Androgyny advocates like our Swedish friends have heard such stories many times, and they have an answer. They acknowledge that sex differences have at least some foundation in biology, but they insist that culture can intensify or diminish their power and effect. Even if Eliza is prompted by nature to interact with a train in a stereotypical female way, that is no reason for her parents not to energetically correct her. Hunter College psychologist Virginia Valian, a strong proponent of Swedish-style re-genderization, wrote in the book Why So Slow? The Advancement of Women, "We do not accept biology as destiny ... We vaccinate, we inoculate, we medicate... I propose we adopt the same attitude toward biological sex differences."

Valian is absolutely right that we do not have to accept biology as destiny. But the analogy is ludicrous: We vaccinate, inoculate, and medicate children against disease. Is being a gender-typical little boy or girl a pathology in need of a cure? Failure to protect children from small pox, diphtheria, or measles places them in harm's way. I don't believe there is any such harm in allowing male/female differences to flourish in early childhood. As one Swedish mother, Tanja Bergkvist, told the Associated Press, "Different gender roles aren't problematic as long as they are equally valued." Gender neutrality is not a necessary condition for equality. Men and women can be different—but equal. And for most human beings, the differences are a vital source for meaning and happiness. Since when is uniformity a democratic ideal?

Few would deny that parents and teachers should expose children to a wide range of toys and play activities. But what the Swedes are now doing in some of their classrooms goes far beyond encouraging children to experiment with different toys and play styles—they are requiring it. And toy companies who resist the gender neutrality mandate face official censure. Is this kind of social engineering worth it? Is it even ethical?

To succeed, the Swedish parents, teachers and authorities are going to have to police—incessantly—boys' powerful attraction to large-group rough-and-tumble play and girls' affinity for intimate theatrical play. As Geary says, "You can change some of these behaviors with reinforcement and monitoring, but they bounce back once this stops." But this constant monitoring can also undermine children's healthy development.

Anthony Pellegrini, a professor of early childhood education at the University of Minnesota, defines the kind of rough-and-tumble play that boys favor as a behavior that includes "laughing, running, smiling, jumping, open-hand beating, wrestling, play fighting, chasing and fleeing." This kind of play is often mistakenly regarded as aggression, but according to Pellegrini, it is the very opposite. In cases of schoolyard aggression, the participants are unhappy, they part as enemies, and there are often tears and injuries. Rough-and-tumble play brings boys together, makes them happy, and is a critical part of their social development.