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WHAT IS HOMOSEXUALITY? And how can its existence be explained? These questions go back at least to Ancient Greece. Plato's Symposium famously offered the poetic image of primordial doubled souls who, as punishment at the hands of Zeus, suffered being split into two. The resulting halves who were rent from a single male-female whole went on to yearn for their respective missing piece. These men and women were heterosexual. But the separated parts of doubles that were all-male or all-female were homosexual, yearning after their own kind.

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Just a fable, of course. And since Darwin put many questions about human traits squarely in the scientific domain, the question "Why homosexuality" has only grown more pointed. The theory of natural selection suggests that attributes inhibiting reproduction tend to end up in evolution's dustbin. And yet in history and anthropology, homosexuality seems to pop up all over.

The evolutionary counterintuitiveness of homosexuality helped secure what in the West became the dominant explanation for it in the 20th century– as a product of nurture, not nature, with psychiatry's picture of an outta-kilter family drama taking center-stage. Yet psychiatry's ideas could never be rigorously tested, and psychotherapy had little success in altering homosexuality– the rationale mostly driving therapists' attempt to understand it. Homosexuality's incomprehensibility gave a boost to moralistic claims that it was an individual choice– one usually deemed bad.

British-born neuroanatomist Simon LeVay is among the keenest proponents of a radically different view– that the causes determining the basic shape of a person's sexual orientation are biological. "There are certain things that tend to go along with being gay or lesbian, and they fall under the general rubric of gender atypicality," he says. The process is mediated, he believes, by prenatal hormones, and probably also by genes. He thinks he's found a key piece of the puzzle– a structure the size of a snowflake in the anterior of the brain's hypothalamus that may be on average two-to-three times larger in heterosexual as in homosexual men, in whom its size is comparable– adjusting for body weight– as in heterosexual women.

LeVay's research, conducted at the Salk Institute in San Diego, was published in Science in 1991, attracting enormous attention. The function of the structure in question– the third interstitial nucleus of the anterior hypothalamus (INAH-3)– is still unclear. But this is only one of a number of biological markers that researchers have found in– for the most part– self-identified American or European gay men or lesbians. These markers range from a higher ratio of ring-finger to index-finger length in lesbians (more like men's) to the function of gay men's inner ears (more like women's), to the greater likelihood of gay males to have had older brothers (maybe more testosterone exposure in the womb, or maybe less).

To its proponents, the emerging biology of sexual orientation is dashing old just-so stories that were grounded on too little evidence and too much hand-waving, when not simply moralistic judgments. Critics argue that the biological evidence so far is flawed and inconclusive. Proponents of a biologically-grounded homosexuality, they say, are conjuring and falling under the thrall of new fables.

I CAUGHT UP WITH the 59-year-old LeVay in February 2003 in his office at Stanford. After attending Cambridge University, LeVay received his doctorate at the University of Göttingen, and served on the faculty of at Harvard Medical School, specializing in the neurology of vision. At Stanford, he was teaching introductory sexology for the first time. LeVay was testing out on his students his own recently-published textbook, which he co-wrote, he says, in part to highlight sexuality's debt to biology. The book is one of eight LeVay has written or co-written in the past ten years– one each about Parkinson's disease, earthquakes, and extraterrestrial life. Three others dwell on homosexuality– one of them a thriller exploring the implications he thinks his research could have for eliminating it. Openly and happily gay himself, that's hardly something LeVay proposes doing.

LeVay's hypothalamus research and writings have made him into a public intellectual around sex. But the study that launched his new career was– as it turns out– among the last laboratory work he did or ever expects to do. In 1993 LeVay left the Salk Institute in order to write and teach.

LeVay undertook his hypothalamus study for personal reasons. LeVay had taken off time to care for his lover, who died from AIDS in 1990. When he returned to the lab, he wasn't sure he wanted to be there. "It had been an extremely traumatic year," LeVay says, parting the curtains a little on his usual English reserve. "I was really not giving a damn about things in the world." Looking for a neurology project that felt personally relevant, he undertook his hypothalamus study as a hobby, funding it from his own pocket.

That study's success and notoriety set LeVay on his new career (and also found him his current lover– a young man, whose family was from India, who saw LeVay on a TV talk show and found himself smitten). But writing books and getting the odd teaching job can be a chancy way to make a living. LeVay's small Stanford office, shared with another instructor, was the sort that universities dole out to graduate assistants. When we walked to the Stanford parking lot, it took him a few minutes to jimmy open the passenger door of his well-worn car, patched in places with duct tape. Partly it's a matter of economy, and partly, added LeVay– a vegetarian who is fond of bicycling– it's a public statement about a California that's ruined its ecology by automobile.

