A recent study done by researchers analysed the brain size of 140 primate species found that those who ate fruit instead of leaves had up to 25 percent more brain tissue even when controlling for body size and the like.”If you are foraging on harder-to-access food, like fruit instead of leaves, then you need to have all the cognitive strategies to deal with that,” says Alex DeCasien who was the lead author of the study. The findings are obviously a big deal especially with the possible relatability to humans and other species. It was long thought that group size was a predictor of brain size, but researcher Stephen Montgomery says that this study refutes that idea. It is crazy to me that diet can have such a huge effect on the brain and it should be a wake up call for us to eat better (and more fruit)

http://www.npr.org/sections/thesalt/2017/03/27/521423216/what-gave-some-primates-bigger-brains-a-fruit-filled-diet