Homosexuality and Biology

An introduction to a muddled and sometimes contentious world of scientific research—one whose findings, now as tentative as they are suggestive, may someday shed light on the sexual orientation of everyone
The Puzzles of Chemistry

In a large room at the UCAL department of anatomy, Roger Gorski and I recently stood facing a dozen black-topped lab tables, each below a ceiling-mounted video monitor. We were about to watch a tape of rats having sex.

Gorski, an eternally cheerful, almost elfin man of fifty-seven, was energetically describing the tape. "There are six couples," he explained, though at the moment I saw only one uninterested-looking white rat. "That's an unaltered female," he said. "They're going to put in another female that has been injected with testosterone." Sure enough, someone's hand reached down into the screen and a second rat landed in the cage. The rats at first edged around each other, but in just a few seconds on the dozen monitors I saw the testosterone-injected female begin to sniff the other female rat and then mount her aggressively. At the lab tables a handful of medical students went on with their work, paying no attention. After a few moments the tape cut to two males, one perinatally castrated and injected with estrogen, one unaltered. After some initial maneuvering the castrated male responded to the advances of the unaltered male by bending his back and offering himself in what was to me indistinguishable from female-rat lordosis—behavior indicating receptivity to sex, pictures of which Gorski had shown me in his office. The altered rat submitted as the other male mounted him. The tape continued with similar scenes. It was quite dramatic.

Such research in animals has led to hypotheses that hormones are, in some way, a cause of homosexuality in human beings. No one, of course, suggests that the sexuality of rats and that of human beings are strictly comparable; some critics of neurobiological research on homosexuality question the utility of animal models entirely. Nonetheless, it was investigations involving animals that got researchers thinking.

Of the scientists who have concentrated on hormonal or psychoendocrinological studies of homosexuality, Günter Dörner, of Germany, is one of the best known. In the 1970s Dorner classified homosexuality as a "central nervous pseudohermaphroditism," meaning that he considered male homosexuals to have brains with the mating centers of women but, of course, the bodies of men. For decades endocrinologists had speculated that because male sex hormones are known to be responsible in human beings for masculine body characteristics and in animals for certain aspects of male sexual behavior, it follows that adult homosexual men should have lower levels of testosterone, or else higher levels of estrogen, in the bloodstream than adult heterosexual men, and that homosexual and heterosexual women should display the opposite pattern. This is known as the "adult hormonal theory" of sexual orientation, and Dorner claimed that some initial studies bore it out.

In 1984 Heino Meyer-Bahlburg, a neurobiologist at Columbia University, analyzed the results of twenty-seven studies undertaken to test the theory. According to Meyer-Bahlburg, a score of the studies in fact showed no difference between the testosterone or estrogen levels of homosexual and heterosexual men. Three studies did show that homosexuals had significantly lower levels of testosterone, but Meyer-Bahlburg believed that two of them were methodologically unsound and that the third was tainted by psychotropic drug use on the part of its subjects. Two studies actually reported higher levels of testosterone in homosexual men than in heterosexual men, and one unhelpfully showed the levels to be higher in bisexuals than in either heterosexuals or homosexuals.

As it came to be widely accepted that adult hormone levels were not a factor in sexual orientation, scientists shifted their attention to prenatal hormone exposure. Many of the glands in a human being's hormone system are busily functioning even before birth—tiny hormone factories that produce the chemicals that help to mold the person who will eventually emerge. Perhaps, it was thought, different levels of prenatal hormones produce different sexual orientations. For obvious reasons, the sometimes brutal hormonal experiments done on monkeys and rats cannot be done on human beings, but nature at times provides a narrow window onto the mysteries of prenatal hormonal effects in ourselves.

Congenital adrenal hyperplasia (CAM) has been called by Meyer-Bahlburg a "model endocrine syndrome" for examining the effects of abnormal amounts of prenatal sex hormones. CAH, which can affect both males and females, is caused by a simple problem: an enzyme defect makes it impossible for a fetus's adrenal gland to produce cortisol, an important hormone. In a normal fetus, as the adrenal gland produces cortisol, the brain stands by patiently, waiting for the signals that the cortisol level is appropriately high and production can be shut off. But in CAH fetuses, which lack the enzyme to create cortisol, the brain doesn't get those signals, and so it orders the adrenal gland to continue production. The adrenal gland continues pumping out what it thinks is cortisol, but it is unknowingly producing masculinizing androgens. It dumps these into the fetus's system, thereby overexposing it to male hormones.

The consequences are most dramatic in females. Once, in his office, Roger Gorski dug into a desk drawer and grabbed a few photographs. "What sex is it?" he asked. I squinted at close-ups of a child's genitals and saw a penis, plain as day. "It's a boy,', I said confidently. Gorski's eyebrows shot up. "Where are the testicles?" he asked. I looked closer. Oops.

This was a CAH baby. In this case, Gorski told me, the doctors had decided at the time of birth that the child was a boy with undescended testicles, a relatively common and minor condition. But in fact I was looking at a genetic female.

With surgery a CAH female's external genitals can be made to look feminine, as her internal apparatus already fully is, and she will be raised as a girl. But hormones may have already had their effect in an area that plastic surgery cannot touch: the brain. Or at least so proponents of the prenatal-hormone theory of sexual orientation would argue. The sexual orientation of CAH females tends to bear them out. A 1984 study by the Johns Hopkins University sex researcher John Money found that 37 percent of CAH women identified themselves as lesbian or bisexual; the current estimate of the proportion of lesbians in the general female population is from two to four percent.

