, and I can still hope she will cooperate in the future,” Pérez-Escudero explains. “I think this connects with real-life situations. If you ask ‘Can you do me a favor?’ it would be very rude that I just say no. Instead, even if I’m confident I will not grant the favor, I will first ask what favor is it, and then present an excuse. My asking here would be a false signal that prevents you from realizing that I’m such a bad person that I would not grant you even the smallest favor.”

Commenting on the new MIT findings, Moshe Hoffman, a research scientist and lecturer at Harvard's Program for Evolutionary Dynamics, says, “The model helps us understand why we trust more those who don't look at the costs and benefits before deciding whether to cooperate, and more generally why we value principled behavior above strategic calculated behavior.”

“This model is a solid contribution to our understanding of principles of behavior, cooperation, and morality, and more generally fits within a wider literature that is important and insightful which uses game theoretic models and models of learning and evolutionary processes to understand puzzling aspects of human social behavior,” Hoffman says. “How else can we understand our social species if we don't try and uncover the hidden function behind what they do think and believe? And what better tools to do that than models of game theory, learning and evolutionary processes?”

This work was supported by an EMBO Postdoctoral Fellowship, Human Frontier Science Foundation Postdoctoral Fellowship, and the Paul Allen Family Foundation.

Are Parasitic Worms in Your Oysters?

While parasites do cause harm to their hosts, they are also a crucial piece of the planet’s ecosystem.

If you’re an oyster lover, seeing a shaggy worm slither across your appetizer is revolting – even though such worms are harmless to people. An internet search using the keywords “oyster” and “worm” will bring up a large cache of images, each one less palatable than the next.

As a biologist, I study invasive species including these mud blister worms. Despite their high gross-out factor, their parasitic lifestyles are fascinating. While parasites do cause harm to their hosts, they are also a crucial piece of the planet’s ecosystem.

Shell-boring worms

Mud blister worms belong to a larger group of segmented worms, collectively known as polychaetes. “Poly” means many and “chaete” means bristles in ancient Greek. Mud blister worms are one of many species that burrow into the shells of animals like oysters, abalone and scallops, where they spend their entire adult life.

Considering the shells of oysters and scallops are made up of calcium carbonate, which has limited nutritional value, it might seem an odd location for a worm infestation. But rather than feeding on the shell itself, these worms create an amazing network of tunnels within the shell’s matrix, using it as a house rather than a food source.

The worms feed by protruding their tentacles out of tiny openings in the shell, where they capture food particles from the surrounding seawater. Unlike other parasites, which feed directly on their hosts, mud blister worms invade their hosts’ outer covering and must have food delivered to them for survival.

How many worms can a single shell harbor? I once counted more than 120 worms emerging from the shell of a heavily infested Pacific oyster. The surface of the oyster looked like any other, but once it was immersed into a special irritating solution, a stunning number of worms began to rise up, just like a creature in a zombie film.

Sibling cannibalism

Adult worms are sedentary, meaning they remain within the tunnels they create and do not actively leave their quarters. The offspring of these worms, however, are free-swimming larvae, which are released into the water column after birth and disperse the species throughout the ocean.

After mating, females produce an egg case containing thousands of eggs, some of which hatch into larvae and some of which do not hatch at all. The latter become “nurse eggs,” or food that nourish the developing offspring. This is where things get interesting.

In one of my earliest studies of these worms, my colleagues and I found that in situations where nurse eggs were depleted, larger larvae often viciously attacked and cannibalized their siblings within the egg case. In other situations, the cannibalism occurred even in the presence of nurse eggs.

The mother is in charge of releasing the larvae, using a pair of tentacles to rupture the egg cases at a time of her choosing. Because she is solely responsible for liberating the offspring from the egg case, she has complete control over which siblings live and which die.

Sibling cannibalism, as brutal as it sounds, is actually quite common across the animal kingdom. Sand tiger sharks, for example, exhibit a similar behavior where siblings fight each other to the death in the womb although, in this case, the mother shark does not exert as much control as a mud blister worm matriarch does.

The evolutionary significance of sibling cannibalism – and why it seems to have emerged in animals as far apart on the tree of life as worms and sharks – is still not fully known and remains an active area of evolutionary biology research.

Threats to humans and the aquaculture industry

Luckily, shell-boring worms pose no threat to humans. Aside from an unexpected protein boost, accidental consumption will not lead to any health problems.

However, these worms are notorious pests in the aquaculture industry. Heavy infestations can cause reduced growth in shellfish, because the mollusk must divert energy from growth to shell repair. In addition, the meat of infested oysters has been reported as having a more “watery” consistency than uninfested oysters. Together, these effects result in a commercial loss for aquaculture farms.

In past years, scientist have proposed the use of chemical compounds and the heat-shocking of oysters to control the worms, but there has yet to be a silver bullet for eradication.

Perhaps one of the most overlooked facts in zoology is that parasitism is the most predominant lifestyle on Earth and plays an important role in maintaining ecosystems by stabilizing food webs and regulating population sizes. Like many marine invertebrates, the larvae of these worms serve as planktonic food for animals higher up in the food chain, thereby contributing to the overall structure of the marine community.

So next time you are at a seafood restaurant and you order a couple of raw oysters, try breaking apart the shells – perhaps after you’ve finished eating. You might discover a few hidden freeloaders.

[You’re smart and curious about the world. So are The Conversation’s authors and editors. You can read us daily by subscribing to our newsletter.]

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Cannibalism: It's 'Perfectly Natural,' A New Scientific History Argues

An illustration from 1875 depicts the survivors of the frigate Cospatrick, which caught fire off South Africa's Cape of Good Hope in November 1874. Of more than 470 people on board, just three ultimately survived, and they were reduced to cannibalism. Hulton Archive/Getty Images hide caption

An illustration from 1875 depicts the survivors of the frigate Cospatrick, which caught fire off South Africa's Cape of Good Hope in November 1874. Of more than 470 people on board, just three ultimately survived, and they were reduced to cannibalism.

Hulton Archive/Getty Images

There are very few scenarios where I could see myself considering the flesh of a fellow human being as food, and the ultimatum "eat today or die tomorrow" comes up in all of them. Most people are probably with me on this.

But Bill Schutt's newest book, Cannibalism: A Perfectly Natural History, reveals that from a scientific perspective, there's a predictable calculus for when humans and animals go cannibal. And far more humans — and animals — have dipped into the world of cannibalism than you might have imagined.

Schutt, a vertebrate zoologist at LIU Post and the American Museum of Natural History, dives into cannibalism's history, its place in the kingdom Animalia and the source of its taboo. The book encompasses a range of stories far wider than the usual canon of gruesome reports.

He makes no secret of his distaste for these over-sensationalized accounts, not because of the gore but because there's just so much more to cannibalism. Instead, he opts for a keenly scientific approach. "I'm taking things that seem grotesque and misunderstood and horrify people and putting it through the eyes of zoologist," he says.

A Perfectly Natural History

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And what does a zoologist see, exactly? He sees the perfect, natural sense of cannibalism, the evolutionary biology of eating one's own kind and, oddly, the wonder of it all. Macabre summaries of men eating men are present, too, but by far the most interesting section on human cannibalism in the book is Schutt's description of the long history of European aristocrats eating human parts as medicine.

"Upper-class types and even members of the British Royalty 'applied, drink or wore' concoctions prepared from human body parts, and they continued to do so until the end of the 18th century," Schutt writes in the book.

We caught up with Schutt to chat about the book. The interview below has been edited for length and clarity:

I hadn't heard of the medicinal cannibalism you described in Europe, starting with the Ancient Greek physician Galen of Pergamon and continuing to the 20th century. That was one thing that really surprised me.

Yeah, especially given the Western taboo around cannibalism, which has been around since the time of the Greeks, to find out that for hundreds of years, for many countries in Europe, pretty much every body part you can think of was used to cure something or the other. That was a complete shock.

I don't even know where some of these [purported cures] came from. That blood was going to cure epilepsy or how human fat could cure skin diseases? The most interesting one to me was mummies, and I think that was a mistranslation. To the Arabs . the word mumia meant this stuff they used to bind wounds and prep mummies. In the translations, the Europeans thought they meant real mummies had medicinal values, so they started grinding [mummies] up.

The Salt

When People Ate People, A Strange Disease Emerged

How did you get to seeing cannibalism as something that was really, very natural?

Cannibalism as a behavior has various functions – from parental care to a reproductive strategy to foraging. If you look at insects, snails, crustaceans, fish, toads, salamanders, there's plenty of cannibalism. When you're talking about cob fish and the million eggs they lay, they're not looking at [their eggs] like juniors. They're like a handful of raisins. It's just food. My favorite is these legless amphibians, or caecilians. The mother provides her skin to the young hatchlings, and they peel her skin like a grape. That to me was wild and amazing, and I'd never heard of it.

When you get to mammals, it's rarer because you're dealing with less offspring and [more] parental care. The cannibalism you do see sometimes takes place because of environmental stresses.

Seems like the decision to cannibalize is a pretty simple calculus. You do it when the need for food outweighs the risk of getting a disease.

Yup. Though when you're starving, I don't know if you're thinking, 'This person might have a disease I could catch.' No, you're just at the end of your rope, and you're going to die.

