Pillard says that it had long puzzled him why transsexuals—men or women who wish to live in bodies of the opposite sex—are so different from gay people: "You'd think they'd be on the far end of the spectrum, the 'gayest of the gay.'" And yet transsexuals are not in fact gay. Whereas gay men, quite comfortably and unalterably, see themselves as men, male transsexuals see themselves as women trapped in men's bodies. Pillard and a colleague, James Weinrich, a psychobiologist at the University of California at San Diego, began to theorize that gay men are men who in the womb went through only a partial form of sexual and psychosexual differentiation. More precisely, Pillard and Weinrich theorized that although gay men do undergo masculinization—they are, after all, fully male physically—they go incompletely if at all through another part of the process: defeminization.
As fetuses, Pillard points out, human beings of both sexes start out with complete female and male "anlages," or precursors of the basic interior sexual equipment—vagina, uterus, and fallopian tubes for women, and vas deferens, seminal vesicles, and ejaculatory ducts for men. These packages are called the Mullerian (female) and Wolffian (male) ducts, and are tubes of tissue located in the lower abdomen. How do the sexual organs develop? It happens differently in men and women.
At the moment of conception an embryo is given its chromosomal sex, which determines whether it will develop testes or ovaries. In female human beings (as in female rats) the female structures will simply develop, without any help from hormones; the Wolffian duct will shrivel up. The process of becoming male, however, is more complex. Where women need none, men need two kinds of hormones: androgens from the testes to prompt the Wolffian duct into development, and a second substance, called Mullerian inhibiting hormone, to suppress the Mullerian duct and defeminize the male fetus.
Pillard speculates that Mullerian inhibiting hormone, or a substance analogous to it, may have brain-organizing effects. Its absence or failure to kick in sufficiently may prevent the brain from defeminizing, thereby creating what Pillard calls "psychosexual androgyny." In this view, gay men are basically masculine males with female aspects, including perhaps certain cognitive abilities and emotional sensibilities. Lesbian women could be understood as women who have some biologically induced masculine aspects.
An experimental basis is provided by research by the psychiatrist Richard Green, of the University of California at Los Angeles, which shows that children who manifest aspects of gender-atypical play are often gay. Green has concluded that an inclination toward gender-atypical play in prepubescent boys—for example, dressing in women's clothes, playing with dolls, or taking the role of the mother when playing house—indicates a homosexual orientation 75 percent of the time. If that is true, it is important, because it would be an example of a trait linked to sexual orientation which does not involve sexual behavior—suggesting how deeply rooted sexual orientation is. Discussing this line of research, Simon LeVay told me, "It's well known from animal work that sex-typical play behavior is under hormonal control. Robert Goy [at the University of Wisconsin at Madison] has done many studies over the years showing that you can reverse the sex-typical play behavior of infant monkeys by hormonal manipulations in prenatal life. [Play] is an example of a sex-reversed trait in gay people that's not directly related to sex. It's not sex, it's play. When you get to adulthood, these things become blurred. It's easier to tell a gay kid than a gay adult--kids are much of a muchness. Most gay men, even those who are very macho as adults, recall at least some gender-atypical behavior as children."