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but always could be reasoned with, who might use words to be hurtful but never raised a hand to hit or punch. I had begun to dread the daily battles with Mikey, and I was happy about the change.
    The cause for this preference for same-sex playmates remains largely unknown, but scientists speculate that basic brain differences may be one reason. Girls’ social, verbal, and relationship skills develop years earlier than boys’. That their styles of communication and interaction are completely different is probably a result of these brain variations. Typical boys enjoy wrestling, mock fighting, and rough play with cars, trucks, swords, guns, and noisy—preferably explosive—toys. They also tend to threaten others and get into more conflict than girls beginning as early as age two, and they’re less likely to share toys and take turns than are female children. Typical girls, by contrast, don’t like rough play—if they get into too many tussles, they’ll just stop playing. According to Eleanor Maccoby, when girls get pushed around too much by boys their age—who are just having fun—they will retreat from the space and find another game to play, preferably one that doesn’t involve any high-spirited boys.
    Studies show girls take turns twenty times more often than boys, and their pretend play is usually about interactions in nurturing or caregiving relationships. Typical female brain development underlies this behavior. Girls’ social agenda, expressed in play and determined by their brain development, is to form close, one-on-one relationships. Boys’ play, by contrast, is usually not about relationships—it’s about the game or toy itself as well as social rank, power, defense of territory, and physical strength.
    In a 2005 study done in England, little boys and girls were compared at four years of age on the quality of their social relationships. This comparison included a popularity scale on which they were judged by how many other children wanted to play with them. Little girls won hands down. These same four-year-old children had had their testosterone levels measured in utero between ages twelve and eighteen weeks, while their brains were developing into a male or a female design. Those with the lowest testosterone exposure had the highest quality social relationships at four years old. They were the girls.
    Studies of nonhuman female primates also provide clues that these sex differences are innate and require the right hormone-priming actions. When researchers block estrogen in young female primates during infantile puberty, the females don’t develop their usual interest in infants. Moreover, when researchers inject female primate fetuses with testosterone, the injected females end up liking more rough-and-tumble play than do average females. This is also true in humans. Though we have not performed experiments to block estrogen in little girls, or injected testosterone into human fetuses, we can see this brain effect of testosterone at work in the rare enzyme deficiency called congenital adrenal hyperplasia (CAH), which occurs in about one out of every ten thousand infants.
    Emma did not want to play with dolls. She liked trucks and jungle gyms and sets to build things with. If you asked her at two and a half years old if she was a boy or a girl, she’d tell you she was a boy and she’d punch you. She’d get a running start, and “the little linebacker,” as her mother called her, would knock over anyone who came into the room. She played catch with stuffed animals, though she threw them so hard it was tough to hang on to them. She was rough, and the girls at preschool didn’t want to play with her. She was also a little behind the other girls in language development. Yet Emma liked dresses and loved when her aunt styled her hair. Her mother, Lynn, an avid cyclist, athlete, and science teacher, wondered, when she brought Emma in to see me, if her being a jock had influenced her daughter’s