The Economist reports about research by Steven Piantadosi and Celeste Kidd from the University of Rochester who tried to explain why humans tend to be intelligent. Their answer: Because human babies are extraordinarily helpless when compared with other animals.

… human infants take a year to learn even to walk, and need constant supervision for many years afterwards [indeed]. That helplessness is thought to be one consequence of intelligence—or, at least, of brain size. In order to keep their heads small enough to make live birth possible, human children must be born at an earlier stage of development than other animals. …

… helpless babies require intelligent parents to look after them. But to get big-brained parents you must start with big-headed—and therefore helpless—babies. The result is a feedback loop, in which the pressure for clever parents requires ever-more incompetent infants, requiring ever-brighter parents to ensure they survive childhood.