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Morality and Evolution 3: Apes and populations

Last updated on 22 May 2014

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Humans are apes, evolutionarily speaking. That is, while we are named distinctly in the vernacular usage from the rest of the apes (chimp, bonobo, gorilla, orang-utan, gibbons x 2), we fall squarely within the great ape clade. As such, we might expect that we share with them a social nature.* As I said in the first post, social living is what apes do. Moreover, apes have and enforce social norms. Usually this is based upon mating strategies that are typical for the species. Gorillas, for instance, live in troops of females with young, and a single male (a silverback) that defends mating rights against other unattached (“bachelor”) males. Common chimps live with in troops of several dozen, with an alpha male and several subordinate males. Bonobos have roughly equal numbers of adult males and females, and the alpha individuals can as easily be female as male. Dominant females also occur in chimps when no male is mature enough to compete for the top position.

In each species, norms are enforced by, in the first instance, the dominant individual (usually a male, as I said), who punishes those who fail to recognise the norms, and, in the case of dual gender social group species like chimps and bonobos, reward their allies. Frans de Waal has documented these behaviours in a series of engaging books [listed below]. Roughly, the smaller the sexual dimorphism (differences in size and traits between the sexes), the more equal the gender mix of a social group, and the more a dominant individual must “negotiate” cooperation within their troop. The reason for this is that if extreme polygyny is the rule, as in gorillas, the bigger the male has to be able to control access to his females. Bonobos, on the other hand are nearly the same size – males are around 10% larger than females, and the gender balance is more equal.

Humans, however, have even less sexual dimorphism than bonobos: the average size dimorphism is around 5% for males over females. And our mating strategies are much more equal: mates tend to be singles rather than multiples. Each species has its own version of the Primate Standard Social Structure (PSSS). Since, if we wish to understand human morality, we must consider how we vary and are the same as our nearest relatives, let me set this up, as I understand it.

The PSSS is based upon primates living in troops of varying size. A troop can be a family group, or it can be a more distantly related kin group. Outbreeding occurs when chances of meeting other troops is rare enough to ensure inbreeding among the troop members, but common enough that mates can be acquired from other neighbouring troops. Each troop tends to have its own range, and in chimps, for example, conflict at the borders of these ranges, and attempts to take control of resources and territory, can be quite vicious. Generally, however, primate interactions within the troop are peaceful, and involve only threat displays rather than actual violence.

Pairwise conflicts and transactions set up a dominance hierarchy among primates. Each individual tracks those troop members they are dominant, subordinate, or equal to, and thus a pyramid of relationships evolves over time, as members mature and challenge for status. High status individuals have first choice of mates, food and other resources like resting places. They permit secondary choices to the next rank, and so on. Generally these dominance hierarchies are fluid and not well defined past the third layer, of “gamma” individuals. As the dominant individual ages, younger members may challenge for the top position, often forming alliances with other individuals. Dominant individuals reward their allies and well-behaved individuals in the troop, and punish those who transgress the status quo (if they can). Grooming is one way that primates tend to reinforce these social relations: the higher the status, the more well groomed by others an individual is.

Troops have a maximum size, based on the working memory constraints for tracking cooperators and defectors. The more complex the signs of these behaviours, the smaller the maximum size of the troop. When the troop exceeds this threshold, it will split, spontaneously.

Now humans are apes, as I said, and so we should expect that humans will behave the same way as the other primates, and especially apes, do, because behavioural repertoires are evolutionarily inherited from species to species, which then adapts its species-typical social structure to suit the mating strategies employed. It is unclear what the sequence of human social adaptation and evolution was, but we can make some inferences from so-called “traditional” or “foraging” societies, which used to be called “hunter-gatherers” before anthropologists realised that most of the food was gathered, and very little hunted, in these societies, depending upon the environment.

Robin Dunbar, an anthropologist at Oxford University, has christened what has come to be known as “Dunbar’s Number”, which is the mean size of social groups in human societies: 100-250, which an average of 150. He has argued that this is due to our cognitive limits at tracking reciprocal relations with individuals. We can only manage to keep track of around 150 social relationships, including family and their mates. Consequently, our social groups are usually about this size. In foraging societies, this is roughly the size of an unstructured tribe or village. By “unstructured”, I mean that there is no fixed social set of institutions that divide labor up and establish a ruling class, to which I shall return later.