THE SPAGHETTI of Los Angeles's freeways, nestled among which LeVay lives in West Hollywood, is nothing compared to the neural tangle of the brain. With its ten billion nerve cells and ten trillion connections (or synapses), the brain is a daunting object of study. But LeVay is confident we can eventually map out its secrets. His optimism is a little ironic, since LeVay is something of a lower-brain fundamentalist. He tends to regard consciousness and reason, so necessary to scientific discovery, as mere shadows cast on a cave wall– to invoke another Platonic metaphor. "Sometimes at lectures I'll show a slide of the brain and point to the cerebral cortex," LeVay relates, "whose main function, I'll quip, is to rationalize what the hypothalamus makes us do anyway". Scientists not excluded: after a career working among Nobel prize winners and other luminaries, LeVay thinks that it's a desire to impress potential mates that drives a lot of research.

The clue to solving the puzzles of the brain, argues LeVay, is found in dividing the number of synapses by the number of neurons– ten trillion by ten billion– which shows each nerve cell is connected on average to only a thousand others. Those connections are mostly local– otherwise the necessary long-distance cabling would require our brains to be huge. (An exception to this clustering pattern are the cerebral and cerebellar cortexes, arrayed with continuously repeating neural interconnections, which suggests that consciousness emerges from some "massively parallel" signal processing.) But underneath the brain's outer layer, whose hyperdevelopment is human beings' defining feature, many brain functions are localized in these neural clumps or nuclei, ranging in size from a grain of sand to a nut. Their functions can potentially be mapped– inadvertently, as when a stroke victim loses the ability, say, to name objects seen out of the right eye. Or they can be mapped deliberately, through imaging and experiment.

The hypothalamus is about the size of a level teaspoonful and sits under the thalamus, near the center of the brain. It is home to a number of nuclei that help regulate appetite, thirst, fear, the impulse to flight-or-fight, body temperature, and mating– along others whose function isn't clear. In his career working on the visual system, LeVay hadn't bothered much with the hypothalamus; only a few optic nerves end here– those going to the superchiasmatic nucleus, which judges brightness of light to regulate circadian rhythm. But as he turned his attention from sight to sex, LeVay focused on the hypothalamus, a region that has a neurological reputation as being, LeVay writes in The Sexual Brain, "haunted by animal spirits and the ghosts of primal urges."

The hypothalamus seemed a promising place to look for biological register of sexual orientation because it was so functionally involved in sexual arousal, and parts of it were found to be sexually dimorphic. The hypothalamus's sexual dimorphism was discovered in 1978 by UCLA's Richard Gorski, who found a small nucleus (less than a millimeter across at its largest) in the front of the rat hypothalamus. Gorski's group called the structure the Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA), and found that it was larger in males than females. The greater size of the SDN-POA in male rats was shown to result from the testosterone burst male mammals give themselves around birth. Castration at birth, or serious maternal stress just before, permanently reduces the size of a male rat's SDN-POA. In gerbils, by contrast, the structure's size has been found to vary with the amount of circulating testosterone.

It was clear that the hypothalamus's preoptic area is involved in mammalian sexual function. Neurons in that region are especially responsible to sex hormones. Male monkeys with lesions in the medial preoptic show impaired heterosexual activity, but continue to masturbate as normal. Male rats and ferrets with damage there show more female-typical sexual behavior. A study involving monkeys at Kyushu University in Japan showed that the rate of firing of many neurons in the medial preoptic corresponded to sexual arousal, reaching a peak of 50 impulses per second as an estrous female approached. At ejaculation, the firing of another closely adjacent cell peaked, suggesting, says LeVay, "a parcellization of sexual function among anatomically distinct regions". In 1989 Gorski and his colleague Laura Allen and Gorski looked for nuclei in the human brain that may correspond to the SDN-POA in rodents, and found four contenders-- INAH's numbers 1 to 4. Two of the four were sexually dimorphic, they reported-- INAH-2 and INAH-3-- with the INAH-3 showing the most marked difference-- being some three-times larger in volume in men. Inspired by their work, LeVay decided to compare the INAH's in homo- and heterosexual men.

With the AIDS epidemic then raging among American gay men, hypothalamus samples from homosexuals could be readily obtained. LeVay got his tissue samples from 41 routine autopsies– 18 were from men who died of AIDS whose medical records indicated they were homosexual, and one was from such a man labeled as bisexual. Of the other 22 samples, 16 were from men (six of whom had died from AIDS), and six from women (1 of whom had died from AIDS). LeVay didn't know about the sexuality of the persons in the non-gay samples, but he presumed that they were heterosexual, with the odds being on his side. LeVay prepared the slides himself, and coded them so that he didn't know which belonged to whom, and proceeded to measure the volume of the four hypothalamic nuclei in question. Contrary to prior results, LeVay found that the INAH-2 to be the same in his male and female samples. The INAH-3, varied with sex as an earlier study had found, being larger in men. And crucially, it varied with sexual orientation– the straight men showing on average an INAH-3 more than twice as large in volume as the gay men. LeVay could claim to have found the first brain structure that correlated to a person's being gay or straight.

