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Is Homosexuality Inherited?

The Sexual Brain

by Simon LeVay
MIT Press/a Bradford book, 168 pp., $25.00; $10.95 (paper)

The Science of Desire: The Search for the Gay Gene and the Biology of Behavior

by Dean Hamer, by Peter Copeland
Simon and Schuster, 272 pp., $23.00


Historians of homosexuality will judge much twentieth-century “science” harshly when they come to reflect on the prejudice, myth, and downright dishonesty that litter modern academic research on sexuality. Take, for example, the lugubrious statements of once respected investigators. Here is Sandor Feldman, a well-known psychotherapist, in 1956:

It is the consensus of many contemporary psychoanalytic workers that permanent homosexuals, like all perverts, are neurotics.1

Or consider the remarks of the respected criminologist Herbert Hendin:

Homosexuality, crime, and drug and alcohol abuse appear to be barometers of social stress…Criminals help produce other criminals, drug abusers other drug abusers, and homosexuals other homosexuals.2

The notion of the homosexual as a deeply disturbed deviant in need of treatment was the orthodoxy until only recently. Bernard Oliver, Jr., a psychiatrist specializing in sexual medicine, wrote in 1967 that

Dr. Edmond Bergler feels that the homosexual’s real enemy is not so much his perversion but [sic] ignorance of the possibility that he can be helped, plus his psychic masochism which leads him to shun treatment….

There is good reason to believe now, more than ever before, that many homosexuals can be successfully treated by psychotherapy, and we should encourage homosexuals to seek this help.3

Such views about the origin of homosexual preferences have become part of American political culture as well. When, in 1992, Vice-President Dan Quayle offered the view that homosexuality “is more of a choice than a biological situation…. It is a wrong choice,”4 he merely reasserted the belief that homosexuality reflected psychological conditioning with little biological basis, and certainly without being influenced by a person’s biological inheritance.

And now we have the much publicized spectacle—Time magazine has taken up the story in a dramatic feature entitled “Search for a Gay Gene”5—of homosexuality’s origins being revealed in the lowly fruit fly, Drosophila6 Males and females of this, one has to admit, rather distant relation adopt courtship behavior that has led two researchers at the US National Institutes of Health to draw extravagant parallels with human beings.

Shang-Ding Zhang and Ward F. Odenwald found that what they took to be homosexual behavior among male fruit flies—touching male partners with forelegs, licking their genitalia, and curling their bodies to allow genital contact—could be induced by techniques that abnormally activated a gene called w (for “white,” so called because of its effect on eye color). Widespread activation (or “expression”) of the white gene in Drosophila produced male-to-male rituals that took place in chains or circles of five or more flies. If female fruit flies lurked nearby, male flies would only rarely be tempted away from their male companions. These findings, which have apparently been reproduced by others, have led the investigators to conclude that “w misexpression has a profound effect on male sexual behavior.”

Zhang and Odenwald go on to speculate that the expression of w could lead to severe shortages of serotonin, an important chemical signal that enables nerve cells to communicate with one another. The authors conjecture that mass activation of w diminishes brain serotonin by promoting its use elsewhere in the body. Indeed, cats, rabbits, and rats all show some elements of “gay” behavior when their brain serotonin concentrations fall. Intriguing and, you might think, convincing evidence.

Yet, although w is found in modified form in human beings, it is a huge (and, it seems to me, a dangerous) leap to extrapolate observations from fruit flies to humans. In truth, when the recent data are interpreted literally we find that (a) the w gene induces male group sex behavior in highly ritualized linear or circular configurations, and (b) while these tend more toward homosexual than straight preferences, they are truly bisexual (as pointed out by Larry Thompson in Time). Zhang and Odenwald force their experimental results with fruit flies to fit their preconceived notions of homosexuality. How simplistic it seems to equate genital licking in Drosophila with complex individual and social homosexual behavior patterns in humans. Can notions of homosexuality apply uniformly across the biological gulf that divides human beings and insects? Such arguments by analogy seem hopelessly inadequate.

