Honestly, I'm quite surprised by how much people here seem to think it's biological. It seems to me there is no way society's slight move toward encouraging gender-neutral playing isn't still strongly offset by the pervasive culture of machismo and femininity. The second your boy steps foot into school, if he's wearing something pink or playing with a doll, other boys will make fun of him. A single incident like that is a plenty strong incentive to stick with same-gendered toys.