(When we talk about “intelligence,” we’re describing something complex and, frankly, sort of nebulous; it’s impossible to really quantify, but that hasn’t stopped generations of scientists from trying. Researchers who study cognition break it down into specific areas: attention, inhibition, cognitive flexibility, speech production and speech comprehension, working memory, and episodic memory. Some of those abilities are associated with particular sections of the brain, but those relationships are often complicated.) So, when looking at brain size and intelligence, the differences among human brains are relatively small compared to the differences between a human brain and any other great ape brain. For example, our closest relatives, the chimpanzees, have brains that average just 400 cubic centimeters; the average adult human brain takes up about 1,350 cubic centimeters.…