The Salt

Bones Tell Tale Of Desperation Among The Starving At Jamestown

It's natural behavior. Scientists have looked at starvation. At a certain point, one of the predictable behaviors that you'll see is cannibalism. It could start with dead bodies and then get to the extreme where you kill somebody and eat them. Then there's the case where some people will just not eat dead bodies and starve to death.

There's plenty of mammals and animals that don't practice the kind of parental care, or sexual cannibalism or that lifeboat strategy. But if you stress any creature out enough, I think the odds are that they'll eat their own kind.

In the book, you describe getting invited over for dinner by one of your sources to eat her placenta. How was that? Was it good?

Yeah, it was really the prep that made it taste good. Granted, the [husband] was a chef and so he knew how to prepare it osso bucco style and used a really nice wine I had brought. It smelled great. It didn't taste bad. I wouldn't do it again. I don't have any regrets I did this.

The Salt

Neanderthal Dinner: Reindeer With A Side Of Cannibalism

She said that after birth, she had all sorts of ups and downs, baby blues and stuff like that. Somebody turned her onto this and she tried and it she said she felt a lot better. She readily acknowledged that it was probably a placebo effect. Which is a real thing.

Would you eat another human – not a placenta? Not because of overcrowding, predation, competition or hunger. Just . because?

No. If I was put into a life or death situation like the guys who got stranded in the Andes, or in a besieged city with no alternatives, then I can't say that I wouldn't consume human flesh. Would I do it again, just for kicks? Why would I? There's no need for that. Not unless it really came down to a real horror situation where there was nothing else to eat. I try not to think about that as a possibility.

Evolution exam #2 Ch. 11

A female's reproductive success is often limited by the number of eggs that she can produce and provision. Females with access to the most resources generally achieve the highest egg (and offspring) numbers and have the highest fitness relative to other females.

Male reproductive success is often limited by the number of eggs that he can fertilize. Males who mate with the most females generally sire the greatest number of offspring and have the highest fitness relative to other males.

Biased operational sex ratios can generate strong sexual selection because the abundant sex (typically males) must compete over access to the limiting sex (typically females).

Males often fight with each other over access to females, and this behavior can generate strong
sexual selection for large body size, weapons, and aggression.

Males compete over harems (groups of females), resources required by females, or display arenas
(leks) visited by females, depending on the species.

Sexual selection affects the sex with the greater variance in reproductive success (usually males).

Intersexual selection (often ca lled female choice) occurs when members of the limiting sex (generally
females) actively discriminate among suitors of the less limited sex (generally males).

Populations Tend to Evolve toward Stable Equilibria

It is hardly ever possible to find an exact solution to the differential equations that arise in evolutionary models. Today, it is straightforward to solve differential equations using a computer, provided that numerical values for the parameters are chosen. However, this is insufficient to understand the system fully, because often there will be too many parameters to explore the full range of behaviors. In any case, the goal usually is to do more than just make numerical predictions. We want to understand which processes dominate the outcome: Which terms matter? How do they interact? What is their biological significance? In this section, we will see how we set about understanding models in this way.

The first step is to identify which parameter combinations matter. For example, we have just seen that, in models of selection, only the scaled time T = st matters, not s or t separately. Often, this can drastically reduce the number of parameters that need to be explored. The next step is to identify equilibria&mdashpoints where the population will remain&mdashand find whether they are locally stable. In other words, if the population is perturbed slightly away from an equilibrium, will it move farther away or converge back to the equilibrium? There may be several stable equilibria, however, so where the population ends up depends on where it starts. In that case, each stable equilibrium is said to have a domain of attraction (Fig. 28.10A). Conversely, there may be no stable equilibrium, so that the population will never be at rest (Fig. 28.10B). However, identifying the equilibria and their stability takes us a long way toward understanding the full dynamics of the population.

In population genetics, in the absence of mutation or immigration, no new alleles can be introduced. In that case, there will always be equilibria at which just one of the possible alleles makes up the entire population. At such an equilibrium, the allele is said to be fixed in the population. The crucial question, then, is whether the population is stable toward invasion from new alleles. This depends on whether the new allele tends to increase from a low frequency, which it will do if it has higher fitness than the resident allele. Thus, analysis is especially simple if, most of the time, the population is fixed for a single genotype. In that case, the fate of each new mutation is examined to determine whether the mutation can invade to replace the previous allele.

Matters are more complicated if, when an allele invades, it sets up a stable polymorphism with the resident allele. Whether new alleles can invade this polymorphic state can still be determined, because such invasion depends only on whether or not a new allele has higher fitness than that of the residents. A substantial body of theory, known as adaptive dynamics, is used to analyze situations like this by making the additional assumption that invading alleles are similar to the resident (Fig. 28.11). This approach is best thought of as a way of exploring the dynamics when a range of alleles is available rather than as an actual model of evolution. (For one thing, when new alleles are similar in fitness to the residents, evolution will be very slow [Fig. 28.11].) In Chapter 20, we looked at other ways of finding what kinds of phenotypes will evolve. In particular, evolutionary game theory focuses on whether a resident phenotype is stable toward invasion by alternatives. Here, the analysis avoids making assumptions about the genetics by simply comparing the fitnesses of different phenotypes. All of these methods are guides to the direction of evolution rather than ways of making exact predictions.

2. Descriptive Evolutionary Ethics: Explaining Morality in the Empirical Sense

Evolutionary explanations for morality in the empirical sense are offered at different levels, and this makes for very different explanatory projects with different implications. Some typical explananda in accounts of &ldquothe evolution of morality&rdquo are:

  1. The general capacity for normative judgment and guidance, and the tendency to exercise this capacity in social life
  2. The capacity for certain sentiments and the ability to detect them in others
  3. The tendency to experience and to be motivated by certain sentiments in certain types of situation
  4. The tendency to make certain particular kinds of moral judgment or inference, or to have certain characteristic moral intuitions (i.e., a &lsquomoral sense&rsquo)
  5. The tendency to exhibit certain particular types of behavior in certain types of situation (as a result of D)
  6. The tendency of societies to exhibit certain particular systems of norms or types of practice (due to D and E).

2.1 Levels and Types of Explanation: Some Possibilities

It is uncontroversial that there will be evolutionary explanations of some sort for the very general capacities and tendencies in A and B: we are evolved creatures, and our psychological capacities, like other complex capacities, are outcomes of evolutionary processes. But this does not by itself settle whether these capacities and tendencies are themselves adaptations, having evolved through natural selection because of their adaptive effects. That is the most common view (further explored below), but there are alternatives.

It is possible, for example, that our capacity to make moral judgments is a spin-off (side-effect or by-product) of our non-moral intellectual capacities, which latter are adaptations. [3] On this view, we tend to make moral judgments because we are intellectual and reflective creatures, not because natural selection has specifically given us this moral capacity and tendency as an adaptation the role of natural selection would be indirect, supplying more general capacities as adaptations, from which specifically moral tendencies spring independently (Ayala 2006 Prinz 2008, Machery and Mallon 2010).

The deeper point, however, is that whichever position one takes on the role of natural selection in the emergence of generic capacities for moral judgment, this does not settle how best to account for other, more specific tendencies, such as C-F. In particular:

  • Even if (1) our capacity and tendency to make moral judgments is an adaptation that evolved through natural selection, it remains possible that (2) the content of particular moral judgments is derived autonomously, i.e., free from causal shaping by particular elements in our evolutionary background&mdashin roughly the way that the contents of our beliefs in physics or philosophy seem to be (see section 2.4 below).
  • Alternatively, even if (1*) our capacity and tendency to make moral judgments is not itself an adaptation, it remains possible that (2*) the content of particular moral judgments is deeply shaped by evolved emotional dispositions that have affected the content of moral judgments since human beings began making them. Such emotional adaptations may thus have influenced cultural norms throughout history and may continue to influence our moral judgments, and hence our behavior, today.

And of course there are other possibilities: 1 might be combined with 2*, or 1* might be combined with 2, and both 2 and 2* could also be weakened to apply only to some judgments, while others are explained in other ways, as suggested earlier. There are, then, many possible stories to be told about the origins of our moral judgments and behavior.

Since our concern is with morality, the crucial issue to begin with is the origins of moral judgment: for morality has not merely to do with certain emotions and behaviors (such as sympathy and altruism) as such, but with the exercise of moral judgment about how one ought to behave in various social circumstances (Joyce 2006, ch. 2 Korsgaard 2006 Kitcher 2006c, 2011 Machery and Mallon 2010). Certain emotions and behaviors are then relevant too insofar as they relate to the exercise of such judgment, but in the absence of moral judgment they seem only to belong to proto-morality.

Still, many discussions employ a looser notion of &lsquomorality&rsquo that refers simply to certain kinds of socially positive emotion or behavior, such as psychological altruism (defined below). Since accounts of the origins of moral judgment rely on these ideas as well, and these emotions or behavioral dispositions are also often appealed to as causal influences on the content of moral judgment, it is worth starting with a look at the issue of psychological altruism, distinguishing it from merely biological altruism.

2.2 Biological and Psychological Altruism

Many discussions of morality and evolutionary biology focus largely on the issue of altruistic feeling and behavior. This can be confusing because in addition to psychological altruism there is also biological altruism, which is found in many species. (See Kitcher 2011, part I, for a comprehensive discussion.) Psychological altruism involves caring about others&rsquo welfare and deliberately benefiting them for their own sake, with no restriction on the type of benefit involved. By contrast, biological altruism has nothing essentially to do with intentions or motives, and it pertains only to &lsquobenefits&rsquo to others that increase their reproductive fitness (boosting their genetic contribution to future generations).