So the end result is that a human troop will tend to be around 150 individuals (or greater – some researched put the number at around 300), and there will be only a pairwise-formed dominance hierarchy in such societies. The rules of cooperation are troop-based: one owes most support to relatives, and any support at all to troop members, not members of other troops, although territorial disputes may be managed in a ritual fashion akin to ape threat displays. One thing that is unique to humans, however, is that we trade resources between troops. In Paleolithic Europe, stone tools have been found as far away as 3000km from where they were mined and shaped. So our contacts between troops must be regulated by some shared norms.

This, too, is primate behaviour in the PSSS. Troops generally live in “bands” of up to several thousand individuals. In Australian aborigines in the southeast of the continent, regular gatherings of troops were held, to exchange mates and resources and undergo ritual behaviours that strengthened ties. Called “corroborees” by Europeans, these meets were social, political, and economic events, and happened regularly (every four or five years for southeastern aborigines).

A morality based solely upon troop (tribe) dynamics, therefore, will tend to treat others well only in proportion to the degree of troop (or band) inclusion, and the norms are those of the troop and band. Humans have one rather obvious added complexity: culture. We have highly diverse local cultures, and therefore cultural norms. It pays to distinguish between our biological (species-typical) dispositions to form norms culturally, and the norms themselves. As apes, we have the dispositions. As cultural agents, we have the cultural norms.

Something happened to human societies in the late Neolithic, so marked it is called the “Neolithic transition”. Agriculture made it possible for social groups to grow well beyond Dunbar’s Number in a very short time, speaking from an evolutionary viewpoint. High density populations of thens of thousands now lived in well marked regional territories, which had to be defended, as arable land and water resources were not uniformly distributed. Some cultures became semi-nomadic herders, while others became crop growers. Urbanisation began to occur, and now there had to be norms of the city (in Greek, polis, from which we get behavioural terms like politics, polite, and police, the norm enforcers).

Now the working memory constraints were well-exceeded, and as a result, norms began to be taught as codified rules. Moreover, thinkers began to identify obligations to those who were not part of one’s social network and familial groups. Out of this, morality was born (the term morality comes from m?s, plural m?res, a Latin word meaning “usage” or “custom”). The problem of morality is, I believe, a problem of urbanisation.

If this is the way to see the origins of morality, and as Sterelny and Fraser argue, morality is about the best ways to reinforce cooperative behaviour, which is the fitness enhancer of moral behaviour. Darwin held that it enabled groups (“villages”) to prosper over other groups, and so he is often thought to have provided a group selectionist account, in which the villages are the beneficiaries of moral conformity. Individual selectionists, however, take an agent based view. An individualist selectionist account, however, as we saw, can only work if the agent is already part of a cooperative society, so the origination of that milieu is left as an accident.

I do not know what I shall next write about. This series is taking a turn I did not anticipate.


de Waal, Frans. 1982. Chimpanzee politics: power and sex among apes. London: Cape.

de Waal, Frans. 1989. Peacemaking among primates. Cambridge, Mass.: Harvard University Press.

de Waal, Frans. 1996. Good natured: the origins of right and wrong in humans and other animals. Cambridge, Mass.: Harvard University Press.

de Waal, Frans. 2001. The ape and the sushi master: cultural reflections by a primatologist. New York: Basic Books.

de Waal, Frans. 2005. Our inner ape: a leading primatologist explains why we are who we are. New York: Riverhead Books.

Dunbar, Robin I. M. 1992. “Neocortex size as a constraint on group size in primates.” Journal of Human Evolution 22:469-493.

Dunbar, Robin I. M. 1998. Grooming, Gossip, and the Evolution of Language. Boston: Twayne Publishers.

Dunbar, R. I. M. 2012. “Social cognition on the Internet: testing constraints on social network size.” Philosophical Transactions of the Royal Society B: Biological Sciences 367 (1599):2192-2201.

Dunbar, Robin I. M., Camilla Power, and Chris Knight. 1999. The evolution of culture: an interdisciplinary view. Edinburgh: Edinburgh University Press.


* Orangs are found in the wild in solitary ranges of males, encompassing several female smaller ranges. Females tend to bear single young, and raise them, but generally do not live in greater than pairs of mother and progeny. However, when orange are placed into groups, as in refuges and sanctuaries, they form social groups readily enough. It is possible that modern orang populations are relict populations living in marginal environments, and have recently adapted to this in the face of human (and possibly other, now-extinct, ape) incursion on their territories.


  1. “Humans, however, have even less sexual dimorphism than bonobos: the average size dimorphism is around 5% for males over females.”

