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Punishing Cheaters Promotes the Evolution of Cooperation

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"Primates Playing Poker" by Nathaniel Gold

"Primates Playing Poker" by Nathaniel Gold

Humans are one of the most cooperative species on the planet. Our ability to coordinate behavior and work collaboratively with others has allowed us to create the natural world’s largest and most densely populated societies, outside of deep sea microbial mats and a few Hymenoptera mega-colonies.

However, a key problem when trying to understand the evolution of cooperation has been the issue of cheaters. Individuals in a social group, whether that group is composed of bacteria, cichlids, chimpanzees, or people, often benefit when cooperating with others who reciprocate the favor. But what about those individuals who take advantage of the generosity of others and provide nothing in return? These individuals could well thrive thanks to the group as a whole and end up with greater fitness than everyone else because they didn’t have to pay the costs associated with cooperating. For decades the idea that cheaters may in fact prosper has been the greatest difficulty in understanding cooperation as an evolved trait.

However, it turns out that cooperation could be a viable evolutionary strategy when individuals within the group collectively punish cheaters who don’t pull their weight. For example, Robert Boyd, Herbert Gintis, and Samuel Bowles published a paper in the journal Science in 2010 with a model showing how, so long as enough individuals work together to punish violators, each cooperative individual in the group can experience enhanced fitness as a result.

Before understanding how their model could explain the emergence of cooperative behavior it is first important to look at the two leading explanations for the evolution of cooperation: William Hamilton’s (1964) theory of kin selection and Robert Trivers’ (1971) theory of reciprocal altruism.

Kin selection proposed that cooperation will emerge in groups that are made up of close relatives. Hamilton’s rule, beautiful in its simplicity, proposed that cooperation occurs when the cost to the actor (c) is less than the benefit to the recipient (b) multiplied by the genetic relatedness between the two (r). This equation is written out simply as rb > c. Kin selection has been one of the most well tested models that seeks to explain the evolution of cooperation and has held up among such diverse groups as primates, birds, and social insects (though Edward O. Wilson has recently challenged kin selection as an explanation in the latter).

To put this into context: an alpha male lion and his brother share half of their genes, so have a genetic relatedness of 0.5. Suppose this brother recognizes that the alpha male is getting old and could easily be taken down. If so, the brother could potentially have eight additional cubs (just to pull out an arbitrary number). But, instead, that brother decides to help the alpha male to maintain his position in the pride and, as a result, the alpha ends up having the eight additional cubs himself while the brother only has five. The brother has lost out on 3 potential cubs. But, even so, because he assisted his brother he has still maximized his overall reproductive success from a genetic point of view: 0.5 x 8 = 4 > 3. He could have attempted to usurp his brother and, perhaps, had the eight cubs himself but he wouldn’t have been in any better of a position as far as his genes were concerned.

Reciprocal altruism follows this same basic idea, but proposes a mechanism that could work for individuals that are unrelated. In this scenario, cooperation occurs when the cost to the actor (c) is less than the benefit to the recipient (b) multiplied by the likelihood that the cooperation will be returned (w) or wb > c. This has been demonstrated among vampire bats who regurgitate blood into an unrelated bats mouth if they weren’t able to feed that night. Previous experience has shown the actor that they’re likely to get repaid if they ever go hungry one night themselves.

Whereas kin selection requires a community of closely related individuals for cooperation to be a successful strategy, reciprocal altruism requires that individuals be part of a single group, with low levels of immigration and emigration, so that group members will be likely to encounter each other on a regular basis. However, neither model can explain the emergence of cooperation in societies composed of unrelated individuals and where there is a constant influx of strangers. In other words, cooperation in human societies.

The more recent model proposed by Boyd et al. seeks to address this very problem. Their paper posits that fitness is enhanced, not by cooperating with close kin or reciprocating a previous act of generosity, but through the coordinated punishment of those who don’t cooperate. In a social group individuals are able to choose whether they want to cooperate or defect. Suppose, for example, that a hunter returns from a successful hunt and must decide whether or not to share their gains with other members of the tribe. According to Boyd’s model, the cost to the cooperator (c) is less than the overall benefit (b) but is still greater than the benefit to each member of the group (n): b > c > b/n. If the hunter chose to cooperate, the meat would be divided so that everyone benefits but the hunter still enjoys a slightly larger share. They would also receive a benefit in the future when other hunters had more success than they did (just as they would under reciprocal altruism).

However, if the hunter refuses to share with other members there are two stages to contend with. The first is the signaling stage in which individuals signal their intent to punish those who refuse to cooperate. This is a common occurrence not just in humans but in many animals, especially primates. Baboons, for example, use threat signals such as staring, eyebrow raising, or a canine display to warn others to change their behavior. In humans this can take a variety of forms including angry looks, hand gestures, and/or harsh words. The cost of such signals are fairly low, but still high enough that it doesn’t pay to signal and fail to back it up with action if necessary.

