Games Primates Play

The evolution and economics of human relationships.

What Your Facebook Account Says About Your Brain

It takes brainpower to win the social chess game.

Why do some people have thousands of friends on Facebook and others have just a few? It turns out that it depends on the size of their brain. A recent study found that people with more friends have a larger orbital prefrontal cortex, the region of the brain in your forehead right above your eyes. This brain region is involved in complex cognitive processes such as thinking about oneself and thinking about what other people might be thinking. Other recent studies found that people with larger social networks (including the number of friends on Facebook) also have a larger amygdala (a brain region involved in emotion regulation).

Establishing and maintaining many social relationships requires a great deal of brainpower. One must remember many names and faces, keep many individual memory files with information about what these people did in the past, and how and when they did it, constantly update information about the quality of the relationships with these people (are they still friends or have they become enemies?), keep track of the quality of the relationships some of these people have with one another (who is sleeping with whom, who is making alliances with whom), and try to predict what these people might do in the future. For humans and other primates that live in complex and highly competitive societies, having these social skills can make the difference between life and death, or between a good life and a bad one.

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Take a primate species like the rhesus macaque for example. Rhesus macaques live in a competitive society in which the life of every individual is intertwined with the lives of many others into a thick web of intricate connections. Every move a rhesus macaque makes on the social chessboard has a domino effect on everybody else’s life, whether they like it or not. Rhesus macaques can’t afford to mind their own business. Being passive can be interpreted by others as an encouragement to exploitation. If rhesus macaques just want to be left alone, they must work hard at it. If their goal in life is not just to survive, but also to be successful, they must find a way to get others to work with them or for them. In a highly competitive society, having enemies is inevitable and, therefore, making friends becomes necessary for survival: individuals must cooperate with one another to compete against others. Living in a large and complex group generates many social problems and to solve these problems good social intelligence and high brainpower are necessary. The theory according to which the intelligence and the large brain that characterize humans, the other great apes, and some other primates evolved primarily to solve social problems is known as the Machiavellian Intelligence theory.

Consistent with this theory, previous studies comparing many different species of monkeys and apes showed that species that live in large social groups tend to have a larger brain, and particularly a larger prefrontal cortex (after controlling for differences in body size) than species that live in small groups. Rhesus macaques, baboons, chimpanzees, and humans are examples of these socially intelligent species with a large frontal cortex. It’s not that other primates are stupid, but they have different kinds of social organizations, and their social intelligence is probably adapted to their lifestyle and needs. Mountain gorillas, for example, live in small groups consisting of a large male and his harem of females and children. A successful male in mountain gorilla society is a strong and quiet type, and a successful female is one who can find such a guy and stick with him. These personality traits don’t encourage the nurturing of political ambitions. The lifestyle of the mountain gorilla would produce someone like King Kong, but not Machiavelli.

There is an interesting twist to the Machiavellian Intelligence theory of brain evolution. According to current primatological theories, whether or not primates live in groups is determined by females and their needs. If females are better off looking for food and avoiding predators on their own, then the species as a whole has a solitary lifestyle. If females need the help of males to raise their offspring, then the species lives in pairs or small family groups. Finally, if females need to cooperate with other females to find and defend their favorite food or to protect themselves from predators, then the species lives in groups. These groups will be large or small, again, depending on the females’ needs. Males have needs too, of course, and those needs are called females. There are some very brilliant minds among the scientists who study primate social evolution, and after decades of intellectual efforts and countless mathematical models, they have concluded that the contribution of males to the evolution of primate sociality is best summarized in the following statement: males go where the females are. So if the females are solitary, the males just follow them around, and if they live in groups with other females, males join those groups. Males’ primary goal in life is always one and the same: sex. Because males eat a lot of food, are potentially dangerous, and don’t help much with the kids, females of many primate species tolerate only a few males within their groups and expect them to make themselves useful by fighting predators and primates from other groups.

The interesting twist in the Machiavellian Intelligence theory is that the association between prefrontal cortex size and group size seen across many primate species is seen in the females and not the males. In other words, the more females that live in the company of other females, the larger the cortex of the species, whereas male group size does not correlate with cortex size. This interesting finding suggests that the evolution of complex intelligence in Old World monkeys and apes, including humans, may be due to the increasing complexity of female social life. In the evolutionary journey that led to big primate brains and complex intelligence, females and males traveled together and eventually reached the same destination, but females were the drivers and males were the passengers. Smart females produce smart kids, and some of them happen to be male. Males are genetically and anatomically similar to females, so as females became smarter, males – at least some of them - became smarter too.

Powell J., Lewis P.A., Roberts, N., García-Fiñana, M, & Dunbar, R.I.M. 2012. Orbital prefrontal cortex volume predicts social network size: an imaging study of individual differences in humans. Proceedings of the Royal Society of London B, in press.

Bickart, K. C., Wright, C. I., Dautoff, R. J., Dickerson, B. C. & Barrett, L. F. 2011 Amygdala volume and social network size in humans. Nature Neuroscience. 14, 163 – 164.

If you enjoyed this blog post and the previous ones, read my book Games Primates Play!

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Dario Maestripieri, Ph.D., is a professor of comparative human development, evolutionary biology, and neurobiology at the University of Chicago.

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