The Bonding Brain

Primate sociality is linked to brain networks for pair bonds.

Social conservatives are fond of linking morality with monogamy and will be quick to condemn the moral crimes of adulterous felatio while ignoring the moral crimes of cutting social programs for poor mothers. However, in a bizarre twist, research suggests that morality and monogamy are closely intertwined, though it's doubtful many conservatives will champion the reasons why.

In the journal Science Robin Dunbar revisits the question with a unique perspective as to why some species (including humans) succeed so well as members of a group.

While it may come as a shock to the Milton Friedman's of the world (proponents of the brand of capitalist theory often referred to as "free market fundamentalism") human beings are a distinctively socialist species. While we come nowhere near the extreme for the natural world (the eusocial bees, ants and termites win the Karl Marx utopian award for selfless behavior on that one) we humans are far and away the most social species of the most social order of the most social class in the animal kingdom (for those of you not up on your Linnaean terminology I refer to primates and mammals respectively). How can I claim such a thing? A very simple measure will suffice: social group size.

Humans have the largest group sizes of any primate. Baboons are known as having the largest group sizes of all non-human primates with an average of about 40-50 individuals and only approach as many as two hundred under extreme circumstances. Humans, in contrast, have an average group size of about 150-200 individuals in hunter-gatherer societies and a maximum group size in the millions under the unique conditions we experience as the result of industrial agriculture. And these large social groups require substantial brain power. All organisms need to successfully predict and navigate their environments and this becomes far more complicated when there are multiple actors interacting in the same social circle.

In the 1990s Robin Dunbar championed an idea known as the Social Brain Hypothesis. He found that mammals who lived in the largest social groups often had the largest neocortex to brain ratio. Since the neocortex is associated with complex and abstract thought he suggested that the demands of group living selected for an increase in neocortex size. In his most recent paper in Science he and Susanne Shultz have suggested that there is even more than simply group size that may have influenced this selective process. When the authors analyzed the mating strategies of those highly social mammals that had the largest neocortex they found that pair bonds were significant in all groups except primates.

All social mammals except primates show connection between brain volume and pair bonding

Pair bonds occur when an animal stays with their partner for extended periods rather than simply meet up during the mating season. Pair bonds are cognitively tricky because monogamy is a risky business. In order to avoid getting stuck with a bad partner (either one with bad genes or one who won't share the costs of reproduction) individuals have to be careful in choosing a good-quality mate. Also, pair bonded individuals have to carefully coordinate their activities to be in synchrony with the other. This may require substantial brain power to predict the other's behavior and adjust your own behavior accordingly. But why is it significant that the social primates don't show this connection between a large neocortex and pair bonding like other mammals do? Well, whenever there is a consistent pattern in nature that is violated in a single case a good scientist will want to know why. Primates are already unique among mammals, so any unique qualities that jump out could help us understand the evolution of our lineage.

As mean group size rises so does neocortex ratio

What Dunbar and Shultz have suggested is that the social brain that was selected under conditions for pair bonds in other species has been coopted and utilized for strangers in primate social groups. As the authors state in their paper:

This would explain why, as primatologists have argued for decades, the nature of primate sociality seems to be qualitatively different from that found in most other mammals and birds. The reason is that the everyday relationships of anthropoid primates involve a form of "bondedness" that is only found elsewhere in reproductive pairbonds.

Primates, and humans in particular, are such good social cooperators because we can empathize with others and coordinate our activities to build consensus. Rather than natural selection being a process of selfish individuals maximizing their own fitness at the expense of others, this "bonding brain" hypothesis suggests that natural selection, at least in primates, was a process of maximizing individual fitness through the promotion of the group as a whole. There is already a vast literature on the proximate mechanisms (the hormonal and neurobiological aspects) that promote both pair bonding and affiliative behavior. This is exciting research and I will continue to follow up on it in these pages.

However, there are problems with this study which I will address in a later post. A paper that I recently co-authored with Evan MacLean, Nancy Barrickman and Chris Wall showed that neither group size nor pair bonding was linked with larger brain sizes in strepsirhine primates (lemurs, lorises and bush babies). This is a potential problem for Dunbar's model as one would expect strepsirhines to be more like other primates or more like other mammals. But to have them in a category unto themselves suggests that this model isn't as universal as currently proposed.

Nevertheless, what has been shown for haplorhine primates by Dunbar and Shultz is truly fascinating. It's important to point out, of course, that this research doesn't imply that monogamy has caused increased social cooperation, merely that the brain mechanisms selected for in the evolution of pair bonds have been extended to additional members of the primate order. But it's unfortunate that so many conservatives are adamantly opposed to understanding evolution. Finally a connection between morality and monogamy has been established by Science and their refusal to understand means they'll miss a terrific opportunity to pound the bully pulpit.


Dunbar, R., & Shultz, S. (2007). Evolution in the Social Brain Science, 317 (5843), 1344-1347 DOI: 10.1126/science.1145463


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You probably misunderstand free market basics if you think this has any relevance on the topic.