SOME OF THE MEDIA accounts following the publication of LeVay's study in Science suggested that a brain structure had been found that 'caused' homosexuality in males. But LeVay himself was much more cautious.

The brain changes its structure in response to experience– and even thought– so it could be that the size difference was a result of homosexual experiences, not their cause. When blind people learn Braille, the area of their brain controlling their reading finger becomes enlarged and more active, for example. But LeVay noted that in male rats, the increased size of the SDN-POA (the area of the hypothalamus seemingly cognate) was determined perinatally, through exposure to androgens, and does not change in later life, even when androgens are sharply reduced via castration.

The size difference LeVay found may have been an effect of HIV or treatments for it, but there was no difference between those with AIDS in LeVay's sample and those without, though samples sizes were tiny.

It was also possible, LeVay granted, that it wasn't so much homosexuality as a preference for receptive anal sex with numerous partners that was registered in the INAH-3 size difference. All his gay subjects had succumbed to HIV, most often transmitted among American gay men in that way. But LeVay argued that surveys showed that such sexual practices did not constitute unrepresentative minorities within the gay population.

LeVay conceded as well that a smaller INAH-3 was not a necessary condition for homosexuality– some of the presumably straight men in his sample had a smaller nuclei than some of his gay subjects. The difference he had found in the INAH-3 between gay and straight men was statistical, not absolute.

Critics revisited all these objections, and pointed out that neuroanatomical studies are notoriously hard to reproduce– LeVay's findings had contradicted Allen's, for instance, on sexual dimorphism in the size of INAH-2. And the function of the INAH-3 remained unknown.

But more than ten years later, LeVay's claims about the INAH-3 and sexual orientation, though they've taken some knocks, have been partly confirmed, and are still in play.

Only one other researcher has tried to duplicate LeVay's research. In 2001, William Byne, of Mount Sinai Hospital in New York, and co-authors, reported a significant volume differential in the INAH-3 between men and women, like that found by Allen and LeVay. There was also a difference in size between gay and straight men as found by LeVay– but the difference was small, and not statistically significant. Byne found, with LeVay, that HIV was not a factor affecting INAH-3 size. But he also found that in the gay samples, the neurons of the INAH-3 were more densely packed– suggesting fewer interconnections among them. (LeVay only measured volume, and did not count neurons.) Byne's finding here seems to bolster LeVay's contention that a less active INAH-3 is implicated in male homosexuality.

More evidence in support of LeVay comes from sheep. In November 2002, researchers at Oregon Health and Science University announced the results of a study of domesticated rams (male sheep) who only court and mount other males. Along with humans, rams are among the few animals where individuals display exclusive homosexuality (as many as 10 percent of rams are "gay"). Comparing the preoptic nucleus in homosexual rams' hypothalamuses with heterosexual rams and ewes (female sheep), researchers found a smaller preoptic nucleus in the gay rams, typical of the size found in females. It's a rare instance, LeVay notes, that scientists, given their dependence on animal models, have discovered a neuroanatomial feature first in humans.

In addition, the heterosexual rams showed higher concentrations of an enzyme aromatase, which converts testosterone to estrogen-- the form, perhaps counterintuitively, in which the male sex hormone is active in the brain. The homosexual rams didn't have less testosterone overall, but in that key part of the hypothalamus they were converting less of it to its active form. This could be evidence for the claim that male homosexuality is related to subtle differences in sex-hormone responses in the brain.

LeVay's findings take their place in a list of other biological markers that have been reported by researchers as distinguishing gay people from heterosexuals. Among them

-- a larger 4th finger to 2nd finger (index finger) ratio in lesbians as compared to straight women

-- a prevalence of older brothers among gay men, with Ray Blanchard at the University of Toronto finding that a first-born male has a 2 percent chance of being gay, but for a boy with 4 older brothers, the likelihood triples, to 6 percent

-- reports of increased homosexual fantasies and behavior in women who were exposed to androgens prenatally

-- more frequent left-handedness or ambidexterity among homosexuals

-- a greater likelihood of homosexuality in sons of mothers who were severely stressed in the second trimester of pregnancy

-- inner ear structure and function in lesbians more typical of males as measured by 1) "otoacoustic emissions", faint clicking sounds produced by the inner ears in response to stimulus and 2) "auditory-evoked potential", the pattern of brain-wave peaks produced with exposure to sound. In gay males, auditory-evoked potential appears to by hypermasculinized-- that is, shifted even further in the direction characteristic of males generally.

