The human brain uses about 25% of the energy your body has to dole out to its various organs. That's a big cost. Explaining why we have such proportionally large brains is the subject of many books.

But in any case, Carl Zimmer writes about recent research which hints as greater fitness of "dumb" individuals within two animal species. One thing though, Carl should be cautious about addressing the idea of intergroup differences in humans before knocking it down (see the last paragraph), Charles Murtaugh can attest to the dangers of even mooting the topic .