If the warning doesn’t provide the appropriate result the next stage is coordinated punishment. According to Boyd’s model a quorum (τ) of punishers is required to work together to target an individual who refuses to cooperate. In such cases there will be a cost (p) to the target and an expected cost to each punisher of k/npa, where np is the number of punishers. Given that an outnumbered target is unlikely to inflict costs on the punishers, the model assumes that a > 1. What this means is that the higher the number of punishers, the lower the cost to each involved. Furthermore, the punishment doesn’t necessarily involve physical attacks. The model allows for punishment to come in the form of gossip, group shunning, or any other nonaggressive action that brings a cost to the uncooperative target.

Punishment is an evolutionarily stable strategy

Punishment is an evolutionarily stable strategy when multiple punishers are involved. Reproduced from Boyd et al. (2010). Click image to enlarge.

According to this model a society would be made up of some combination of punishers Wp and nonpunishers Wn. If there were only a single punisher in the group (τ = 1), what is known as the “Lone Ranger” condition, the fitness cost would outweigh the benefit and punishers would decline in the population. However, for larger values of τ punishment does pay and therefore increasing the number of punishers increases their fitness.

This model has had some empirical support. For example, last year Boyd and Sarah Matthew found that punishing desertion promoted cooperation in raiding parties among the Turkana pastoralists in East Africa. Likewise, Lauri Sääksvuori and colleagues published their results in Proceedings of the Royal Society suggesting that this form of enforced cooperation could emerge through competitive group selection. While further empirical tests are needed to confirm Boyd’s model, it has the benefit of demonstrating how cooperation could evolve even in large societies where kinship is low and immigration is high: the very factors that were previously thought to confound the evolution of cooperation.

However, given that many indigenous systems are based on restorative justice (in which offenders are brought into a relationship with the victim and must make restitution to regain the society’s trust) it’s unclear how accurate a model focusing exclusively on punishment would be for understanding the evolution of human cooperation. Nevertheless, Coordinated Punishment now joins other recent approaches, such as Generalized Reciprocity, that seek to reexamine how the common good could emerge out of the selection for individual fitness.


Boyd, R., Gintis, H., & Bowles, S. (2010). Coordinated Punishment of Defectors Sustains Cooperation and Can Proliferate When Rare, Science, 328 (5978), 617-620. DOI: 10.1126/science.1183665

This post has been adapted from material that originally appeared at

Eric Michael Johnson About the Author: Eric Michael Johnson has a Master's degree in Evolutionary Anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics.

Follow his work on Facebook and Google+. Follow on Twitter @primatediaries.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. CherryBombSim 6:52 pm 08/16/2012

    So, what behavior is Boyd, et al. claiming to be the target of selection here? Is it the behavior of the punishers who gang up on anyone not willing to share? Is it the behavior of the person who shares under duress? In either case, it would seem that individuals are acting to increase individual fitness, and the apparent altruism is an epiphenomenon.

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  2. 2. marclevesque 7:48 pm 08/16/2012

    “Punishing Cheaters Promotes the *Evolution of Cooperation*”

    I’m wondering if it is accurate to say, since the beginning of life as we know it and in the context of evolution, that the questioning, if not foremost, then at least equally, should also be about, how did individualism evolve ?

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  3. 3. danarel 3:02 am 08/17/2012

    another awesome post Eric. Thank you.

    The research on this is fascinating and is the main area of my studies in anthropology.

    Cheaters can prosper in groups if they control their cheating and are not caught, if caught, if the society has a mode of punishment, than of course that individual we pay a price. Looking at this via natural selection makes such perfect sense. Even if a cheater does prosper, if they are able to reproduce and base on the gene or meme that brings out their cheating behavior, than you could end up with a whole species or society of cheaters where everyone loses. Therefor, to prosper, it all comes down to tit for tat. Much like proposed by Richard Dawkins in The Selfish Gene. Societies working together will prosper as a whole even if for selfish reasons.

    This does not allude to Group Selection but still falls right into Kin Selection. Altruism is still in fact selfish behavior and self serving. I pick the bugs off you, you off me and we both win, but if I can trick you into cleaning me, and me not cleaning you, I may gain short term success, but it will not last.

    The punishment of cheaters, to me, makes perfect evolutionary sense.