-- increased density of fingerprint ridges on the left thumbs and little fingers of homosexual men, an asymmetry typical of females.

a larger suprachiasmatic nucleus, a structure in the hypothalamus acting as the body's internal clock, found in one study to have almost twice as much volume in homosexual men compared to heterosexual men, and with more than twice as many neurons

-- a larger anterior commissure-- one of the two bundles of nerve fibers connecting the two brain hemispheres-- found UCLA's Laura Allen to be 18 percent larger in homosexual than heterosexual men

-- a history in male homosexuals of having been 'sissy boys' at least in early childhood, around ages 3 to 5-- avoiding sports and rough-and-tumble play and showing feminine behavior even in the face of hostility and discouragement

Many of these findings point to hormonal involvement in shaping sexual orientation-- finger length, fingerprint patterns, and the formation of the inner ear, for example, are all influenced by the levels and activity of androgens in the womb.

In addition, there are hints at a genetic contribution to homosexuality. J. Michael Bailey of Northwestern University reports that a male with a gay brother is 3 to 7 times more likely than average to be gay himself, while a woman with a lesbian sister is 4 to 8 times likelier to be homosexual. In a study of twins, at least one of whom was gay, Michael Bailey of Northwestern University Richard Pillard of Boston University found that 52 percent of identical twins to both be gay, and 22 percent of same-sexed fraternal twins, but only 11 percent of adopted brothers.

BUT THE ATTEMPT to ground patterns of desire in biology hasn't convinced everyone. Many of the studies showing these biological markers have not been reconfirmed. In almost all of them, the subjects were Western, self-identified gay men and lesbians. In most cases they were recruited through gay newspapers and community centers. These people may not be representative of all those with homosexual feelings and experiences. In a study of twins, at least one of whom was gay, Michael Bailey of Northwestern University Richard Pillard of Boston University found that 52 percent of identical twins to both be gay, and 22 percent of same-sexed fraternal twins, but only 11 percent of adopted brothers. The rate of heritability for homosexuality that Bailey and Pillard's twin study implied was confounded by an unexpectedly low percentage of concordance for homosexuality among non-twin brothers (only 9.2 percent were gay, compared to 11 percent of non-biologically related adoptive brothers). This undercuts the main finding that genes could account for 50 percent of male sexual orientation, since biological brothers are so genetically close. Could it be that gay twins who were more aware of sexuality, because both happened to be gay, were likelier to join Bailey and Pillard's study in the first place? That's suggested by another twin study, which used data from Australia's national twin registry instead of self-selected volunteers and found a rate of heritability for homosexuality among identical twins of only 20 percent.

Genetic explanations for personality traits and diseases have, time and again, proven overstated. Scientists have reported that genes have been found to be responsible for, say, schizophrenia or alcoholism, only for the claim to be withdrawn (to little notice) later on when further research disproved the link.

Such exaggerated claims have also been made for homosexuality. Dean Hamer's study of gay brothers offered, he wrote, "the first concrete evidence that 'gay genes' really do exist." The study received enormous attention when it was published in Science in 1993. Hamer was looking for shared genetic markers on the X chromosome (in males, supplied always by the mother). Hamer found a small region of the X chromosome– called Xq28– that seemed to have these shared markers. In 33 of 40 pairs of gay brothers, Hamer found shared common sequences (not the same ones, however) in Xq28, which contains several hundred genes. LeVay and Hamer co-wrote an article in Scientific American, and speculated that the difference LeVay found in the INAH-3 between gay and straight men might be connected to Hamer's genetic findings.

But today that claim seems like overreach. Hamer's study was seriously flawed. It failed to inquire whether its subjects' heterosexual brothers also shared the gene markers, essentially depriving his study of a control group. Hamer also assumed a rate homosexuality among males (2 percent) that is probably too low, which meant he overestimated the significance of the shared markers he found at Xq28. According to Neil Risch, the Yale biostatistician who developed the method Hamer employed, Hamer's small sample size probably made his findings worthless. Hamer became the subject of an National Institutes of Health investigation when an unnamed associate who worked on the study alleged in 1994 that Hamer had misrepresented his data. The NIH ultimately did not find any fraud, but the allegations and evidence were never made public. Researchers at the University of Western Ontario could not replicate Hamer's findings, and found no evidence of male homosexuality being X-linked (though the question continues to be debated). Any apparent preponderance of homosexual relatives on the mother's side is likely due to women knowing more about their relatives than men, some researchers suggest. A follow-up study, in which Hamer collaborated, found that the connection to Xq28 so statistically insignificant that the finding, had it come first, would have been unpublishable. Hamer's study today is largely discredited scientifically, yet it continues to be cited in the popular literature.