Though psychological altruism is different from biological altruism, there are a variety of possible explanations of the evolution of psychological altruism that appeal to the same factors that explain the origins of biological altruism, namely:

  • Kin selection or &lsquoinclusive fitness&rsquo theory (Hamilton 1964)
  • Selection pressures leading to teamwork, reciprocal altruism (Trivers 1971 Maynard Smith 1982 Axelrod 1984) and indirect reciprocity (Alexander 1987 Joyce 2006) and
  • Group selection (Sober and Wilson 1998).

It would take us too far afield to survey all these biological accounts in detail, the background for which is already treated in the entry for biological altruism there are also many excellent and accessible summaries of such accounts and their application to psychological altruism (e.g., Joyce 2006, ch. 1). We will settle here for one detailed illustration of one way in which biological altruism can be given an evolutionary explanation, followed by a sketch of the ways in which this and other evolutionary mechanisms might likewise explain the emergence of psychological altruism&mdashkeeping in mind that this is all just one part of explaining morality proper, to which we return in the next sub-section.

The very idea that biological altruism can come about through natural selection may initially seem puzzling. One way of characterizing natural selection, after all, is in terms of &ldquoselfish genes&rdquo: natural selection occurs when a variant of a gene (an allele) at a given locus tends to cause a modification of a bodily or behavioral trait (a phenotypic trait) in a way that, in the overall environment, tends to cause that variant of the gene to increase its relative frequency in the next generation this then increases the representation of the associated trait modification as well. Typically this happens when the phenotypic modification is one that causes the organism to have greater reproductive success: if, in the overall context, allele A causes its carrier to have a trait T that causes the organism to have more offspring than other organisms in the population who carry rival allele A* and display alternative trait T*, then A will be inherited and carried by more organisms in succeeding generations and that means that T will likewise be displayed by more organisms (Dawkins, 1989). But how do we get from &ldquoselfish genes&rdquo increasing their representation in the gene pool, via improving the reproductive success of their carriers, to such things as cooperation and biological altruism? It turns out there are many ways.

To take one dramatic example, consider social insect colonies, and in particular, the Hymenoptera (bees, ants and wasps). In these colonies we find such an extreme degree of cooperation&mdashdivision of labor (queen, workers, soldiers, etc.), food-sharing, information sharing&mdashthat it is tempting to view the entire colony as a single functioning organism. Indeed, in the case of stinging worker honey bees, there is not only cooperative labor but also, when necessary, the ultimate sacrifice in defense of the hive (at least where the invader is a mammal, stinging of which proves fatal to the bee as the barbed sting is torn out upon being deposited in the victim). How can such striking cooperation and self-sacrifice be explained in evolutionary terms?

One important fact to notice is that the colony is one large family: typically, the workers are sisters&mdashdaughters of the queen. Return to the idea of &ldquoselfish genes&rdquo described above: what ultimately increases an allele&rsquos representation in the gene pool is its having some phenotypic effect that causes copies of that allele to be in more organisms in succeeding generations. That normally happens when the phenotypic effect causes the organism to have greater reproductive success, but it can equally happen if it causes the organism&rsquos close kin to have greater reproductive success: for close kin are likely to carry copies of that same allele, which means that greater reproductive success for kin likewise propagates copies of the allele. So while an allele that causes an organism to engage in sex more often may thereby spread, so might an allele that causes an organism to help a sibling to reproduce (as by aiding survival) either way, that allele will be helping to propagate copies of itself in the next generation, which in turn means that the helping behavior it causes will likewise spread over time (Dawkins 1989, 171&ndash77).

This mechanism of &ldquokin selection&rdquo can explain how worker bees evolved such apparently &lsquoselfless&rsquo traits, focused on aiding the queen&rsquos survival and reproduction. In fact, the situation is even more interesting and extreme: workers have evolved to serve the queen&rsquos reproduction at the complete expense of their own, as they are sterile. This extreme biological altruism, however, may be explained by the same principles, with the addition of the fact that due to a genetic peculiarity of the Hymenoptera (their haplodiploidy), sisters are more closely related to each other genetically than they would be to their own offspring. [4] This means that natural selection will actually favor worker traits that help their mother reproduce (thus making more sisters, who are especially likely to carry copies of the gene for that same trait, making it spread), over traits in workers aimed at personal reproduction. This could explain how worker sterility evolved, as traits focused on helping the queen took precedence over personal reproduction, and it explains how even suicidal behavior could have been selected for as propagating the genes that cause it (Dawkins 1989, 174&ndash75). (It is worth noting, however, that the theory of kin selection and inclusive fitness is highly complex and has received renewed critical scrutiny in recent years. For a recent special collection of articles on the topic see Royal Society Open Science on Inclusive Fitness).

This is only one example of one way in which biological altruism can evolve: there are others, which are not restricted to kin and are explained using game-theoretic or group dynamic models (again, see the entry on biological altruism for details). The crucial point is just that any kind of genetically-based trait can evolve if it happens to have the right kind of feedback effect on the genes that influence it. This brings us, then, to the sort of trait we are more interested in: a disposition for psychological altruism, as defined above.

Again, while it may initially seem puzzling that evolution should give rise to psychological altruism, rather than merely to selfishness, there is nothing paradoxical about it: a genetically-based disposition for psychological altruism will evolve just in case such a trait, in the relevant circumstances, promotes the propagation of the genes that bring it about (and does so more effectively than alternative traits produced by rival alleles). And this can again happen in various ways.

The basic idea is that psychological altruistic dispositions can evolve as proximate mechanisms or &ldquomodules&rdquo for promoting the biological advantages relevant to the forms of selection listed above. In some cases, these adaptive psychological mechanisms will involve specifically targeted or conditional altruistic motivations, involving capacities for discrimination to focus benefits on kin or on reciprocators. In other cases, the selection pressures will give rise to less discriminating altruistic sentiments and tendencies as the simplest and most cost-effective mechanisms for promoting adaptive cooperative behaviors in a given environment. If our hominin ancestors tended to live in circumstances where the opportunities for &lsquowasting&rsquo altruism in non-fitness-enhancing ways (e.g., on &lsquooutsiders&rsquo) were sufficiently few and far between, then a simple, undiscriminating (though limited) sense of concern and altruism may have promoted fitness at far less cost than more discriminating forms, evolving more readily. One advantage of this hypothesis is that it might help explain some sorts of concern and altruism that are otherwise hard to make sense of in evolutionary terms.

For example, suppose you receive a letter from UNICEF soliciting contributions for health and nutrition programs for children in Darfur, and you are moved to send a check. This is not merely selective altruism toward kin or likely reciprocators, but altruism toward strangers who are in no position to reciprocate, and it might therefore seem puzzling from a purely biological point of view: such &lsquoindiscriminate altruism&rsquo isn&rsquot biologically adaptive in the way more selective altruism might be your helping children in Darfur isn&rsquot helping to propagate your own genes, so it may seem mysterious how such a trait could have evolved through natural selection. But a trait that is not presently adaptive may once have been. In the environment in which our hominin ancestors lived, where there was little positive contact with outsiders, even relatively indiscriminate altruism would tend to benefit kin or potential reciprocators, and so might have been a simple adaptive mechanism on the whole.

If so, then your presently non-adaptive altruistic behavior in our current global environment could in principle be an expression of an evolved, indiscriminate altruistic tendency&mdashan adaptation that is largely no longer adaptive, and so would amount to a kind of &lsquomisfiring&rsquo of formerly adaptive instincts (Dawkins 2006, 220&ndash21 Kitcher 2006b). It would be in this way on a par with our taste for fatty foods&mdashoriginally evolved for its adaptive effects though it is no longer beneficial in today&rsquos fast food economy&mdashthough in the case of altruism the &lsquomisfiring&rsquo has happier results. Of course, it&rsquos also possible (as discussed in section 2.4) that your altruistic tendency isn&rsquot itself an adaptation at all, but is instead rooted in values that you&rsquove developed, through moral reflection in your cultural context, independently of specific evolutionary influences. Or it may be some combination of the two.

This area of inquiry remains largely speculative, since it is one thing to develop models for how psychological altruism could in principle evolve, and quite another to show convincingly that a given form of natural selection has in fact played the relevant role in actual human evolutionary history. There may be doubts, for example, whether there was sufficient pair-wise engagement in iterated prisoner&rsquos dilemma games to explain the evolution of reciprocal altruism according to some of the most familiar game-theoretic models (Kitcher 2006a,b). In any case, it is time to return to the issue of moral judgment, which is crucial to the explanation of morality proper, and goes beyond mere altruistic feeling and behavior. The following sub-section describes one leading hypothesis.

2.3 Explaining the Origins of Morality: From Psychological Altruism to the Evolution of Normative Guidance

Kitcher (2006a,b 2011) has proposed a three-stage account of the evolution of morality. It begins with the evolution of an early but fragile form of psychological altruism among hominins in the context of &ldquocoalition games&rdquo in mixed adult groups. The social structure would have been similar to that of contemporary chimpanzees and bonobos, where cooperation among the relatively weak (or those in weak stages of life) is beneficial to them, but strategic calculation is infeasible. These conditions may have led from simpler forms of biological altruism developed through kin selection to the evolution of a psychological altruistic disposition involving &ldquoa blind tendency to respond to the preferences of another animal with whom you might engage in cooperative activity,&rdquo as a simple but effective mechanism for promoting advantageous participation in coalitions and subcoalitions (Kitcher 2006b). (This is the &lsquoindiscriminate&rsquo altruism discussed in the previous sub-section.)

