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The Social Brain and Brain Size

Sociability, and gossip, may be the best explanations for our large brains.

Key points

  • The size of a primate's brain is correlated with the size of its social group.
  • Imitation of others is a well-developed capacity of humans, suggesting that it is a uniquely human phenomenon—at least among primates.
  • Language capacity may have arisen as a way of sharing information about the motivations of other group members.

Large brains use a lot of energy. Yet brain size has little connection to intelligent problem-solving. Some of the best problem-solvers are birds, who have tiny brains. Do our large brains help us to keep track of complex social relationships?

Brain Size and Group Size

Efforts to establish functional explanations for brain size have generally failed. This is remarkable. What could be so mysterious about a large, energy-guzzling organ? It is as though animals with long necks like giraffes only ate grass.

There is one exception, however: The size of a primate's brain is correlated with the size of its social group. Primates living in more complex societies have larger brains.1

This would suggest that increased brain tissue helps primates to cope with added social complexity in larger groups. Even here, it may be too soon to celebrate scientific success.

The generalization may work for primates, but it need not hold up for other groups of animals. The most social invertebrates—namely, ants and bees, which have tens of thousands of indviduals to a colony—have tiny brains.

Even within the primate order, orangutans are an outlier. Although highly encephalized (i.e., having large brains relative to body size) and highly intelligent, they are mostly solitary. A young orangutan interacts almost exclusively with its mother. One answer to this puzzle is to imagine that, before they were such an endangered species, orangutans were thicker on the ground and lived in larger groups.

Humans Are More Social Than Other Primates

If the social brain idea has any validity, then one might expect humans—as the most highly encephalized of the brainy great-ape family of species—to be unusually social as well. What is the evidence for this?

One approach compared small children with immature apes.2 Sociability was tested in terms of two key dimensions. The first looked at imitation: Could a subject pay close enough attention to a trained individual to repeat what they had done when it was tested in the same situation? The second asked whether the subject could exploit social cues to find a hidden reward if another individual witnessed it being hidden.

Human children were much better at imitation than apes were. They also did better at using social prompts, such as gazing in the correct direction, or pointing, to locate a hidden prize. Apes were surprisingly weak at this task and often ignored social cues when searching for the reward.

Imitation of others is a well-developed capacity of humans leading to the suggestion that it is a uniquely human phenomenon—at least among primates.3 One would hesitate to make this claim if birds were included in the comparison. Mockingbirds can imitate the songs of dozens of other birds, and parrots imitate their parents' calls while still in the nest. Of course, they are also good mimics of human language.

The Gossip Angle

Perhaps the most obvious criterion of human sociability is our complex vocal communication system that is infinitely expressive and generates new meanings, and new words, continuously. Other primates have vocal communication systems, including the unison calls of mated pairs of gibbons.

Vervet monkeys have a well-developed vocal repertoire, and some make different alarm calls for ground predators compared with aerial ones.

We do not know exactly when our language capacity arose, although the pharynx became enlarged sometime in the last 100,000 years. It is not clear whether language was unique to Homo sapiens or whether extinct human species also used spoken language.

Within this time frame, hunter–gatherer societies were becoming more complex, and group sizes expanded to as many as 150 individuals. People also became more mobile given that items such as personal ornaments and tools that were used for trading or gift-giving sometimes moved hundreds of miles from their origins.

Language may have been helpful in coordinating activities within the local group. Another advantage of language communication was in sharing knowledge about other individuals and their hidden motivations. This notion is supported by research finding that hunter–gatherers spent about two-thirds of their daytime conversation gossiping about other members of the group. (Fireside speech, in the evening, was dominated by storytelling.)

Conclusion

Increasing sociability could help explain why primates, as a group, have larger brains (adjusted for body size) than most other orders of mammals. One finds that species having the largest brains also have the largest social groups. Humans have much larger brains relative to body size than other primates, and this may reflect a more complex social life. In this context of intense sociability, language capacity may have arisen as a way of sharing information about the motivations of other group members.

References

1. Healy, S. D. (2021). Adaptation and the brain. New York: Oxford University Press.

2. Henrich, J. (2015). The secret of our success: How culture is driving human evolution domesticating our species and making us smarter. Princeton, NJ: Princeton University Press.

3. Clay, Z., Tennie, C. (2017) .Is overimitation a uniquely human phenomenon? Insights from human children as compared to Bonobos. Child Development. DOI:10.1111/cdev.12857

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