By contrast, the work of Simon LeVay, Dean Hamer, and a small group of researchers concerned to distinguish biological and genetic influences on sexual behavior has discredited much of the loose rhetoric that has been used about homosexuality. In August 1991, LeVay, a neuroscientist who now directs the Institute of Gay and Lesbian Education in southern California, published in the magazine Science findings from autopsies of men and women of known sexual preference. He found that a tiny region in the center of the brain—the interstitial nucleus of the anterior hypothalamus (INAH) 3—was, on average, substantially smaller in nineteen gay men who died from AIDS than among sixteen heterosexual men.7

The observation that the male brain could take two different forms, depending on one’s sexual preference, was a stunning discovery. The hypothalamus—a small, intricate mass of cells lying at the base of the brain—was long believed to have a role in sexual behavior, but direct evidence that it did so was weak. Yet LeVay expressed caution. Although his data showed that human sexual preference “is amenable to study at the biological level,” he noted that it was impossible to be certain whether the anatomical differences between the brains of gay and straight men were a cause or a consequence of their preference.8

In the thirteen persuasive essays that make up The Sexual Brain, LeVay takes account of the current biobehavioral controversy over the science of sex. From the union of wiry sperm and bloated ovum to the child-roaring practices of mammals and humans, for which mothers are largely responsible, he writes (metaphorically), the “male is little more than a parasite who takes advantage of [the female’s] dedication to reproduction.” He goes on to draw from a wide range of sources to support his contentious assertion that “there are separate centers within the hypothalamus for the generation of male-typical and female-typical sexual behavior and feelings.” He argues that a connection—the details of which remain mysterious—between brain and behavior exists through hormones such as testosterone.

The most convincing evidence he puts forward to support his view comes from women with congenital adrenal hyperplasia. This condition, in which masculine characteristics, such as androgenized genitalia, including clitoral enlargement and partially fused labia, become pronounced in women, is caused by excessive testosterone production and leads, in adulthood, to an increased frequency of lesbianism affecting up to half of all the women who have the condition. The theory, still unproven, that is proposed to explain these behavioral effects of hormones is that one or more chemical signals act during a brief early critical period in the development of most males to alter permanently both the brain and the pattern of their later adult behavior. Unless this hormonal influence is switched on, a female pattern of development will follow automatically.

What might be the origin of biological differences underlying male sexual preference? In 1993 Dean Hamer and his colleagues at the National Cancer Institute discovered a preliminary but nevertheless tantalizing clue.9 Hamer began his painstaking search for a genetic contribution to sexual behavior by studying the rates of homosexuality among male relatives of seventy-six known gay men. He found that the incidence of homosexual preference in these family members was strikingly higher(13.5 percent) than the rate of homosexuality among the whole sample (2 percent). When he looked at the patterns of sexual orientation among these families, he discovered more gay relatives on the maternal side. Homosexuality seemed, at least, to be passed from generation to generation through women.

Maternal inheritance could be explained if there was a gene influencing sexual orientation on the X chromosome, one of the two human sex chromosomes that bear genes determining the sex of offspring.10 Men have both X and Y chromosomes, while women have two X chromosomes. A male sex-determining gene, called SRY, is found on the Y chromosome. Indeed, the Y chromosome is the most obvious site for defining male sexuality since it is the only one of the forty-six human chromosomes to be found in men alone. The SRY gene is the most likely candidate both to turn on a gene that prevents female development and to trigger testosterone production. Since the female has no Y chromosome, she lacks this masculinizing gene. In forty pairs of homosexual brothers, Hamer and his team looked for associations between the DNA on the X chromosome and the homosexual trait. They found that thirty-three pairs of brothers shared the same five X chromosomal DNA “markers,” or genetic signatures, at a region near the end of the long arm of the X chromosome designated Xq28.11 The possibility that this observation could have occurred by chance was only 1 in 10,000.

LeVay takes a broad philosophical perspective in his discussion of human sexuality by placing his research in the context of animal evolution. Hamer, on the other hand, has written, with the assistance of the journalist Peter Copeland, a more focused popular account of his research. He conceived his project after reflecting on a decade of laborious research on yeast genes. Although the project was approved by the National Institutes of Health after navigating a labyrinthine course through government agencies, it remained rather meagerly funded.

