Nature? Nurture?

The way kids play is determined by a range of factors. Influences as diverse as hormones and pop culture combine in a complex stew in which cause is often indistinguishable from effect. Understanding this not only let me come to terms with my own sons' behavior but also pointed the way for me to help them be boys who follow their heart rather than the herd. What I found:

Born to play Barbie? In a blow to every parent who thought vigilance and encouragement could bring forth girls who play with trucks as easily as dollhouses, a child's taste in toys is now thought to be influenced in part by testosterone levels in the fetus during pregnancy. Although amounts vary individually even within the same gender, higher concentrations may predispose the majority of boys to behave more aggressively.

Megan Decker of Bozeman, Montana, noticed the differences between her kids early on. When the family recently spent a few days at a remote cabin, Olivia, 3, decided she had to have a doll -- but her mom hadn't packed one. "So she found some oarlocks wrapped in duct tape, and it was her 'baby' all weekend. It had no baby characteristics at all, but it was better than nothing," says Decker. Five-year-old Jonathan, meanwhile, frequently makes guns out of whatever materials are nearby.

The urge to imitate. Biology can go only so far in explaining kids' preferences, though. They play pretend versions of whatever social routine is at hand, and Mom and Dad -- a child's first playmates -- inevitably have a role in defining gender norms for them.

Studies show that most of us unconsciously treat our kids differently from birth, handling a son more roughly, talking to our daughters more frequently. And the boy whose testosterone helped hardwire his brain also has a father and uncles who model male behavior for him every day and both knowingly and unknowingly encourage it. Ditto for mothers, daughters, and female behavior. Sometimes our signals can be so subtle that they're easy to miss. My husband, Haywood, was perfectly comfortable with the fact that Sam had a doll, but it was never something they played with together. Invariably, they did things Haywood himself enjoyed: tossing a baseball, playing hide-and-seek. Without meaning to, Haywood had reinforced the idea that dolls aren't a guy thing.

At other times our messages are more overt. Susan Gilbert, author of A Field Guide to Boys and Girls, remembers when her son, David, then 2, wanted a toy kitchen. Her husband -- who frequently cooks for the family -- surprised her by saying, "But that's a girl toy."

The culture factor. TV, movies, video games, and books tend to reinforce the idea that certain activities -- such as dancing -- are for girls and others -- like seeking adventure -- are for boys. When your daughter watches a commercial in which several boys are oohing and ahhing over a remote-control motorcycle, she'll most likely pick up on the ad's subtext: This isn't a toy for girls.

Often, even classically gender-neutral items are given design cues that signal which sex they're intended for. "The only toddler cars we could find were either lavender and pink or black and red cop cars with lightning bolts," says Rich Brotherton, a dad of two in Austin. Nonetheless, the chicken-and-egg question remains: Does toy marketing drive demand, or are toy companies simply responding to what kids already want?

A very young worldview. The most severe gender police of all are probably kids themselves. Starting around age 3, they instinctively figure out the world by sorting virtually everything they encounter into mental categories. And until they begin to develop more complex reasoning skills, usually by age 7, there's little room for ambiguity in that way of thinking. To many, boys wear pants and get dirty and girls have long hair and throw tea parties. And everyone -- themselves included -- must behave in said fashion to qualify as a member of their sex. That's why a girl who's always been a bit of a tomboy may suddenly insist on wearing dresses when she gets to preschool; she wants to clarify -- in her own mind -- which gender she belongs to.

The power of peers. The predisposition to viewing things without nuance is then magnified by groups of similarly inclined kids, whether at the playground or in school. "When my neighbor's son was four, he would say, 'Pink is my favorite color,'" says Adele Faber, coauthor of How to Talk So Kids Will Listen & Listen So Kids Will Talk. "Now that he's gone to kindergarten, he sees the way things go in the boy world: Pink is a girl's color. So he professes to hate it."

Even when our kids begin to outgrow that kind of rigid thinking, they don't escape the need to feel they fit in. "Most children band together in same-sex groups, and they get teased about being with those of the opposite sex," says Eleanor Maccoby, Ph.D., a psychologist and author of The Two Sexes: Growing Up Apart, Coming Together. "The social taboos kick in, and they want to feel like they belong." That explains why a boy may become more traditionally masculine in the company of his friends.

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