IS HOMOSEXUALITY too culture-bound to support genetic or physiological explanations? In some societies homosexual relations are extremely common, while in others they are rare. But almost everywhere, same-sex eroticism occurs readily in schools, monasteries, prisons, armies, and other single-sex institutions, which suggests that homosexuality involves some general human capacity for bisexuality, not specialized biology.

The sex lives of animals can be read to bolster this view. Homosexuality is common in the animal world– with some 15 to 30 percent of species showing evidence of it, according to Bruce Bagemihl, author of Animal Exuberance. Animals engage as well in many kinds of nonreproductive sexual behavior– masturbation, incest, and sex with and among juveniles. Rams are the exception: in most other cases, animals who engage in same-sex activities also practice heterosexuality. Nature may make the impulse to sex so strong it overflows its cup, Bagemihl argues.

In humans, in any case, the biological evidence conflicts– most of it points to homosexuals being, in some subtle ways, intersexed, with females somewhat hormonally mascunlized and males less masculinized. But some reports suggest– at least in males– that homosexuality can result from hyper-masculinization (the function of the inner ears, as measured by 'auditory-evoked potential', and some of the data about digit-length ratios).

Culturally, at least, the equation of male homosexuality and intensified masculinity has a long history. In Plato's fable, homosexuality follows from a prior state of pure, super-saturated maleness or femaleness– those all-male or all-female doubled beings. It is heterosexuality, on this view, whose origins are intersex– rooted in prior hermaphrodite doubles.

Homosexuality is often part of the social structures involved in masculinization. Whereas women's social role always follows, at least in part, from her central involvement in childbirth and nursing, the male's role is not so fixed. "Many non-Western societies see (or have seen) homosexuality as a phase boys go through on their way to becoming men," notes University of Washington sociologist Pepper Schwartz. Plato would have agreed– ancient Greek male homosexuality was embraced as sort of a training ground for masculinity, a way of inculcating elite youth in the ways and alliances of manhood.

Cultures that value an intensified masculinity tend as well to valorize male sexual opportunism– a basic characteristic of male sexuality that follows from the male's lighter burden in reproduction. Much of the homosexuality in Mediterranean (or Mediterranean-influenced) societies results from a tendency for males to welcome any opportunity for penetrative sex. Antonio de Moya, a sociologist in Santo Domingo, says that a Dominican male propositioned by another for sex in exchange for money or favor– so long as he is not asked to be penetrated– loses face if he declines the offer: a rejection is taken as indication that he's afraid of the other man, or lacks a sufficiently carefree, hard-edged virility. Freud contended that among the ancients, sexual drive mattered more than object, whether it be man, women, boy, girl, or goat.

LeVay himself says he feels himself lucky to have grown up in a culture remarkably accepting of the fact of homosexuality, if not exactly the idea. Same-sex relations were illegal in the UK in the 1950s and scandals were often in the newspapers. But homosexuality flourished in the shadows, as it had for centuries. The peak of his sex life, LeVay says, was when he was 14 and attending an all-boys boarding school, the sort of place Britain's elite traditionally educated its sons. Hormones were flowing and same-sex play was rife. Homosexuality was quite ingrained as well at Cambridge, LeVay says. He remembers feeling surprise a few years ago when British actor Ian McKlellan, with whom LeVay attended university, made news for coming out publicly as gay. It was common knowledge at Cambridge at the time, and McKlellan and LeVay had even shared the same boyfriend.

LeVay sees no contradiction between what was in his youth the wide participation of his peers in homosexual relations, and the clear, narrow notion of homosexuality for which he argues for a biological basis today. Only a few of the boys at school were gay, he contends, while the others did it just for physical release, and went on to marry, father children, and be ordinarily heterosexual.