Taken together, the scientific papers of both LeVay and Hamer and the books that their first reports have now spawned12 make a forceful but by no means definitive case for the view that biological and genetic influences have an important—perhaps even decisive—part in determining sexual preference among males. LeVay writes, for example, that “…the scientific evidence presently available points to a strong influence of nature, and only a modest influence of nurture.” But there is no broad scientific agreement on these findings. They have become mired in a quasi-scientific debate that threatens to let obscurantism triumph over inquiry. What happened?

To begin with, we must ask what LeVay and Hamer have not shown. LeVay has found no proof of any direct link between the size of INAH 3 and sexual behavior. Size differences alone prove nothing. He was also unable to exclude the possibility that AIDS has an influence on brain structure, although this seemed unlikely, since six of the heterosexual men he studied also had AIDS. Moreover, Hamer did not find a gene for homosexuality; what he discovered was data suggesting some influence of one or more genes on one particular type of sexual preference in one group of people. Seven pairs of brothers did not have the Xq28 genetic marker, yet these brothers were all gay. Xq28 is clearly not a sine qua non for homosexuality; it is neither a necessary nor a sufficient cause by itself.

And what about women? Although the genitalia of women as well as men are clearly biologically determined, no data exist to prove a genetic link, or a link based on brain structure, with female sexual preferences, whether heterosexual or homosexual. Finally, neither study has been replicated by other researchers, the necessary standard of scientific proof. Indeed, there is every reason to suppose that the INAH 3 data will be extremely difficult to confirm. Only a few years ago INAH 1 (located close to INAH 3) was also thought to be larger in men than in women. Two groups, including LeVay’s, have failed to reproduce this result.

  1. 1

    See Perversions: Psychodynamics and Therapy, edited by Sandor Lorand and Michael Balint (Ortolan Press, 1965; first edition, Random House, 1956), p. 75.

  2. 2

    Quoted in Kenneth Lewes, The Psychoanalytic Theory of Male Homosexuality (Simon and Schuster, 1988), p. 188

  3. 3

    See Bernard J. Oliver, Jr., Sexual Deviation in American Society (College and University Press, 1967), p. 146.

  4. 4

    See Karen de Witt, “Quayle Contends Homosexuality Is a Matter of Choice, Not Biology,” The New York Times, September 14, 1992, p. A17.

  5. 5

    See Larry Thompson, “Search for a Gay Gene,” Time (June 12, 1995), pp. 60-61.

  6. 6

    See Shang-Ding Zhang and Ward F. Odenwald, “Misexpression of the White (w) Gene Triggers Male-male Courtship in Drosophila,” Proceedings of the National Academy of Sciences, USA, Vol. 92 (June 6, 1995), pp. 5525-5529.

  7. 7

    See Simon LeVay, “A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men,” Science (August 30, 1991), pp. 1034-1037.

  8. 8

    The suprachiasmatic nucleus, also located in the hypothalamus, is larger in homosexual men than in either heterosexual men or women. The anterior commissure of the corpus callosum (a band of tissue that connects the right and left hemispheres of the brain) is also larger in gay men.

  9. 9

    See Dean H. Hamer et al., “A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation,” Science (July 16, 1993), pp. 321-327.

  10. 10

    The normal complement of human chromosomes is forty-six per individual, two of which are designated sex chromosomes. In the male, the sex chromosomal makeup is XY, while in the female it is XX. If a gene for homosexuality (Xh) was transmitted through the maternal line, one can see how the subsequent offspring would be affected.

    X Xh

    X XX XXh

    Y XY XhY

    Suppose the unaffected female carrier for homosexuality (XXh) produced offspring with a non-Xh male (XY). Half of all female children would be carriers of Xh (like their mothers), while half of all male offspring would carry Xh unopposed by another X. The Xh trait—homosexuality—would then be able to express itself.

  11. 11

    By chance, one would expect each pair of brothers to share half their DNA. So, assuming that there was no gene for homosexuality, one would expect twenty of the forty pairs of brothers to share the X chromosome marker.

  12. 12

    LeVay has recently completed a second book in collaboration with Elisabeth Nonas—City of Friends—that surveys gay and lesbian culture; it will be published by MIT Press in November. He is currently working on Queer Science, a study of how scientific research has affected the lives of gays and lesbians.

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