    Nonetheless, some of us “naked apes” fall head over heals for that 5% difference. Okay, yes, and some do not….

  2. Again, a nice post.

    But I think a lot of these evopsych explanations should be taken with a grain of salt. I don’t think much of what you discussed here has really been proven to be more than just-so stories, particularly as it applies to humans.

    If humans should behave like apes, then apes should behave like monkeys and monkeys should behave like prosimians and prosimians should behave like mammals, reptiles, amphibians, fish, etc.

    Yes, we inherited a lot from apes, but we also developed a number of our own innovations. In particular, we developed various techniques of passing complex culture with modification from generation to generation which has allowed behavioral change to take off in a way that goes beyond what is possible with only genetic evolution.

    So I’m not saying that anytthing you have written here is wrong. It probably isn’t. But I think it is probably stated with too much confidence.

    • Did you note the point that each species has its own version of the PSSS? There is no reason to expect that the implementation of the PSSS will be identical in any two species. In fact, there is good reason to think not.

      But that doesn’t undercut the point that we share the same dispositions as the other closely related species do. In fact, I think that culture plays out those dispositions in environmentally relative ways, so there is no reason to think that there is even a generic human cultural set of rules.

      • I did indeed note the point. I just think that the jury is out on how much we can learn about human morailty by studying other primates. I’m certainly open to the idea that it may be a great deal, but I’m also open to the converse.

        I’m inclined to agree with you that we share the same dispositions. Again, I’m only disagreeing with the tone of apparent confidence with which you discuss the applicability of these findings.

  3. colinhutton colinhutton

    John : Excellent 3-part series – as also recent series on speciation. Thank you.

  4. Richard Peachey Richard Peachey

    The devil — no, my inner ape — or, um, my inner reptile? — wait, could have been my inner fish — made me do it!

  5. “I do not know what I shall next write about. This series is taking a turn I did not anticipate.”

    Fantastic. Sounds like we should have dinner again soon!

  6. Do you have a specific citation for the 5% figure? It surprised me and when I looked for evidence I found this entry in wikipedia about human height:

    The average stature ratio (male to female) looks to be about 1.08

    (Nitpicky, I know. Just wanted to get this right since it’s a pretty interesting difference among our ape brethren.)

  7. AK AK

    Some points (in no particular order):

    – Human mating customs run the gamut from polygyny to monogamy.

    – Chimpanzee/bonobo dominance is frequently not a result of “Pairwise conflicts and transactions” but involve complex interactions, including alliances, among more than two individuals.

    – AFAIK, related to the last point, each adult (or sub-adult) member of the band appears to feel a personal stake in the outcome of every dominance interaction.

    – In both chimpanzees and bonobos, the vast majority of females relocate to new bands as part of becoming mating adults.

    – Many chimpanzee (and, probably bonobo) bands appear to have bodies of customs/traditions, transmitted through personal learning

    I don’t have time to dig up ref’s, although there’s considerable literature in support of the points I’ve made. I also remember a discussion (since searched for but not found) of an occurrence in a band of bonobos, wherein a young male attacked a female with infant, and EVERY ADULT IN THE BAND jumped to intervene.

    Since I’ve never been able to track down the original reports of that incident, I can only guess at the implications for how cultural norms work in our closest relatives, but it’s certainly interesting.

  8. Individual selectionists, however, take an agent based view. An individualist selectionist account, however, as we saw, can only work if the agent is already part of a cooperative society, so the origination of that milieu is left as an accident.

    As I mentioned in a comment on the first post of this series, I think you are underestimating the extent to which evolutionary game theory has developed, For the individual selectionist, this usually comes down to the inclusive fitness vs. group selection debate, which is an interesting discussion to have but not always relevant (unless you really want grand unifying theories).

    I would recommend the Hammond & Axelrod model as an opening to built intuition for how ethnocentric biases and group identity can evolve without an a priori affinity towards groups or being “already part of a cooperative society”. Once you have this in place, you can start to build group selection arguments on top of it. As before, we can tie these things back to religion and use it to ask questions about morality, in particular to question Rifkin’s expanding moral circles.

    Again, all of the above is without punishment, memory, or image scoring/reputation (the typical candidates I see in words-based theories on this topic); all it requires is spatial structure and even that is not always strictly necessary.

  9. Per moral norms, I suppose there is more to morality than following norms. For example, if morality is reduced to following norms, then every reformation of norms is initially immoral. However, reformers of norms typically feel that their reformation is moral.

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