Perhaps the readiness of homosexuality to bubble forth in the right circumstances, and the variety of its cultural forms and social uses, puts it in the category of phenomena that are irreducible to the small scale of genes and neurons. No one would try to explain a rainstorm or a sunny day by counting the interactions of molecules. The weather, rather, seems explicable– to the extent that it is– only in terms of much grander entities– warm fronts, rates of humidity, air pressure, and wind patterns. Of course, no one doubts that the grossest weather systems and the properties of air molecules are connected along a seamless causal chain. Indeed, chaos theory proposes, famously, that butterfly wings flapping in Mexico can cause thunderstorms in Nigeria. But at this point, it's not clear that the possible links between biological processes and individuals' sexual predispositions can be woven into a meaningful account of sexuality's larger patterns– patterns that influence, in turn, the kinds of sexuality individuals display. By ignoring the culture and meaning of sexuality, are biologists doomed simply to tell us more fables? Simon LeVay weighs in. **

It's hard to imagine how something so basic to one's character as whether one desires men or women could be determined by a brain structure like the INAH-3, which is about the size of a snowflake. If you were an evil brain scientist, what's the most havoc you could wreak on someone's character by altering such a small part of their brain?

You could probably kill them if you put the lesion in the right place, somewhere that controls respiration, for example. Or you could put somebody in a coma if you put a tiny lesion in a well-chosen place in the brain stem. You could make animals, at least, insatiably hungry, or make them not eat at all, and you could adjust their body temperatures up and down.

But those aren't really changes to someone's character or personality.

If you're talking about a human, there have been reports of very small lesions in the amygdala pacifying people who were unusually aggressive. In Germany, there were lesion studies on rapists that supposedly put an end to their abnormal sexual behavior–probably stopping their sex drives all together. These, however, were probably larger lesions in the hypothalamus than what we're talking about here. Even in the cerebral cortex you can cause strange cognitive problems with a tiny lesion, such as producing an inability to see colors.

Your work is based on a size difference in the INAH-3 between gay and straight men. But that size difference has been only partly confirmed. Meanwhile, there are raging controversies about whether there are morphological differences– between men and women, or homosexuals and heterosexuals– in other brain structures, such as the corpus callosum. Why can't these questions get resolved?

It's hard to say. There just aren't that many people who do this kind of work. There haven't been enough studies really to nail it down. And yes, there are problems with subjectivity in the measurements, disagreements about exactly what defines the boundaries, say, of a cell group.

Whatever the fate of your findings– for now the size difference in the INAH-3 joins a list of other possible biological markers distinguishing homo- and heterosexual people– inner ear structure, digit length in lesbians, fingerprint ridge density in gay men. These seem like impossibly disparate phenomena. What's the underlying continuity?

The unifying paradigm is the idea of prenatal sex hormones, particularly testosterone. A lot of the data are explained with the hypothesis that either there are differences in circulating hormone levels between fetuses that become gay and fetuses that become straight, or that there is some difference in the way the brain or the body responds to those hormones. So it might be that the receptors– molecules in the cell that recognize the hormone and then cause genes to be turned on or off– might be different. Or else there could be differences in the way that genes subsequently respond to these hormones. And this would then predispose the brain to develop in a certain way. And also the body– with fingerprints, finger lengths, the inner ear, and so on, developing in a slightly different fashion.

The most manifest effect of sex hormones are on the genitalia. You don't seem to find, say, smaller penises in gay men or larger clitorises on lesbians.

The hormonal effects that seem important for sexual orientation probably come into play at an earlier phase of fetal development than the phase at which external genitals are forming, which is fairly late. There are no obvious differences in the genitalia related to sexual orientation in gay men or lesbians. The bodily differences that have been described are very subtle ones. The relevant hormonal differences are probably not very large, and they may just have an effect at a certain critical time. Of course if you look at personal ads in gay newspapers, it's a different story. I looked through the local gay magazine, and figured out the average penis length mentioned in the personals, and it was something like nine inches, with a range between 7 and 11 inches. That definitely would be larger than the average for heterosexual men, which is around five-and-a- half inches. But I don't trust those data very much!

Sexuality has many dimensions besides whether you like women or men. Are you suggesting that all of these are biologically based or localizable in the brain?

Sometimes in connection with identical twins who've been reared separately you hear these incredible stories about how, when they finally met, they found they both had married one- legged cocktail waitresses called Lucille, or something like that. I'm very skeptical of these stories, and of the implication that biology can really spell out your life and interests in this minute detail. And I'd doubt that biology has a really detailed influence on your sexual attractions. But who knows? It's something that needs to be explored more. Every aspect of our mind has to have some representation in the brain. The fact, say, that you like one musical composer rather than another is represented in the brain anatomically in some sense, because you can cool the brain down– just switch it off basically– and then turn it back on, and you still have the same preference. Of course it would be a horrendously difficult project– and at present impossible– to find that structural representation for composer preference. Even if you found the exact circuitry, it probably wouldn't tell you anything more useful than a whole different approach ignoring the brain altogether– one, say, that looked at someone's personal history and experiences with music. Many of the precise aspects of what we find attractive in partners probably cannot usefully be studied at the brain level. Though even here there are some principles of attractiveness that seem to be universal. Symmetry, for example, is a very important variable affecting whose face is seen as attractive, and we know about possible neural mechanisms that are important in recognizing symmetry.