Because this disposition was both limited and unstable, however, and competed with powerful selfish drives, there was a continual threat of social rupture and loss of cooperative advantage. This in turn would have made ongoing peacemaking a necessity, thus limiting the size of viable cooperative units and the scope of cooperative projects, as well as imposing significant costs through the devotion of time and energy to peacemaking activities. An example would be extensive mutual grooming going well beyond what is necessary for hygiene, of the sort found in chimpanzee societies.

The next phase, according to this hypothesis, was a transition to much larger groups with more extensive cooperative activities, through the evolution of a capacity for emotionally laden normative guidance, without which such arrangements would not have been possible. (See also Gibbard 1990, ch. 4.) With the emergence of a capacity to make and follow normative judgments, reinforced by coevolved reactive emotions such as guilt and resentment, and the development of rules and social practices promoting and enforcing group loyalty and cooperation, a new psychological mechanism came into being for reinforcing the previously unstable altruistic tendencies and promoting large-scale social cohesion and stability. The advantages of membership in coalitions and subcoalitions would be conferred on hominins who had facility with such normative guidance&mdashincluding a strong sense of obligation and tendency toward social compliance&mdashand who thus acted consistently on these altruistic tendencies, acquiring the reputation for being good coalition partners and participating in a broader array of cooperative projects.

If this hypothesis is correct, it might explain not only the origins of our general capacity for normative judgment and motivation, but also the widespread tendency for social rules or norms historically to emphasize such things as group identity, loyalty and cohesion, and to focus largely on the regulation of violence and sex and it might help to explain widespread dispositions toward social conformity, concern with reputation and social standing, tendencies for group-wise scorning or punishment of the disloyal, and the power of emotions such as resentment, guilt and shame.

In the final phase, this sort of &ldquoproto-morality&rdquo of norms and reactive emotions would then be supplemented over thousands of years with various paths of cultural evolution, leading to the development and fleshing out of the much more sophisticated systems of moral beliefs, practices and institutions with which we are familiar, from the earliest historical examples right up to our present moral cultures (Kitcher 2006a, 2011). There is, of course, a great deal of leeway in the last part, concerning the details of the transition through cultural evolution from hominin proto-moralities to contemporary moral systems, which allows for some very different possible stories. Some accounts will attribute great influence to evolved, &ldquodomain-specific psychological modules&rdquo (such as a dedicated mechanism that automatically generates a feeling of concern when confronted with suffering) in shaping the content of our moral feeling, judgment and behavior, both through shaping our cultures and more directly other accounts, while not denying some such influence, will emphasize very different factors as influencing the content of at least some of our moral beliefs (such as the &ldquocapacity for open-ended normativity&rdquo cited by Buchanan and Powell 2015, 2018, 2019). This brings us to the next major topic.

2.4 Autonomous Moral Reflection and the Explanation of Reasoned Moral Judgment and Behavior

So far, we have focused on scientific projects that treat morality in the empirical sense as calling simply for causal explanation, as by appeal to evolutionary influences. This is unexceptionable with regard to the origins of the general human capacity for moral judgment: clearly some causal explanation is required, and an evolutionary explanation is plausible. But things are much more complicated when we consider the explanation of the actual content of moral judgment, feeling and behavior.

We have had a taste above of the way in which scientists might propose to explain our particular moral attitudes or judgments by appeal to specific evolutionary causes as filtered through cultural developments. Human beings have a strong, emotionally-laden sense of basic fairness, resentment of cheaters, and a desire that they be punished, all of which finds expression in both cultural norms and individual moral judgments. You might experience such feelings if you&rsquove been the victim of a scam, morally condemning the perpetrators. And some of these psychological traits may have analogues in other species. For example, Sarah Brosnan and Frans de Waal (2014) argue that &ldquoevidence indicates that [inequity aversion, i.e., negative reactions to unequal rewards for similar tasks] is widespread in cooperative species under many circumstances&rdquo--though some have disputed this and offered alternative hypotheses to explain the behavior, based on further research (Engelmann, Clift, Herrmann, and Tomasello 2017). [5] In the simplest case, an animal protests when it sees a companion receive a superior reward for a similar task, as in a well-known study with brown capuchin monkeys, though similar effects have now been observed even in non-primates, such as dogs and crows. In the more complex case, chimpanzees sometimes react negatively to inequity even where they are the ones receiving the greater reward. A natural evolutionary hypothesis for the simpler case is that such a disposition protected against &lsquowasting&rsquo cooperative efforts where they were not fitness-enhancing. Similarly, a natural explanatory hypothesis for the more complex form of inequity aversion is that it evolved in creatures with sufficient predictive capacities and emotional control because it preserved beneficial cooperative relationships that could be threatened by such inequities, thus proving fitness-enhancing on the whole given the benefits of continued cooperation (Brosnan and de Waal, 2014). All of this might suggest good prospects for causal explanation of human moral traits in terms of evolved psychological traits. Indeed, Brosnan and de Waal explicitly hypothesize that the evolved complex response observed in champanzees is what &ldquoallowed the development of a complete sense of fairness in humans, which aims not at equality for its own sake but for the sake of continued cooperation&rdquo (Brosnan and de Waal, 2014).

Caution is needed here, however. Our moral judgments and resulting behaviors cannot just be assumed to be mere causal upshots of some such biological and psychological forces, on a par with the cooperative activity of bees or the resentment felt by capuchin monkeys (or even the broader unease apparently felt by chimpanzees) over unequal rewards for equal work. When a rational agent makes a judgment, whether in the sphere of morality or in such areas as science, mathematics or philosophy, the proper question is not in the first instance what caused that judgment to occur, but what reasons the person had for making it&mdashfor thinking it to be true. It is those reasons that typically constitute an explanation of the judgment. They explain by bringing out what the person took (rightly or wrongly) to be the justification for the belief in question&mdashthe considerations showing the belief likely to be true. All of this complicates the explanatory project in relation to the thoughts, feelings and actions of rational agents.

It is helpful to illustrate the general point first with other kinds of judgment, and then to return to morality. Consider some judgments in mathematics, philosophy and science:

M: There is at least one prime number between 5,000 and 10,000.

P: If identity claims such as &lsquowater = H2O&rsquo are true, then they are necessary truths (i.e., true in all possible worlds).

S: There is no such thing as absolute simultaneity.

How do we explain someone&rsquos believing something like M, P or S? We normally need to know her reasons for believing it to be true, which we can then go on to assess as good or bad reasons for such a belief. Either way, we typically take her reasons&mdashand the reasoning associated with them&mdashto explain her belief, which is why we engage seriously with her reasons as such in critical discussion, and go on to inquire into their merits. If, for example, we ask a person why she believes M, and she cites a proof she has found convincing, showing that for any natural number n > 1 there is at least one prime number between n and 2n, we normally take this as a sufficient explanation for why she believes M. What we do not normally do is to appeal directly to independent causes, such as evolutionary or other biological or psychological influences to explain people&rsquos beliefs in these areas. Such independent explanations for beliefs can sometimes be correct, as in the case of someone given a post-hypnotic suggestion, in which case we may regard her judgment as merely caused and the reasons offered as mere rationalization. But this is not the norm.

The reason why we normally explain beliefs such as M, P or S by appeal to the reasons the person gives for them is that we normally assume that the person is capable of intelligent reflection and reasoning and has arrived at her belief for the reasons she gives as a result of that reflection (whether or not the belief is ultimately correct). We assume in general that people are capable of significant autonomy in their thinking, in the following sense:

This assumption seems hard to deny in the face of such abstract pursuits as algebraic topology, quantum field theory, population biology, modal metaphysics, or twelve-tone musical composition, all of which seem transparently to involve precisely such autonomous applications of human intelligence. Even if there are evolutionary influences behind our general tendency to engage in certain kinds of mental activity, or behind some of our motives in these pursuits (e.g., if a given musician or poet is motivated to compose music or poetry in order to impress a potential mate), this would not show that the activity is governed in its details by such influences (cf. Buchanan and Powell 2015, 2018, and 2019). And again, this assumption of autonomy is borne out in our normal explanations of beliefs such as P above: we take a person at her word that what explains her believing P, for example, is that due to philosophical arguments she&rsquos read and has found convincing she takes &lsquowater&rsquo to work like a proper name and thinks that it follows from this that &lsquowater&rsquo refers to whatever molecular compound it picks out in the actual world in all possible worlds in which the compound exists.