Libertarians are pro-voluntary action. They're not anti-society or anything like that.

In this particular case, libertarianism states that people should be able to voluntarily join and act in groups, instead of being coerced into doing so.

Republicans are just retarded.

I'm inclined to agree with you on libertarian economics in theory. However, in practice the so-called "free trade" agreements that have been passed--to the applause of libertarians on the right--have done exactly what you are opposed to: coercion. The coercion is one that US libertarians, conservatives and liberals favor since it happens to others and benefits the US economy. However, the huge groundswell of support for leftist candidates in Latin America is in direct response to NAFTA, CAFTA, FTAA, etc. that have been imposed despite their opposition by the majority of the populations there.

Adam Smith would be outraged by the symbiosis that exists between the American government and multinational corporations based in the US. Wealth of Nations was written precisely to oppose such collusion between England and companies such as the East Indian Company.

Chuckle. What's the first example that comes to mind of promoting the good of the group as a whole, and how do you feel about it?

Organizing unions in the railroads, mills and mines.

Or a bit earlier -- the enclosure of the common fields.

Or in between -- the revolt of the 13 Colonies against Britan.

Or now -- organizing unions for janitors.

People give up a lot to better the lot of the group.

By Hank Roberts (not verified) on 24 Jul 2009 #permalink

The first thing that comes to mind is a hunter-gatherer group such as the Aka of the Central African Republic. In the United States I would say labor unions and church groups. Not only are these organizations bound to each other in order to create a world they want to live in, they've even developed an artificial family as a way to help promote their cohesiveness. In both kinds of organizations other members are often referred to as brother or sister and church groups refer to the priest as father. This serves to create a kind of family bond that promotes collective action.

I got as far as your "while ignoring the moral crimes of cutting social programs for poor mothers" and read no more.

If anyone is committing "moral crimes" -- it is poor unmarried mothers having lots of children they can't possibly afford -- fathered by non-fathers.

Doesn't the far-left ever learn anything? The welfare programs Bill Clinton ended had tragically undermined the black family -- yet you can't wait to reinstate them.

By hugh prestwood (not verified) on 26 Jul 2009 #permalink

Addressing you on your own level, Hugh, then we are to suppose that imprisoning 10% of the Black male population and allowing a good measure more to be killed in gang violence stemming from illegal drug trafficking has nothing at all to do with the destruction, as you put it, of the Black family?

The destruction of decent jobs for the unskilled labor force, the failure of urban school systems to produce a workforce capable of assuming skilled jobs, the death toll of the illegal drug trade and the draconian prison terms doled out in response to the illegal drug trade (but meted out rather selectively, one might add) are irrelevant to the maintenance of the social structure, hugh tells us, but WIC is the family-killer.

By not a gator (not verified) on 27 Jul 2009 #permalink

@ 5&6: I worried for a second that we were getting a bit too off topic, but then thought better after I realized this IS exactly on topic.

The entire prospect of evolutionary psychology in my mind undermines the fundamental basis for free will, which is possibly THE main pillar of both conservatism and libertarianism. Hugh's emphasis on personal responsibility of poor (black? - the demographic element of this debate is really fascinating) mothers went right to this - elevating the concept of individual accountability over social accountability. And back to the point of "coercion", doing "nothing" is often the most coercive action of all.

But that's the logical outcome of denying contra-causal free will: everything has a cause, i.e. is coercive in nature.

The trick is in recognizing what our social goals are, and then optimizing our system for achieving them. This may mean more government, it may mean none at all. Some social programs have been terrible. Some have been fantastic. But that is the REAL, worthwhile debate.

Modern science has affirmed over and over that we are indeed social creatures, entirely caused by our biology & environment. No one wants poor mothers. No one wants dishonesty. We just have to decide how this is achieved.

Rousseau redux: Man is born in chains, and... for better or worse, will remain in chains regardless of whether we like it or not.

(...of course we CAN teach him to be what we want him to be.)

Moebius - I'm not sure what you mean. Conservatism's clarion call is personal responsibility: the rich and poor both deserve their lot in life. This is rooted in the believe that belief that everyone is able to do anything, despite the limitations of biology or environment - which is contra-causal free will defined.

This was certainly classical enlightenment liberalism. But science and social movements have brought us past this sort of magical thinking. Plenty of modern liberals think they believe in this sort of free will, but by definition their emphasis on social planning and moral non-absolutism lends itself to accepting determinist theory.

I am surprised by this article and the attribution of these ideas to Dunbar as he rehashes ideas of others. The first to speculate about the cognitive demands of pair bonding were bird people, specifically: Nathan J Emery, Amanda M Seed, Auguste M.P von Bayern, and Nicola S Clayton 2007 Cognitive adaptations of social bonding in birds. Philos Trans R Soc Lond B 362(1480): 489â505.

By Frans de Waal (not verified) on 01 Aug 2009 #permalink

I recently came across your blog and have been reading along. I thought I would leave my first comment. I don't know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.