There are so many aspects of sexual desire, but you're assuming that a general preference for males or females is pretty much a binary, either-or choice in terms of brain structure.

I think it's because sexual orientation is a much more basic and profound aspect of our minds than other aspects of desire– or something like musical preferences. It appears as if nature has really decided to apportion some distinct part of the brain, some distinct mechanism, to sexual orientation. With these other preferences, on the other hand, that's probably not true, and instead what's involved are pretty broadly distributed circuits. Those other preferences are more likely an accidental spin-off of circuitry that really has been designed for other purposes. There's a lot of human mental life that doesn't have a lot of survival value, but is probably sparked off of other systems that are more crucial for our survival. Take the hypothalamus– the really important, basic, instinctual life functions are represented down there– hunger, thirst, keeping your body temperature right, being aggressive in the right situations, and sexual functions. It's an old control center. You're not going to find musical preferences in the hypothalamus.

Homosexuality seems to occur in so many varied contexts in humans and animals– and seems to be used, among other things, to encourage alliances, abet gender differentiation, and bonding within an age-cohort. Don't these uses point to homosexuality being determined on a level more general than the brain?

If you look among the primates that are closely related to us, there are immense differences. In some animals, like Bonobo chimpanzees, the uses of sexuality are very exuberant in terms of social bonding, alliance creation, conflict resolution, and so on. These functions go way beyond mating and reproduction, and are very complex socially. And in some other very closely related species, you really don't have anything like that– sex is basically just about mating. I think for humans it's between the two extremes.

In your writings you seem tantalized by the possibility that socialization plays very little role at all in people's sexuality or character.

It would be ridiculous to assert that sexual orientation is entirely a matter of biological predisposition, because we know that homosexual feelings and behaviors are organized differently in different cultures. But I'm a biologist. That's what interests me. The socialization side I'm sure is important. But it doesn't interest me as much. I am intrigued by the idea that we're basically fated to be the way we are much more than we ever imagined.

You've written that the "male is little more than a parasite who takes advantage of [the female's] dedication to reproduction." Couldn't you say, maybe more positively, that the male's function is to be a random variable, a wild-card, helping to keep stirred-up the pot of sexual diversity?

The deepest truth about the sexes and evolution is that being a male is basically a gamble. Nature takes risks in male development that it doesn't take so much in female development. And so you end up with a wider scatter of results. Comparing males and females on some dimension, for example, you often find not so much a difference in the average but a difference in the spread. That is, there may be more men at the top extreme but also the bottom extreme. You may get more men who are gifted in some trait but also more who are extremely bad at it. The basic reason for nature to take risks with males is their capacity to have a very large numbers of offspring, whereas females are biologically limited as to how many offspring they can have. It's not worthwhile for nature to take tremendous risks with female development, because there's not a lot to be gambled and won, while there's a lot to be lost. Whereas in males there's potentially a lot to be won, because so many males don't have offspring. Particularly if you go back into more traditional societies, you've got tremendous imbalances, where some men are having all the offspring and others are just completely excluded. And of course that encourages just this sort of risky developmental strategy, physiologically and psychologically.

Is there a link between that characteristic male genetic and physiological risk-taking and male homosexuality?

If you're talking about exclusive homosexuality, it's a lot more common in men than in women, so that fits in with this overarching picture of sex differences. And that leads to a possibly negative view of homosexuality– that it's a crap shoot and these are the guys who lost out. I don't think I lost out particularly– I think I did rather well in ending up gay. But if you're talking exclusively about evolutionary theory, there is no really convincing account that homosexuality is an adaptation, that it has some evolutionary advantage. So it's more likely probably that it's a disadvantage, connected with lowered reproduction.

Researchers looking– so far not successfully– for genetic markers linked to homosexuality are divided as to whether male homosexuality is X- linked. That is, whether males are more predisposed to it because it's a trait connected to genes on the X chromosome, to whose effects males are more exposed, having only one. Do you have an opinion?