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  4. 4. naya8 10:59 am 08/17/2012

    Coordinated punishment,reciprocal altruism and any other cultural input are all behaviors; these are not traits but mere estratigies of the brain.Good evolved brain will choose a good estratigy to survive.Brain is the promoter of lif and survival. We should speak about brain evolution and not behavior evolution. I tkink that thw best model is:

    Micro-evolution (short term):
    genes—> brain-microstructure—> trait—-> behavior

    Macro-evolution (long-term):
    insisting behavior (culture)—-> trait—> brain microstructure —-> genes

    All evidences of real life support this model.

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  5. 5. danarel 12:39 am 08/18/2012

    not exactly true naya8,

    studies have shown that certain personality traits are in fact genetic (

    this could include traits such as “cheater” that is passed on via reproduction.

    even if not, memes are learned passed on, and if you parent teaches you selfish cheating behavior, its just as good an inheriting it. You will then cheat, and reproduce and pass that on as well.

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  6. 6. naya8 10:30 am 08/18/2012

    danarel:You emphasize what i have said that traits are genetic. When behavior comes to be a trait? as I said also when insisting behavior pushes the equilibrium that is existing in every biological step toward the conducting of that trait, this depends in large on the microstructure of the brain that is genetic.Not every one respond to the same trigger in an identical way for this depends on the brain itself.There is no such thing mems; this is not scientific but phylosophic.

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  7. 7. danarel 11:49 am 08/18/2012

    Naya8, that is not correct. A meme itself can be philosophical, but the idea of a meme is not, and has been studied scientifically since Dawkins coined the idea in the 70s.

    Memes have been studied to be passed on via natural selection just a gene is. I would call it a controversial theory, but that does not mean its irrelevant.

    maybe, i lose a little of what you mean as I am assuming english it not your first language, but there are behavioral traits. so you cannot just dismiss cheating as being a behavior and not a trait.

    Of course not everyone is going to react to their own gene the same, but that applies to every gene being passed on. If i have 4 kids, they will not all get my traits 100% but it doesn’t mean that gene is still not being passed on generation after generation.

    Altruism is the same thing as cheating, its a behavioral trait used to survive, and its not there for the good of the group, but for the good of the individual. Altruism is still a selfish act, even if it does help others. Altruistic behavior only works if others in the group display such behavior as well, if you are the only helpful one in your group and everyone else takes advantage of it, you will not thrive in that environment. You need other to reciprocate for your own good as well. Tit for tat.

    Unless you are also under the belief that altruism is not a passed on trait.

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  8. 8. naya8 2:07 pm 08/18/2012

    danarel: you are right English is not my first language, even not the second, but i will try my best to let you understand properly my idea.I think that science is about facts not about worshiping people who hypothized these facts. Even if Dawkins coined the idea of mems but it is not explainable scientificaly. I can replace mems by “ganglia complex” or “neurons complex”. In fact u did’nt get my idea. I don’t think that any trait is passed gegeticaly as a trait. if you examine the model I suggest you will realize that the unit that is transmitted geneticaly is a unique structure of the brain that it could be “ganglia group”or “neuron group” that could be found in any part of the brain.What I am trying to explain is that not the traits that are passed through generations but the “small parts of the brain that are responsuble to that trait”. For example I am resembling my father in his tendency to get angry not becouse the trait of anger has passed to me, bt for I get that region of the brain from him. I resemble my mother in not believing in god not because this trait passed to me from her bt for I get that part of her brain responsible for this triat. In brief I can say that we inherit our brain like a mosaic from our parents and that micro-structure of the brain we get will behave the same as it did in our parents, all this happens not because of traits passing but for of whol parts of brain passing through generations.

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  9. 9. danarel 1:36 am 08/19/2012

    naya8: the idea of memes have been explained scientifically many many times over. there is even a area of science called memetics. many scientific books and journal articles published on the meme theory.

    now you are arguing that traits are not passed on through genes, but through the brain? I’m sorry, but if something is passed on from parent to child, its genetic, if you have your moms personality trait to lose your temper, is that learned or genetic. if you are saying you got that part of her brain, how do you think it got there? it didn’t magically fly there after birth! it was passed on via reproduction and survived generation to generation via natural selection.

    so your own argument is that traits ARE being passed on via evolution.

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  10. 10. naya8 4:19 am 08/19/2012

    danarel: I suoppose that you are finding difficulties in understanding my theory.May be it’s because of my poor English; or may be i am not profitional in explaining. I don’t assume that traits are passed on via evolution, they are passed on via genes. However, there is no direct connection between genes and traits, for the connection occures via brain miro-structure. I assume that genes affect brain structure and this in turn affect the possibility of conducting a trait.Second; i dont think that the ultimate michanism of evolution is natural selection but accumolating changes.
    danarel: my mail is it’s my pleasure to continue our argument there.

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