Few would deny the autonomy assumption altogether. To do so in the name of providing alternative evolutionary causal explanations of our beliefs would risk self-defeat: for if we lack the relevant intellectual autonomy across the board, then even the biologist&rsquos beliefs about evolutionary biology and its implications would just be attributable to such biological causes, rather than to reasons that provide real warrant for such beliefs within a rational framework with truth-tracking integrity. The challenge to the autonomy assumption is therefore more likely to come in a selective form. [6]

This brings us back to moral judgment. As with M, P and S, people typically have reasons for their moral judgments, and whether or not we agree with them, we typically take those reasons to explain why they believe what they do. Consider again the moral judgment mentioned earlier:

Just as with M, P and S, someone making this judgment will have reasons that she takes to justify this claim, ultimately tying into her overall conception of right and wrong. And we tend to take this at face value as an explanation of why she believes MJ, unless we have special reason to suspect distorting causes, such as prejudice leading to misperception and rationalization. We tend to think that a person has the moral beliefs she does as a result of background moral reflection and reasoning, within her cultural context. In other words, we tend to treat a person&rsquos moral beliefs much as we treat her reasoned mathematical, scientific, or philosophical beliefs, applying the autonomy assumption in seeking to explain why she believes what she does.

One potential lesson from evolutionary biology, however, is that even if the autonomy assumption equally applies in principle to the sphere of moral judgment, it may be a mistake just to assume that most moral judgment and behavior is in fact a result of the exercise of such autonomous reflection, reasoning and judgment. The autonomy assumption, after all, says only that we have the capacity for relevantly autonomous reflection and judgment it does not imply that we always exercise it. Perhaps the human capacity for autonomous thinking is exercised only in some cases, while in others the process that leads to moral belief is largely influenced by evolved psychological dispositions, such as emotional adaptations. While such a situation may not tend to arise in relation to our mathematical, philosophical, or scientific thinking, our evolutionary history may have given rise to emotional dispositions that play a significant role in at least some of our moral thinking, feeling and behavior. (See section 2.5 below.)

One plausible story, then, is that while many of our more reflective and reasoned moral judgments involve autonomous exercises of domain-general intelligence, many other less reflective moral judgments are largely attributable to evolutionary influences&mdashboth through direct conditioning of people&rsquos moral judgments by evolved, domain-specific psychological dispositions and through background influences on cultural factors. On this hypothesis, we cannot treat moral judgments as a homogeneous set, but must recognize that they can come about in very different ways, requiring a plurality of explanatory models. While a model of autonomous reasoning might apply to some moral beliefs and behaviors, a model appealing to evolutionary causal influences will apply to others. And often some combination of models may be necessary, as in cases where both evolutionary influences and our own independent moral reflection might lead to similar judgments, overdetermining them.

For example, even if it is true that evolutionary pressures favoring caring for offspring have strongly influenced our attitudes towards our children, it does not follow that this is the complete explanation for why we believe we have special obligations to care for our children and for why we behave as we do toward them. It may be part of the explanation, while another part may have to do with an autonomous recognition of the appropriateness of special parental care: we see that we have good reasons to take special care of our children, which ought to motivate us even if we weren&rsquot already motivated by instinctive feelings. We might also employ autonomous, domain-general comprehensive moral reasoning to recognize that our instinctive feelings shouldn&rsquot always be followed, e.g., in cases where considerations of justice constrain certain pursuits of benefits for one&rsquos own children, providing genuine overriding reasons to refrain from unjust actions. (We will consider another plausible example of overdetermination involving judgments about the much discussed &lsquoTrolley cases&rsquo in section 3.)

2.5 The Challenge to the Autonomous Reflection Model: Post Hoc Rationalization of Emotionally Caused Judgment

Advocates of an autonomous reflection model of moral judgment grant that various causal influences&mdashlikely including evolutionary ones&mdashoften play some role in moral thought and feeling. But opponents who hold a deflationary view of the role of autonomous reflection seek to press the challenge here more fully. To expand the potential reach of the evolutionary influence model of moral judgment, they typically combine that causal model with what may be called the &lsquomere rationalization hypothesis&rsquo with regard to the reasons people give for moral judgments that are really shaped by evolutionary factors.

On this view, our giving of reasons for our moral beliefs in such cases is interpreted as mere post hoc rationalization. Rather than engaging in autonomous reflection and reasoning, and coming to believe certain moral propositions for the reasons that emerge from that reflection (as with M, P and S), what is happening instead according to this hypothesis is that (1) our moral beliefs are simply caused by emotions or &lsquomoral instincts&rsquo we have largely due to our evolutionary background, and (2) we then invent rationalizations for these resulting beliefs in order to try to make sense of them to ourselves, unaware of their real causal origins (Haidt 2001 Wheatley and Haidt 2005 Knobe and Leiter 2007 Leiter 2007).

For example, when someone judges that abortion is wrong, it may be that &ldquothe anti-abortion judgment (a gut feeling that abortion is bad) causes the belief that life begins at conception (an ex post facto rationalization of the gut feeling)&rdquo (Haidt 2001, 817). On this model, our moral reasonings and justifications (or at least those to which the model is supposed to apply) are just so much window dressing for beliefs that are more like post-hypnotic suggestions than they are like M, P or S just as with post-hypnotic suggestions, people go to great lengths to provide rationalizations in an attempt to render their moral beliefs intelligible to themselves. The difference is that instead of a hypnotist, the causal agents are emotional dispositions stemming from our evolutionary past.

The basic idea that some moral beliefs are susceptible to this sort of debunking explanation is not new. Moral philosophers have long recognized that people have often been led to moral judgments based on self-interest or prejudice, for example, and then rationalized their views, inventing justifications for positions held due to other causes. A plausible example would be a judgment such as:

While such a judgment was not uncommon just a few generations ago, most readers of this article will recognize MJ2 to be not only false, but also likely to have stemmed from racial prejudice, with much of what was said to justify it being mere post hoc rationalization for a view more attributable to the causal influence of prejudice than to the workings of autonomous reflection and reasononing. Most philosophers today would say something similar about a moral belief still held by many people, especially within traditional religions:

The reason why philosophers tend to find a &lsquomere rationalization hypothesis&rsquo plausible for such beliefs as MJ2 and MJ3 is that (i) the justifications offered for them have consistently failed to stand up to critical reflection, and (ii) there are plausible alternative explanations for why people have really come to hold such beliefs, such as that they have misconstrued personal feelings of disgust as perceptions of objective moral wrongness, and projected those feelings onto the world as &lsquomoral impurity&rsquo. [7] Together, these considerations lend support to the hypothesis that the justifications offered are mere rationalizations, and that the beliefs are best explained by appeal to emotional causes.

The central question is: How broadly does the deflationary model of the &lsquomere rationalization hypothesis&rsquo apply to moral judgments, and to what extent are evolutionary influences implicated in those cases?

At one extreme, someone might deny that the autonomy assumption applies to the moral domain at all: we either lack these capacities in the domain of moral thought, or at least never exercise them. Such a claim seems to have little plausibility, however. Why should it be that human intelligence and innovation know virtually no bounds in other domains&mdashas illustrated by feats of autonomous inquiry and creativity in quantum field theory, algebraic topology, modal metaphysics, or symphonic composition&mdashand yet when it comes to moral thinking we remain stuck in ruts carved out for us by evolution, slavishly following patterns of thought prescribed for us by evolved, domain-specific mechanisms, with all of our cultural developments providing mere variations on those themes?

The very fact of human self-consciousness makes such a picture unlikely: for as soon as we are told that our thinking is constrained along evolutionarily given paths, our very awareness of those influences provides the opening to imagine and to pursue new possibilities. If you are told, for example, that you are evolutionarily conditioned to favor your group heavily over outsiders in your moral judgments, you are able, as a reflective agent, to take this very fact into account in your subsequent moral reflection, deciding that this favoring is unwarranted and thus coming to a new, more egalitarian moral view. [8] Analogously, it may be true that we possess dedicated mechanisms for &lsquoreading&rsquo faces as trustworthy or threatening, and often make split-second judgments on this basis but this doesn&rsquot preclude our ability to revise such judgments, as when we reflect on the behavior of someone with &lsquotrustworthy facial features&rsquo and realize that he is actually a scoundrel.

It is noteworthy that the leading proponent of the mere rationalization hypothesis, Jonathan Haidt (2001), does not take the extreme position of denying the autonomy assumption. He grants that such capacities exist and are sometimes exercised, singling out the moral deliberations of philosophers as likely examples (2001, 828&ndash29). His claim is just that such exercise of autonomous reasoning in the production of moral judgment is rare in the vast majority of cases, our judgments result from immediate &lsquointuitions&rsquo reflecting emotional causation. But there are many questions to raise even about this qualified claim (despite its generously letting philosophers, at least, off the hook).

Even where our judgments are based on immediate &lsquointuitions&rsquo, it may be that these intuitions are often themselves informed by prior acts of reflection and reasoning (Pizarro and Bloom 2003), or are partly cognitive responses to objective features of situations that may outrun our ability (prior to philosophical reflection) to articulate them&mdashmuch as a chess master may &lsquosee&rsquo that a certain move is best prior to being able to articulate exactly why (Kamm 1996, 2007). Similarly, the input from the autonomous reasoning of philosophers and some religious leaders may well influence the background ethical sensibilities of whole societies, thus influencing moral judgment even where there is no lengthy reasoning occurring in the particular case. There are also important philosophical worries about the methodologies by which Haidt comes to his deflationary conclusions about the role played by reasoning in ordinary people&rsquos moral judgments.

