His Brain, Her Brain

Scientific American relates a study that demonstrates that "gender" may not be just cultural:

The researchers presented a group of vervet monkeys with a selection of toys, including rag dolls, trucks and some gender-neutral items such as picture books. They found that male monkeys spent more time playing with the 'masculine' toys than their female counterparts did, and female monkeys spent more time interacting with the playthings typically preferred by girls. Both sexes spent equal time monkeying with the picture books and other gender-neutral toys.

More than that, day-old babies respond differently based on their sex.