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monkeys showed significantly greater interest in the feminine toys. The two sexes did not differ in their preference for the neutral toys. Alexander and Hines’ article contains pictures of a female vervet monkey examining the genital area of the doll in an attempt to determine whether it is male or female, as a girl might, and of a male vervet monkey pushing the police car back and forth, as a boy might. If children’s toy preferences were largely formed by gender socialization, as the Standard Social Science Model claims, in which their parents give “gender-appropriate” toys to boys and girls, how can these male and female vervet monkeys have the same preferences as boys and girls? They were never socialized by humans, and they had never seen these toys before in their lives.
    As these two studies (and numerous others) show, the sex differences in behavior, cognition, values, and preferences are largely innate; universal across cultures; and, in many cases, constant across species. 4 If the sex differences were the result of social and cultural practices such as gender socialization, then they should by definition vary by culture and society. In fact, however, in every human society (and among many other species), males on average are more aggressive, violent, and competitive, and females on average are more social, caring, and nurturing. What is constant in every culture and society (sex differences in behavior) cannot be explained by what is variable across cultures and societies (cultural and social practices). A variable cannot explain a constant; only a constant can explain a constant.
    A Consequence, Not a Cause
    Rather than the results of lifelong gender socialization, sex differences in behavior, cognition, values, and preferences are part of innate and distinct male and female human natures; men and women are hardwired to be different. Male and female human brains are different, just like male and female reproductive organs are different. Gender socialization helps to accentuate, solidify, perpetuate, and strengthen the innate differences between men and women, but it does not cause or create such differences. In other words, men and women are not different because they are socialized differently; they are socialized differently because they are different. Gender socialization is not the cause of sex differences; it is their consequence.
    If gender socialization is not the cause of sex differences, then what is? What is the constant that explains the universal sex differences? It turns out that two simple biological facts lead to a whole array of sex differences: anisogamy and the internal gestation of fertilized eggs within the female body. Anisogamy means that the female sex cell (egg) is larger in size and fewer in number than the male sex cell (sperm). (This, by the way, is the biological definition of male and female. The female of any sexually reproducing species is defined as the sex that produces the larger sex cell, and the male, by default, is the other sex.) Anisogamy means that the egg is biologically far more valuable than the sperm; in nature, the sperm is abundant (practically infinite) in supply and biologically less costly to produce than the eggs. A quick rule of thumb in biology, which can explain a lot of sex differences in many species, is: Sperm is cheap.
    The internal gestation of fertilized eggs within the female body means, among other things, that the female can produce far fewer offspring than the male can. It takes a woman at least nine months, usually a few years, to produce one child (because a woman is usually infertile while she nurses her baby); it takes a man fifteen minutes. A woman can normally have at most twenty to twenty-five pregnancies in her entire lifetime, usually far less; there is no limit to the number of children men can potentially produce. The operative term here, of course, is potentially .
    Anisogamy and the internal gestation within the female