To take just one example, Haidt cites a study where people made negative moral judgments in response to &ldquoactions that were offensive yet harmless, such as&hellipcleaning one&rsquos toilet with the national flag.&rdquo People had negative emotional reactions to these things and judged them to be wrong, despite the fact that they did not cause any harms to anyone that is, &ldquoaffective reactions were good predictors of judgment, whereas perceptions of harmfulness were not&rdquo (Haidt 2001, 817). He takes this to support the conclusion that people&rsquos moral judgments in these cases are based on gut feelings and merely rationalized, since the actions, being harmless, don&rsquot actually warrant such negative moral judgments. But such a conclusion would be supported only if all the subjects in the experiment were consequentialists, specifically believing that only harmful consequences are relevant to moral wrongness. If they are not, and believe&mdashperhaps quite rightly (though it doesn&rsquot matter for the present point what the truth is here)&mdashthat there are other factors that can make an action wrong, then their judgments may be perfectly appropriate despite the lack of harmful consequences.

This is in fact entirely plausible in the cases studied: most people think that it is inherently disrespectful, and hence wrong, to clean a toilet with their nation&rsquos flag, quite apart from the fact that it doesn&rsquot hurt anyone so the fact that their moral judgment lines up with their emotions but not with a belief that there will be harmful consequences does not show (or even suggest) that the moral judgment is merely caused by emotions or gut reactions. Nor is it surprising that people have trouble articulating their reasons when they find an action intrinsically inappropriate, as by being disrespectful (as opposed to being instrumentally bad, which is much easier to explain).

To return to our central question: it remains unclear just how much of human moral judgment is susceptible to the mere rationalization hypothesis, or when it is, how much of a role is played by evolutionary influences on emotions. The mere rationalization hypothesis is best supported when we have independent reason to reject an appeal to autonomous reflection, as when a moral judgment is implausible in itself and we have a very likely debunking causal explanation of why someone might nonetheless be led to believe such a thing. Such is the case with MJ2 and MJ3 above, and perhaps many other traditional moral judgments focused on sexual taboos, notions of &lsquoimpurity&rsquo, in-group loyalty (from tribalism to nationalism), hierarchical authority relations, and rigid gender roles. But this does not provide grounds for a deflationary approach across the board.

Many moral beliefs&mdashfor example, concerning the moral irrelevance of sexual preference, the moral equality of persons of all races and nationalities, or moral obligations even to future generations in far away countries&mdashare much more plausible candidates for being upshots of autonomous moral reflection and reasoning. Indeed, many philosophers take them to be plausible candidates for moral truths, grasped through reflection that reveals good reasons for believing them, which is therefore what explains our moral beliefs and behavior in these cases.

These claims may, of course, be disputed. Even if it is granted that our judgments are often the products of our reasoning, pace Haidt (2001), it remains possible that our reasoning itself is distorted by evolutionary influences. This may be especially worrisome given the role played by intuitions in moral reasoning (unlike with mathematical reasoning, say), even in determining our acceptance of premises in arguments: insofar as there are concerns that our intuitions may be distorted by evolutionary influences, there will equally be worries about our moral reasoning (Sinnott-Armstrong 2005).

It remains unclear, however, just how far such worries really extend, even if it is granted that moral intuition and reasoning are often subject to such distorting influence. It is hard to see, for example, how some people&rsquos moral belief that we should impoverish ourselves and limit our own reproduction in order to help distant strangers, or even that we should cease having children altogether, could be explained in terms of the distortion of their reasoning by evolutionary influences. The striking range of moral beliefs itself, both across and within cultures, seems like good evidence of significant input from autonomous moral reflection&mdashmoral thought going beyond exercises of evolved &ldquoneurocomputational systems&rdquo or &ldquosocial contract algorithms&rdquo or &ldquoinference engines&rdquo with domain-specific goals and special inference procedures designed for solving particular adaptive problems (Cosmides and Tooby 2008). Even if there are indeed such special modules devoted to reasoning about obligation and entitlement, as posited by evolutionary psychologists, the question is how much they explain, and whether they really exclude significant input from autonomous moral reasoning.

These issues remain challenging and controversial. But the controversies are as much ongoing philosophical ones as scientific ones, and it is therefore unlikely that scientific results will settle them. Science will plainly not settle, for example, whether or not there are moral truths and if there are, they will likely play an explanatory role with regard to at least some of our moral beliefs&mdashsomething we will miss if we approach these issues from an exclusively scientific point of view.

A duly cautious claim about the explanatory role of evolution with respect to morality in the empirical sense might therefore be:


For the understanding of human nature, the evolutionary roots of human moral behaviour are a key precondition. The self-sacrificing behaviour within family is a social norm in most cultures. In the paper we apply the basic idea of evolution stability for our new population genetics model to investigate when the self-sacrificing behaviour within family is evolutionary rational. In evolution, from eusociality [1], through parental care [2] to sib cannibalism [3], interaction occurs between family members, i.e. between parents and their offspring and between siblings. For modelling this selection situation, there are basically three different approaches, all of which are connected to the present paper.

The first one uses the classical evolutionary game theory model [4, 5], based on the fact that interaction takes place between two individuals and the pay-off depends on the strategies of the players. For instance, if cooperation is evolutionarily stable in a well-mixed population [6, 7], then it should also be stable among siblings. However, classical evolutionary game theory is based on the following two assumptions, neither of which hold in our case. The first assumption is that the interaction is well-mixed (i.e. the rate of interaction between different phenotypes is proportional to their relative frequency in the whole population). Clearly, interaction within the family is not well-mixed. On the one hand, it has been pointed out [8] that if the interaction rate is high enough within the same asexual phenotype, then the maximizer phenotype (maximizing the average fitness coming from the interactions within the same phenotype) overperforms the classical evolutionarily stable strategy, ESS. (We note that there are a number of evolutionary studies on non-well-mixed interactions [9,10,11,12]. Moreover, our problem is connected to n-person games [12, 13], since the family can be considered as a genetically well-defined group. The second basic assumption of classical evolutionary game theory is that the population is asexual. However, parental care occurs in sexual families. We note that the relationship between the predictions of asexual and sexual evolutionary models is not a straightforward issue [14, 15].

The second approach uses kin selection theory. Hamilton [16] considered interactions between “neighbours” in a non-overlapping sexual viscous population, he measured the average genetic relatedness between neighbours and used a gene pool approach. Moreover, in this model interactions take place between two individuals. Under these assumptions, Hamilton’s rule claims that the frequency of altruistic genes should increase in a sexual population if rB > C, where r is the genetic relatedness of the recipient to the actor, B is the additional reproductive benefit gained by the recipient of the altruistic act and C is the reproductive cost to the individual performing the act. Observe that Hamilton’s rule applies to an interaction between recipient and actor, and only focuses on the degree of their genetic relatedness, furthermore it does not take into account the genotype-phenotype mapping. We note that Hamilton’s rule was validated by Rowthorn [17] in a sexual model where two alleles code different levels of altruism. Moreover, van Veelen [18] found that the mean inclusive fitness is maximized if there is no pair of alleles for which there is over- or underdominance. We also note that in a sexual model for parent-offspring conflict [19, 20], Hamilton’s rule could not be validated by Bossan et al. [21].

We call the attention to the fact that now we will strictly focus only on the genes that determine the altruistic interactions within a monogamous family, disregarding the competition between families. In this case, the distribution of altruistic genes in groups of families has no direct effect on the evolutionary success of the genes considered within the family. Thus our model cannot be built upon any of the earlier life history models [22, 23], or Price’s equation [24]. We note that the models based on Price’s equation support Hamilton’s rule [25, 26].

Before describing the third modelling approach and setting up ours, we recall some recent results we consider starting points of the present study. In Garay et al. [3] a kin demographic selection model was introduced for the study of the selection situations when interactions take place only within a family of an asexual aging population with overlapping generations. Here fitness was the long-term growth rate of the phenotype, and it was shown that cannibalism can be considered as a mutualistic kin strategy, if the sib cannibal either decreases its developmental time or increases fecundity in the adult stage. There are two further applications of this kin demographic selection model. In Garay et al. [27] the juvenile honest food solicitation and parental investment was studied. It was shown that honest begging results in decreased variance of collected food among siblings, which leads to a higher mean number of surviving offspring. Furthermore, in Garay et al. [28] the evolutionary roots of human morality were studied. It was found that the biological version of the Fifth commandment, called the Fifth rule (“During your reproductive period, give away from your resources to your post-fertile parents”) can spread by means of natural selection, by increasing the survival rates of the family members. All three studies cited above considered asexual populations, thus the question arises: What is the mathematical condition for the evolutionary stability of altruistic behaviour within the monogamous family in a diploid sexual population?