Not really, except that one possible evolutionary theory would say that a gene for sexual attraction to men would be a gene for homosexuality in men and a gene for heterosexuality in women. The evolutionary significance of such a gene would be what it does in women, rather than what it does in men. And such a gene would be particularly likely to be preserved on the X chromosome, simply because of the fact that women have two of them, and men only have one. So that means that natural selection operates on the gene twice as often in women as it does in men. If you think of that gene on a chromosome going down through history, body after body after body, twice the time, on average, it's in a female body rather than a male one. So natural selection is working harder in women on those genes than it is in men. You could argue that this would be how you would get a gene for male homosexuality preserved– because it's on the X chromosome and it's basically being selected as a gene for heterosexuality. Now that doesn't mean that a woman who doesn't have that form of the gene is not heterosexual. It would have to be a sort of hyper-heterosexuality, because most women would not have it– it's not that common a gene. It might make a woman somewhat more sexually active with men than otherwise, thereby increasing her chances of having offspring. But this is just a theoretical idea– there's no evidence, for example, that the female relatives of gay men have more offspring than do the female relatives of straight men, which is the kind of prediction you would make if that theory was right. So it's pretty speculative.

Your basic picture is that homosexuals are, on some deep and subtle level, more like the other sex on average. But isn't that view confounded by the substantial differences in the way sexual desire works in men and women, with female sexuality being more connected to emotional context instead of a fixed visual ideal? Gay men are more like straight men on those terms.

Female sexuality is indeed a bit more fluid– there's more bisexuality, less genital focus, less visual focus– and so it's probably much harder to pin down into a simple model. But in spite of that, there is evidence that hormones play a role in a fairly simple way. In congenital adrenal hypersplasia (CAH), where the female fetus is androgenized, these girls turn out much more often to be lesbians. Whether that sort of causal mechanism says anything about the sexual orientation of the majority of women I don't know– CAH is a rare syndrome so it may not be relevant for why most women have the sexual orientation they do.

But lesbianism and male homosexuality may not be homologous phenomena.

There are tons of reasons to be cautious about drawing a simple analogy between the two. I think perhaps the most striking thing is the great ease of determining a man's sexual orientation, in contrast to the total impossibility of doing comparable studies with women. You can readily measure a man's physiological arousal with a cuff around his penis when you show him pictures of naked men or naked women. You can measure physiological arousal quite easily in women, by what's called photo- plethysmography, where you measure color changes in the walls of the vagina. But you cannot predict a woman's sexual orientation on that basis, apparently. That's kind of surprising– it really makes you think that you have to regard sexual orientation as a little bit more high-level, more cognitive, in women than in men.

Your work bolsters the case for homosexuality being a distinct natural type. You've argued that this encourages social acceptance for homosexuals. But making sexual orientation so distinct has opened a Pandora's box culturally– we now increasingly say that those who break sex laws– say, by having sex with a minor– should be permanently eliminated from society because they are a distinct and diabolical kind of person.

I think we're going completely the wrong way in terms of how we deal with sex-offenders. If that's a bad outcome of this way of thinking than I would deplore that. You have to think about whether something is really an identity, but also what that identity implies. I don't think anyone's sexual behavior is so ingrained in their genes, or is biologically fated– certainly not to the point that one should just automatically assume that they're going to reoffend if they've committed some sexual offense. Which is not to say that it's not worth discussing whether everything that now counts as a sexual offense ought to. I tried to have some discussion about that at Stanford but didn't get very far. I don't think there's any particular biological research on sexual attraction to children that would speak one way or the other, but let's just assume now that there are genes now or whatever predisposing to pedophilia or even ephebophila [attraction to adolescents]. What would happen if those genes were found? I think it would possibly change the moral discourse. Because right now people think of pedophiles as morally reprehensible individuals. I think that might change if people were persuaded that there were biological or inborn imperatives that predisposed to those desires. But on the other hand of course naming people as inevitably inclined to engage in a certain behavior might just encourage the sort of repressive legal strategies that you mention. So it's two-edged. To the extent, say, pedophiles themselves can-influence the discussion, I think that's an important thing. That's people listen to them and their views on what they're sexuality means.

Ironically, making same-sex behavior the sign of a deep- seated identity makes it harder, generally, for people to engage in it, because it implies they are a certain kind of person. Doesn't this obstruct the various social uses to which same- sex affection and bonding have been put?

I remember when I was a teenager the first time I went on a trip to the south of Europe, to Sicily. It was common there for men to walk around hand-in-hand. In London you absolutely never saw anything like that. You go back to times when homosexual or heterosexual identity was not well understood, or just not commonly spoken about, and you see more homosocial intimacy than you see in the West today. In 18th-century Germany, for example, there were all these guys writing love letters to each other. Some of those relationships may have been sexual, but some of them almost certainly were not. It's the same thing between women, when you go back to 19th-century Europe or America. Yet these intimate relationships between women became harder to sustain as lesbians asserted themselves. So I think yes, the promulgation of gay identity makes people very suspicious of intimacy between males or between females. That's very sad, actually.

Author Profile:  Bill Andriette
Bill Andriette is features editor of The Guide
Email:	theguide@guidemag.com