Here's a great excerpt from an article reported on by LifeSiteNews titled "Child expert: Those who insist boys should play with dolls are ‘mistaken’":
Studies have repeatedly showed that children are indeed ‘hard-wired’ in their toy preferences as boys and girls, and that they are very different.
In one study of toddlers, when researchers offered boys a truck or a doll, most boys chose the truck.
Another study looked at nine-month-olds “and they found similar results,” said Dr. Sax. “Nine-month-old boys strongly preferred ‘boy toys,’ such as balls, trains, and cars. Nine-month-old girls preferred ‘girl toys,’ such as dolls and baby carriages.”
Boys and girls have their toy preferences at this young age despite the fact that they aren’t old enough to have arrived at a self-awareness of their gender. In this way, the assertion that children are taught what toys they are supposed to play with is debunked; they know it innately from the core of their beings.
So, science shows that sex and behaviour is hardwired into who we are. Did we really need "science" to tell us this? Unfortunately, yes.