To answer the latter question, we will follow the third modelling approach, namely we will use a population genetic model that can take account of the genetic background of the inheritance of the altruistic phenotype, and the frequency dependent interactions between genotypes, at the same time [29,30,31]. The advantages of a population genetic model for the evolution of altruism were already demonstrated back in the 1980’s, [32,33,34]. Maynard Smith [30] has already pointed out that the personal fitness model has the virtue of “incorporating the evolution of altruism into the corpus of population genetics as an example of frequency-dependent selection.” It seems to us that this research line, in comparison with the kin and group selection approaches, is less elaborated. The reason for this, as we see it, is that the study of population genetics models may be mathematically rather involved, although recently these models gained more attention [17, 35]. Furthermore, based on Hamilton [36] Michod [37, 38] pointed out that inbreeding (e.g. sib mating) may facilitate the evolution of altruism [39]. However, the incest has been one of the most widespread of all cultural taboos, both in present and in many past societies [40]. What is more, in Israeli kibbutzim, it was observed that there was no sexual activity and no marriage between those who are not relatives but grew up in the same peer group. Among marriages of second generation adults in all kibbutzim, no intra-peer group marriage occurred in fact, a continuous exposure among peers aged 0–6 years results in sexual avoidance and exogamy [41]. Based on all these, here we only focus on the exogamous, monogamous families (in particular, we will not consider either polygamy, or sexual selection). We note that, for the sake of simplicity, we do not investigate either the stability of monogamy [42], or the effect of the cultural evolution [43]. The reason for the latter is our hypothesis that the evolutionary roots of the moral may originate from the times before the evolution of language. Since we are strictly interested only in what happens within monogamous families, our population genetics model is rooted in the models of Hull [44] and Haldane and Jayakar [45], where the juveniles’ survival rate depends on the genotypes of their parents. The basic assumptions of the here introduced population genetics model are quite different from those of Hamilton [16, 32, 37], and are as follows: 1. We consider a large enough, non-ageing, sexual population with overlapping generations [17, 21]. 2. The mating system is exogamous and monogamous [46] without promiscuity and in each reproductive season the pair formation is random. 3. Interactions take place only within each family, and they determine the survival rate of the family members. Thus we consider the following population cycle: Firstly, the pairs of parents are formed randomly, so the mating system is not viscous. Secondly, the family has offspring, whose phenotype is determined by the genotypes of the parents according to Mendelian inheritance. Thirdly, the survival rate of each family member is determined by the phenotypes of family members. Then the cycle starts again. In spite of the fact that the random mating happens between zygotes in the whole population, in our phenotypic selection situation each individual’s survival rate is determined by her family members, and the genotype composition of each family depends on the genotypes of monogamous pairs of parents, thus our selection situation motivates us to use the mating table approach as a starting component of the population genetics model introduced above. A complete dynamical characterization of genotype distribution changes is not our goal. We are interested only in the evolutionary stability of a genotype distribution in a sexual population: Under what conditions is a homozygote state stable?

In the present paper we focus on the following “self-sacrificing” norm: “Risk your life to save your family members, if you want them to save your life.” In biological terms, this norm of self-sacrificing means that the actors risk their own lives for the lives of their family members, i.e. the altruistic interactions change the survival rate of the family members. (In Additional file 1: SI A, we give a fairly general method for the biological modelling of a moral norm.) We note that the nomenclature of “self-sacrificing” goes back to the Haldane quip [47]: “Legend has it that in a pub one evening Haldane told his friends that he would jump into a river and risk his life to save two brothers, but not one, and that he would jump in to save eight cousins, but not seven.” We consider a monogamous family in which male and female differ only in sex, i.e. they are the same from all other points of view. Furthermore, we consider three interactions within the family (see in Fig. 1).

The arrows visualize interactions, + and –stands for gain and cost, respectively

The first interaction is parental investment, i.e. when parents, at the expense of decreasing their own survival rate, increase the survival rate of their offspring. We note that, in human, birth is risky, in our terminology self-sacrificing, e.g. in 2015 the global maternal mortality ratio was 216 deaths per 100 000 live births [48]. For simplicity, we consider two levels of parental investment: a provider parent invests more than a non-provider parent. The second interaction is sib altruism, i.e. the altruistic juvenile individuals, at the expense of decreasing their own survival rate, increase the survival rate of their siblings, while the non-altruistic siblings do not help others. The third interaction is offspring gratitude, i.e. juvenile individuals, at the expense of decreasing their own survival rate increase the survival rate of their parents, while non-grateful siblings do not. We remind that from psychological point of view, human gratitude appears during childhood [49,50,51]. We use the term “offspring gratitude” to emphasize the difference from other kinds of gratitude as e.g. “upstream reciprocity” [52, 53], and the Fifth rule [28]. Now we can make precise what we understand under “self-sacrificing life history strategy”: an individual having this phenotype is altruistic and grateful in juvenile age, and is a provider parent during adult age. Our question can now be formulated as follows: Under what conditions will the self-sacrificing life history strategy be evolutionary stable in sexual populations, if mutation is rare enough?

In this paper we introduce a new population genetics model, with three main simplifying assumptions: during pre-human and early human evolution there was no family planning, the offspring numbers are fixed (i.e. the offspring number in families does not depend on the interactions within the family), and the populations are non-aging. We conclude our study with a discussion section.

We thank Shimpei Ishiyama, Katriona Guthrie-Honea, Jessica L. Verpeut, and David J. Reiss for comments on the manuscript, and all Brecht lab members for valuable discussions. We thank the experts who replied to our request for more information or additions to the data set: Prof. Dr. Jan Dressler, Prof. Dr. Christine Erfurt, Prof. Darnell Hawkins, Prof. Dr. Kathleen Heide, Dr. Marieke Liem, Prof. Dr. Friedemann Pflin, Prof. Dr. med. Klaus Püschel, Prof. Dr. Stefanie Ritz-Timme, Prof. Bill Schutt, Dr. Asser Hedegard Thomsen, Prof. Dr. Michael Tsokos, Prof. Dr. Marcel A. Verhoff, and Peter R. de Vries.

Andrade, M. C. B. (1996). Sexual selection for male sacrifice in the Australian Redback spider. Science 271, 70�. doi: 10.1126/science.271.5245.70

Antfolk, J., Lieberman, D., Harju, C., Albrecht, A., Mokros, A., and Santtila, P. (2018). Opposition to inbreeding between close kin reflects inclusive fitness costs. Front. Psychol. 9:2101. doi: 10.3389/fpsyg.2018.02101

Arens, W. (2004). The man-eating myth: Anthropology and anthropophagy. Nachdr. Oxford: Oxford Univ. Press.

Arijón, G. (2008). Stranded: I’ve come from a plane that crashed on the mountains [motion picture]. France: Zeitgeist Films.

Barker, F., Hulme, P., and Iversen, M. (eds.) (1998). Cannibalism and the colonial world. Cambridge. New York: Cambridge University Press.

Beauregard, E., and DeLisi, M. (2018). Stepping stones to sexual murder: the role of developmental factors in the etiology of sexual homicide. J. Crim. Psychol. 8, 199�. doi: 10.1108/JCP-02-2018-0010

Billingsley, J., Antfolk, J., Santtila, P., and Lieberman, D. (2018). Cues to paternity: do partner fidelity and offspring resemblance predict daughter-directed sexual aversions? Evol. Hum. Behav. 39, 290�. doi: 10.1016/j.evolhumbehav.2018.02.001

Birkhead, T. R., Lee, K. E., and Young, P. (1988). Sexual cannibalism in the praying Mantis Hierodula Membranacea. Behav. Ther. 106, 112�. doi: 10.1163/156853988X00115

Boduszek, D., Debowska, A., and Willmott, D. (2017). Latent profile analysis of psychopathic traits among homicide, general violent, property, and white-collar offenders. J. Crim. Just. 51, 17�. doi: 10.1016/j.jcrimjus.2017.06.001

Boduszek, D., Hyland, P., and Bourke, A. (2012). An investigation of the role of personality, familial, and peer-related characteristics in homicidal offending using retrospective data. J. Crim. Psychol. 2, 96�. doi: 10.1108/20093821211264414

Bruijn, E. F. -D., and Sevenster, P. (1982). Parental reactions to Young in sticklebacks (Gasterosteus aculeatus L.). Behaviour 83, 186�.

Burgess, A. W., Commons, M. L., Safarik, M. E., Looper, R. R., and Ross, S. N. (2007). Sex offenders of the elderly: classification by motive, typology, and predictors of severity of crime. Aggress. Violent Behav. 12, 582�. doi: 10.1016/j.avb.2007.02.006

Bygott, J. D. (1972). Cannibalism among wild chimpanzees. Nature 238, 410�. doi: 10.1038/238410a0

Chan, H. -C. O., and Heide, K. M. (2009). Sexual homicide: a synthesis of the literature. Trauma Violence Abuse 10, 31�. doi: 10.1177/1524838008326478

Chan, H. C. O., Heide, K. M., and Beauregard, E. (2010). What propels sexual murderers: a proposed integrated theory of social learning and routine activities theories. Int. J. Offender Ther. Comp. Criminol. 55, 228�. doi: 10.1177/0306624X10361317

Clemens, A. M., Wang, H., and Brecht, M. (2020). The lateral septum mediates kinship behavior in the rat. Nat. Commun. 11:3161. doi: 10.1038/s41467-020-16489-x

Daly, M., and Wilson, M. (1982). Homicide and kinship. Am. Anthropol. 84, 372�. doi: 10.1525/aa.1982.84.2.02a00090

Dawkins, R. (1982). The selfish gene. New York: Oxford University Press.

DeLisi, M., Kosloski, A., Sween, M., Hachmeister, E., Moore, M., and Drury, A. (2010). Murder by numbers: monetary costs imposed by a sample of homicide offenders. J. Forensic Psychiatry Psychol. 21, 501�. doi: 10.1080/14789940903564388

DeSantis, D. T., and Schmaltz, L. W. (1984). The mother-litter relationship in developmental rat studies: cannibalism vs caring. Dev. Psychobiol. 17, 255�. doi: 10.1002/dev.420170306

Dixon, C. (2011). The crossbow cannibal: The definitive story of Stephen Griffiths—the self-made serial killer. London: John Blake.

