The Science of Sex: Gay and Straight Twins

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How Oprah screwed me

The science of sex: sounds like an oxymoron, doesn’t it? There’s no accounting for taste, after all, and sex is all taste — and smell and feel and sight and sound. How can one expect to reconcile the magical mystery of sex with the icy precision of the test tube and the microscope? It’s not easy, to be sure, but in this column I’ll take up the challenge. If I can’t make sex scientific, maybe at least I can make science sexy.


A few years ago I was invited to appear on the Oprah Winfrey show. The topic was: How do people come to be gay or straight? I had previously published a research study in which I described differences in brain structure between gay and straight men, so my role on the show was to present the “biological” argument: the idea that a person’s sexual orientation results from genes, prenatal brain development and the like.


Once on the show, however, I found that Oprah had stacked the deck against me. The other guests were five sets of identical twins, all of whom were “discordant” for sexual orientation: one twin in each pair was gay or lesbian and the other straight. That made it seem highly unlikely that genes or prenatal biology could have anything to do with a person’s sexual orientation. After all, identical twins have the same genes and develop at the same time in the same uterus. Surely differing life experiences — of which these folks had plenty — must have been why they came to be so different in their sexuality.


I made the best of a bad situation. I told the audience that, based on the results of several studies, the twins on the show were not representative: if one of a pair of identical twins is gay, there’s a very good chance (up to about 50% in male twin pairs) that the other twin will be gay too. That’s not true for fraternal twins, however, who don’t share the same genes; they are no more likely to have the same sexual orientation than regular brothers or sisters. This difference between identical and fraternal twins is one of the key pieces of evidence that genes play a significant role in setting up a person’s sexual orientation, at least in men. (The role of genes in women is still uncertain.)


The fact that there are any pairs of identical twins with opposite sexual orientation is a challenge to biological theories, certainly. But it’s also a challenge to environmental theories. After all, would parents or family or friends really treat two identical twin children so differently as to drive one child toward homosexuality, the other toward heterosexuality? In fact, the life experiences of identical twins don’t usually diverge significantly till adolescence or later. Yet several of the gay or lesbian twins on the show said that they felt different from early childhood, as if they had been “born that way.”


Recently anthropologist Lynn Hall, as part of her graduate research at Temple University, made a discovery that helps to resolve this paradox. Hall’s work built on an earlier Canadian study that reported on differences between the fingerprints of gay and straight men. The Canadians had counted the ridges on the fingers of the left and right hands of their subjects, and had found a tendency for the gay men to be “left-shifted” in comparison to straight men — in other words, to have more ridges on the fingers of the left hand. Since the left-right balance of finger-ridge counts is influenced by testosterone levels during fetal life and fingerprints never change after the second trimester of pregnancy, the Canadian study suggested that prenatal hormone levels had influenced the men’s sexual orientation as well as their fingerprint patterns.


“Identical” twins share similar but not identical fingerprint patterns. Hall therefore wondered whether the fingerprints of gay and straight identical twins would differ in a consistent way. They did. When she counted the fingerprint ridges in nine sets of identical male twins who were discordant for sexual orientation, she found that the gay twin in every pair was left-shifted with respect to his straight twin brother. Furthermore, Hall also found a fingerprint difference in females: the lesbian twins had fewer total ridges on the fingers of both hands than did their straight twin sisters.


Hall’s findings remain to be replicated: two further studies are currently in the works. But if they are confirmed (as I expect they will be), they will take a lot of the focus away from postnatal life events as influencing sexual orientation, especially in twins, and suggest that it’s already decided which twin will be gay and which straight three months before they are born. Still, pinning down the exact mechanisms that cause the brains of the two genetically identical fetuses to develop differently will require much further study.


In spite of the science, one shouldn’t discount a more mundane explanation for the differing sexual orientation of some identical twins, as I learned during lunch after the Oprah show. One of the gay twins leaned over and whispered in my ear: “You know, my brother is gay,” he told me. “He just can’t deal with it.”


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© 2000 Simon LeVay and, Inc.