When offered the choice of playing with either a doll or a toy truck, girls will typically pick the doll and boys will opt for the truck. This isn't just because society encourages girls to be nurturing and boys to be active, as people once thought. In experiments, male adolescent monkeys also prefer to play with wheeled vehicles while the females prefer dolls — and their societies say nothing on the matter.
The monkey research, conducted with two different species in 2002 and 2008, strongly suggested a biological explanation for children's toy preferences. In recent years, the question has become: How and why does biology make males (be they monkey or human) prefer trucks, and females, dolls?
New and ongoing research suggests babies' exposure to hormones while they are in the womb causes their toy preferences to emerge soon after birth. As for why evolution made this so, questions remain, but the toys may help boys and girls develop the skills they once needed to fulfill their ancient gender roles.