Do girls play with dolls and boys play with trucks merely because society imposes such behavior? Or is it genetics at work? Experiments by psychologist Gerianne Alexander of Texas A&M put nature before nurture.

Dr. Alexander presented 44 young male vervets and 44 young female vervets with a range of playthings including cars, balls, dolls, and picture books, and let them choose freely.

The boy monkeys spent more time actively rolling cars and tossing balls; the girl monkeys preferred dolls, examining the bottoms carefully just the way their distant human cousins do with their dolls; gender-neutral toys were chosen equally.

Alexander believes evolutionary programming for active hunting and nurturing childcare roles underlies such innate preferences—which, in the case of humans, society reinforces.