Monkeys living in larger groups need to work a lot harder on their interpersonal skills, so they tend to have larger brains. In fact, the relationship [between brains and group size] is so close, that were you to give a skull to a researcher who worked with monkeys, even though they didn’t know what kind of monkey it belonged to, they would be able to accurately predict the size of group in which it lived.

Monkeys living in larger groups need to work a lot harder on their interpersonal skills, so they tend to have larger brains.  In fact, the relationship [between brains and group size] is so close, that were you to give a skull to a researcher who worked with monkeys, even though they didn’t know what kind of monkey it belonged to, they would be able to accurately predict the size of group in which it lived.