When my son Sam was a toddler, I banned toy guns and swords. I bought him a doll and a toy vacuum cleaner instead. Boys play aggressively, I believed, because they have so many aggressive-looking toys to play with. In my theory, a kid with fun and charming alternatives wouldn't miss the weapons, and he'd grow up to be a sweet and caring man to boot.

So I was surprised that Sam never once spontaneously cuddled his doll and only reluctantly joined me when I said brightly, "Let's give the baby a bottle!" And I was shocked when at ½, he picked up a cracker, bit off a corner, pointed the long end at me, and shouted, "Pow! You're dead!"

Many parents who think they've done all they can to make sure their children won't be shaped by gender stereotypes are mystified by such behavior. A friend who's a lawyer recalls when her 3-year-old daughter, in response to a "you can be anything you want" speech, said, "Then I'll be a princess, and you'll have to buy me lots of beautiful clothes."

Why is it that so many kids blossom into such walking, talking clichés when they play? And how does this affect the grown-ups they'll become?