Elgar, M. A., and Crespi, B. J. (eds.) (1992). Cannibalism: Ecology and evolution among diverse taxa. Oxford [England]. New York: Oxford University Press.

Fish, A., and Borowski, J. (2014). Albert Fish in his own words: The shocking confessions of the child killing cannibal. Chicago, IL: Waterfront Productions.

FitzGerald, G. J., and van Havre, N. (1987). The adaptive significance of cannibalism in sticklebacks (Gasterosteidae: Pisces). Behav. Ecol. Sociobiol. 20, 125�. doi: 10.1007/BF00572634

Fouilloux, C., Ringler, E., and Rojas, B. (2019). Cannibalism. Curr. Biol. 29, R1295–R1297. doi: 10.1016/j.cub.2019.09.068

Fowler, A., and Hohmann, G. (2010). Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale. Am. J. Primatol. 72, 509�. doi: 10.1002/ajp.20802

Fox, J. A. (2007). Uniform Crime Reports [United States]: Supplementary Homicide Reports, 1976�. 2007-07-25.

Fox, J. A., and Zawitz, M. W. (2010). Homicide trends in the United States.

Fox, L. R. (1975). Cannibalism in natural populations. Annu. Rev. Ecol. Syst. 6, 87�. doi: 10.1146/

Garza, S., and Waldman, B. (2015). Kin discrimination in polyphenic salamander larvae: trade-offs between inclusive fitness and pathogen transmission. Behav. Ecol. Sociobiol. 69, 1473�. doi: 10.1007/s00265-015-1959-0

Gillies, H. (1976). Homicide in the west of Scotland. Br. J. Psychiatry 128, 105�. doi: 10.1192/bjp.128.2.105

Goodall, J. (1977). Infant killing and cannibalism in free-living chimpanzees. Folia Primatol. 28, 259�. doi: 10.1159/000155817

Hamai, M., Nishida, T., Takasaki, H., and Turner, L. A. (1992). New records of within-group infanticide and cannibalism in wild chimpanzees. Primates 33, 151�. doi: 10.1007/BF02382746

Hamilton, W. D. (1964). The genetical evolution of social behaviour. II. J. Theor. Biol. 7, 17�. doi: 10.1016/0022-5193(64)90039-6

Hausfater, G., and Hrdy, S. B. (2017). Infanticide: Comparative and evolutionary perspectives. Abingdon: Routledge.

Kapo, I., Kazaferi, A., Vyshka, G., and Xhemali, B. (2018). A case of schizophrenia presenting with anthropophagy. AJMHS 49:6.

Kolata, G. (1986). Anthropologists suggest cannibalism is a myth. Science 232, 1497�. doi: 10.1126/science.3715456

Lehti, M., Kivivuori, J., Bergsdóttir, G. S., Engvold, H., Granath, S., Jónasson, J. O., et al. (2019). Nordic homicide report: Homicide in Denmark, Finland, Iceland, Norway and Sweden, 2007�. Helsingin yliopisto, kriminologian ja oikeuspolitiikan instituutti Available at: (Accessed February 11, 2020).

Lester, D., White, J., and Giordano, B. (2015). Cannibalism. Omega (Westport) 70, 428�. doi: 10.1177/0030222815573732

Liberski, P. P., Gajos, A., Sikorska, B., and Lindenbaum, S. (2019). Kuru, the first human prion disease. Viruses 7:11. doi: 10.3390/v11030232

Lieberman, D., Tooby, J., and Cosmides, L. (2007). The architecture of human kin detection. Nature 445, 727�. doi: 10.1038/nature05510

Lorenz, K., Wilson, M. K., and Huxley, J. (2002). On aggression. London: Routledge.

Mehlis, M., Bakker, T. C. M., and Frommen, J. G. (2008). Smells like sib spirit: kin recognition in three-spined sticklebacks (Gasterosteus aculeatus) is mediated by olfactory cues. Anim. Cogn. 11, 643�. doi: 10.1007/s10071-008-0154-3

Nishida, T., and Kawanaka, K. (1985). Within-group cannibalism by adult male chimpanzees. Primates 26, 274�. doi: 10.1007/BF02382402

Parr, L. A., and de Waal, F. B. M. (1999). Visual kin recognition in chimpanzees. Nature 399, 647�. doi: 10.1038/21345

Parrado, N., and Rause, V. (2013). Miracle in the Andes: 72 days on the mountain and my long trek home. New York: Three Rivers Press.

Pfennig, D. W. (1997). Kinship and cannibalism. Bioscience 47, 667�. doi: 10.2307/1313207

Pfennig, D. W., Reeve, H. K., and Sherman, P. W. (1993). Kin recognition and cannibalism in spadefoot toad tadpoles. Anim. Behav. 46, 87�. doi: 10.1006/anbe.1993.1164

Pfennig, D. W., Sherman, P. W., and Collins, J. P. (1994). Kin recognition and cannibalism in polyphenic salamanders. Behav. Ecol. 5, 225�. doi: 10.1093/beheco/5.2.225

Polis, G. A. (1980). The effect of cannibalism on the demography and activity of a natural population of desert scorpions. Behav. Ecol. Sociobiol. 7, 25�. doi: 10.1007/BF00302515

Rajs, J., Lundström, M., Broberg, M., Lidberg, L., and Lindquist, O. (1998). Criminal mutilation of the human body in Sweden𠅊 thirty-year medico-legal and forensic psychiatric study. J. Forensic Sci. 43:16183J. doi: 10.1520/JFS16183J

Raymond, S., Léger, A., and Gasman, I. (2019). The psychopathological profile of cannibalism: a review of five cases. J. Forensic Sci. 64, 1568�. doi: 10.1111/1556-4029.14099

Ryan, S., Willmott, D., Sherretts, N., and Kielkiewicz, K. (2017). A psycho-legal analysis and criminal trajectory of female child serial killer Beverley Allitt. Eur. J. Curr. Leg. Issues 23, 1�.

Sahlins, M. (2003). Artificially maintained controversies: global warming and Fijian cannibalism. Anthropol. Today 19, 3𠄵. doi: 10.1111/1467-8322.00189

Sawin, P. B., Petropolus, S. F., Denenberg, V. H., and Ross, S. (1960). Genetic, physiological, and Behavioral background of reproduction in the rabbit. Behaviour 15, 71�. doi: 10.1163/156853960X00106

Sherretts, N., Boduszek, D., Debowska, A., and Willmott, D. (2017). Comparison of murderers with recidivists and first time incarcerated offenders from U.S. prisons on psychopathy and identity as a criminal: an exploratory analysis. J. Crim. Just. 51, 89�. doi: 10.1016/j.jcrimjus.2017.03.002

Sugg, R. (2015). Mummies, cannibals and vampires: The history of corpse medicine from the renaissance to the Victorians. 2nd Edn. Abingdon: Routledge.

Tal, I., and Lieberman, D. (2008). “Kin detection and the development of sexual aversions: toward an integration of theories on family sexual abuse” in Family relationships: An evolutionary perspective. eds. C. A. Salmon and T. K. Shackelford (Oxford: Oxford University Press), 205�.

Tanaka, C. T., Berger, W., Valen๺, A. M., Coutinho, E. S. F., Jean-Louis, G., Fontenelle, L. F., et al. (2017). The worldwide incidence of neonaticide: a systematic review. Arch. Womens Ment. Health 20, 249�. doi: 10.1007/s00737-016-0703-8

Tanaka, T. (2018). Hidden horrors: Japanese war crimes in world war II. 2nd Edn. Lanham: Rowman and Littlefield.

Teixeira, E. H., Meneguette, J., and Dalgalarrondo, P. (2012). Matricໝio, seguido de canibalismo e automutilação de pênis e mão em paciente esquizofrênico motivado por delírios religiosos. J. Bras. Psiquiatr. 61, 185�. doi: 10.1590/S0047-20852012000300011

Thomsen, A. H., Leth, P. M., Hougen, H. P., Villesen, P., and Brink, O. (2019). Homicide in Denmark 1992�. Forensic Sci. Int.: Synergy 1, 275�. doi: 10.1016/j.fsisyn.2019.07.001

Uniform Crime Reporting (UCR) Program Federal Bureau of Investigation (2020). Available at: (Accessed February 11, 2020).

Villa, P. (2005). Cannibalism in prehistoric Europe. Evol. Anthropol. 1, 93�. doi: 10.1002/evan.1360010307

Westermarck, E. (1922). The history of human marriage. New York, NY: Allerton Book Company.

Whitfield, J. T., Pako, W. H., Collinge, J., and Alpers, M. P. (2008). Mortuary rites of the south fore and kuru. Philos. Trans. R. Soc. B 363, 3721�. doi: 10.1098/rstb.2008.0074

Keywords: cannibalism, kin, homicide, evolution, mental health

Citation: Oostland M and Brecht M (2020) Kin-Avoidance in Cannibalistic Homicide. Front. Psychol. 11:2161. doi: 10.3389/fpsyg.2020.02161

Received: 22 April 2020 Accepted: 03 August 2020
Published: 31 August 2020.

Pekka Santtila, New York University Shanghai, China

Angelo Zappalà, Istituto Universitario Salesiano Torino Rebaudengo (IUSTO), Italy
Dominic Willmott, Manchester Metropolitan University, United Kingdom
Jan Antfolk, ౛o Akademi University, Finland

Copyright © 2020 Oostland and Brecht. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.


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