One possible clue as to whether the prenatal-hormone theory of sexual orientation is a profitable line of inquiry involves something called luteinizing-hormone (LH) feedback. The brain releases several hormones, including LH, which initiate the development of an egg in a woman's ovary. As the egg develops, the ovary releases increasing amounts of estrogen, stimulating the brain to produce more LH, which in turn promotes the production of still more estrogen. The process is called positive feedback. In men, estrogen usually acts to suppress the production of luteinizing hormone—it results in negative feedback. These differences in LH feedback in human beings, together with the discovery that male rats hormonally altered after birth will display both positive LH feedback and same-sex sexual behavior, led some researchers to a hypothesis. They speculated that gay men, their brains presumably not organized prenatally by testicular hormones, just as women's are not, would show a positive LH feedback, like that of a heterosexual female, rather than the negative feedback of the typical heterosexual male. If such feedback were to be found consistently in homosexual men—by means of chemical analysis of the blood after injection with estrogen—could this not be taken as evidence that some decisive prenatal hormonal event, with important bearing on subsequent sexual orientation, had indeed occurred?

This line of inquiry has given rise to an active field of study that as yet has little to show for itself. The uncertainties are of two kinds. The first one involves the following question: Do LH feedback patterns of the sort being sought in fact exist in human beings? The second comes down to this: Even if LH feedback patterns of the sort being sought do exist, will they really tell us anything about events that occurred before birth? Unfortunately, neuroscientists lack unequivocal answers to both questions, despite considerable efforts. Different studies have yielded conflicting data. No one has yet come up with what one neurobiologist facetiously terms a "gay blood test."

In an article published in 1990 in the Journal of Child and Adolescent Psychopharmacology, Heino Meyer-Bahlberg surveyed the work done so far on hormonal research in general and concluded: "The evidence available to date is inconsistent, most studies are methodologically unsatisfactory, and alternative interpretations of the results cannot be ruled out." On the other hand, Meyer-Bahlberg went on, "not all potential avenues to a psychoendocrine explanation of homosexuality have been exhausted."

Among the unexhausted avenues is one being explored by Richard Pillard.

A PSYCHIATRIST AT THE BOSTON UNIVERSITY SCHOOL of Medicine, Richard Pillard is a tall, pleasant man in his fifties with a neatly trimmed moustache and a relaxed manner. Even when talking seriously, he remains goodnatured. When we spoke one afternoon in his Boston townhouse, he joked that he is uniquely equipped to investigate whether homosexuality has a biological basis: he, his brother, and his sister are gay, and Pillard believes that his father may have been gay. One of Pillard's three daughters from a marriage early in life is bisexual. This family history seems to invite a biological explanation, and it made Pillard start thinking about the origins of sexual orientation.

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."

The Pillard-Weinrich theory also accords with what Green refers to as male "vulnerability" during the process of sexual differentiation. A considerably larger number of male embryos come into existence than female embryos, and yet males and females come into the world in about the same numbers. Therefore, phenomena linked to sex must reduce the number of males who survive to term. Many disorders are, in fact, more common in men than women, and some of these could result from problems originating in masculine differentiation. Although good statistics do not exist, it appears that there may be two gay men for every gay woman, which would be consistent with the vulnerability theory.

It is important to remember that although homosexuals and heterosexuals may be "sex-reversed" in some ways, in other ways they are not. For example, neither gay nor straight men tend to be confused on the subject of what sex they are: male. LeVay says, "It's not just that you look down and see you have a penis and you say, 'Oh, I'm a boy. Great.' I think there must be some internal representation of what sex you are, independent of these external signals like the appearance of your body. I think most gay men are aware of some degree of femininity in themselves, yet there is no reversal of gender identity." Gay men and straight men also seem to display an identical strong drive for multiple sexual partners; lesbians and straight women seem to be alike in favoring fewer sexual partners.

The evidence from hormonal research may circumstantially implicate biology in sexual orientation, but it is far from conclusive. William Byne raises a warning flag: "If the prenatal-hormone hypothesis were correct, then one might expect to see in a large proportion of homosexuals evidence of prenatal endocrine disturbance, such as genital or gonadal abnormalities. But we simply don't find this." Moreover, the hormonal research does not answer the question of ultimate cause. If hormones help to influence sexual orientation, what is influencing the hormones?

Presented by

Never Tell People How Old They Look

Age discrimination affects us all. Who cares about youth? James Hamblin turns to his colleague Jeffrey Goldberg for advice.

Join the Discussion

After you comment, click Post. If you’re not already logged in you will be asked to log in or register.

blog comments powered by Disqus

Video

Never Tell People How Old They Look

Age discrimination affects us all. James Hamblin turns to a colleague for advice.

Video

Would You Live in a Treehouse?

A treehouse can be an ideal office space, vacation rental, and way of reconnecting with your youth.

Video

Pittsburgh: 'Better Than You Thought'

How Steel City became a bikeable, walkable paradise

Video

A Four-Dimensional Tour of Boston

In this groundbreaking video, time moves at multiple speeds within a single frame.

Video

Who Made Pop Music So Repetitive? You Did.

If pop music is too homogenous, that's because listeners want it that way.

More in Technology

More back issues, Sept 1995 to present.

Just In