Girls' and boys' brains: How different are they?

When my son was a toddler, his best friend, a girl, gave him a sparkling dancing Katrina doll for a birthday present. He'd apparently admired the doll at her house, but once he got it he never played with it – until the day I found him chasing his little brother around with the doll, which he'd managed to twist into the shape of a gun.

Boys will be boys? Proof that gender differences are hardwired? Not so fast. Like most parents, I have just as many tales illustrating the influence of nurture on my son's behavior. At preschool one day, as he was playing dress up with two girlfriends, he donned a scarlet tutu. Within a minute an older, cooler boy guffawed, "Boys don't wear dresses!" He never put on girl clothes again.