|Researcher Alan Sanders signs up Daniel Velez Rivera on Boston Common for a study using gay brothers to search for the genetic basis for homosexuality. (Illustration / Chris Buzelli; Globe Staff Photo / David Kamerman)
In May, Swedish researchers reported finding important differences in how the brains of straight men and gay men responded to two compounds suspected of being pheromones - those scent-related chemicals that are key to sexual arousal in animals. The first compound came from women's urine, the second from male sweat. Brain scans showed that when straight men smelled the female urine compound, their hypothalamus lit up. That didn't happen with gay men. Instead, their hypothalamus lit up when they smelled the male-sweat compound, which was the same way straight women had responded. This research once again connecting the hypothalamus to sexual orientation comes on the heels of work with sheep. About 8 percent of domestic rams are exclusively interested in sex with other rams. Researchers found that a clump of neurons similar to the one LeVay identified in human brains was also smaller in gay rams than straight ones. (Again, it's conceivable that these differences could be showing effect rather than cause.)
In June, scientists in Vienna announced that they had isolated a master genetic switch for sexual orientation in the fruit fly. Once they flicked the switch, the genetically altered female flies rebuffed overtures from males and instead attempted to mate with other females, adopting the elaborate courting dance and mating songs that males use.
And now, a large-scale, five-year genetic study of gay brothers is underway in North America. The study received $2.5 million from the National Institutes of Health, which is unusual. Government funders tend to steer clear of sexual orientation research, aware that even small grants are apt to be met with outrage from conservative congressmen looking to make the most of their C-Span face time. Relying on a robust sample of 1,000 gay-brother pairs and the latest advancements in genetic screening, this study promises to bring some clarity to the murky area of what role genes may play in homosexuality.
This accumulating biological evidence, combined with the prospect of more on the horizon, is having an effect. Last month, the Rev. Rob Schenck, a prominent Washington, D.C., evangelical leader, told a large gathering of young evangelicals that he believes homosexuality is not a choice but rather a predisposition, something "deeply rooted" in people. Schenck told me that his conversion came about after he'd spoken extensively with genetic researchers and psychologists. He argues that evangelicals should continue to oppose homosexual behavior, but that "many evangelicals are living in a sort of state of denial about the advance of this conversation." His message: "If it's inevitable that this scientific evidence is coming, we have to be prepared with a loving response. If we don't have one, we won't have any credibility."
AS THE 21-YEAR-OLD COLLEGE JUNIOR IN A HOSPITAL JOHNNY slides into the MRI, she is handed controls with buttons for "strongly like" and "strongly dislike." Hundreds of pornographic images - in male-male and female-female pairings - flash before her eyes. Eroticism eventually gives way to monotony, and it's hard to avoid looking for details to distinguish one image from the rest of the panting pack. So it goes from "Look at the size of those breasts!" to "That can't be comfortable, given the length of her fingernails!" to "Why is that guy wearing nothing but work boots on the beach?"
Regardless of which buttons the student presses, the MRI scans show her arousal level to each image, at its starting point in the brain.
Researchers at Northwestern University, outside Chicago, are doing this work as a follow-up to their studies of arousal using genital measurement tools. They found that while straight men were aroused by film clips of two women having sex, and gay men were aroused by clips of two men having sex, most of the men who identified themselves as bisexual showed gay arousal patterns. More surprising was just how different the story with women turned out to be. Most women, whether they identified as straight, lesbian, or bisexual, were significantly aroused by straight, gay, and lesbian sex. "I'm not suggesting that most women are bisexual," says Michael Bailey, the psychology professor whose lab conducted the studies. "I'm suggesting that whatever a woman's sexual arousal pattern is, it has little to do with her sexual orientation." That's fundamentally different from men. "In men, arousal is orientation. It's as simple as that. That's how gay men learn they are gay."
These studies mark a return to basics for the 47-year-old Bailey. He says researchers need a far deeper understanding of what sexual orientation is before they can determine where it comes from.
Female sexual orientation is particularly foggy, he says, because there's been so little research done. As for male sexual orientation, he argues that there's now enough evidence to suggest it is "entirely in-born," though not nearly enough to establish how that happens.
Bailey's 1991 twin study is still cited by other researchers as one of the pillars in the genetic argument for homosexuality. But his follow-up study using a comprehensive registry of twins in Australia found a much lower rate of similarity in sexual orientation between identical twins, about 20 percent, down from 50 percent. Bailey still believes that genes make important contributions to sexual orientation. But, he says, "that's not where I'd bet the real breakthroughs will come."
His hunch is that further study of childhood gender nonconformity will pay big. Because it's unclear what percentage of homosexuals and lesbians showed CGN as children, Bailey and his colleagues are now running a study that uses adult participants' home movies from childhood to look for signs of gender-bending behavior.
Cornell psychologist Daryl Bem has proposed an intriguing theory for how CGN might lead to homosexuality. According to this pathway, which he calls "the exotic becomes erotic," children are born with traits for temperament, such as aggression and activity level, that predispose them to male-typical or female-typical activities. They seek out playmates with the same interests. So a boy whose traits lead him to hopscotch and away from rough play will feel different from, and ostracized by, other boys. This leads to physiological arousal of fear and anger in their presence, arousal that eventually is transformed from exotic to erotic.
Critics of homosexuality have used Bem's theory, which stresses environment over biology, to argue that sexual orientation is not inborn and not fixed. But Bem says this pathway is triggered by biological traits, and he doesn't really see how the outcome of homosexuality can be changed.
Bailey says whether or not Bem's theory holds up, the environment most worth focusing in on is the one a child experiences when he's in his mother's womb.
LET'S GET BACK TO THOMAS AND PATRICK. BECAUSE IT'S UNCLEAR why twin brothers with identical genetic starting points and similar post-birth environments would take such divergent paths, it's helpful to return to the beginning.
Males and females have a fundamental genetic difference - females have two X chromosomes, and males have an X and a Y. Still, right after conception, it's hard to tell male and female zygotes apart, except for that tucked-away chromosomal difference. Normally, the changes take shape at a key point of fetal development, when the male brain is masculinized by sex hormones. The female brain is the default. The brain will stay on the female path as long as it is protected from exposure to hormones. The hormonal theory of homosexuality holds that, just as exposure to circulating sex hormones determines whether a fetus will be male or female, such exposure must also influence sexual orientation.
The cases of children born with disorders of "sexual differentiation" offer insight. William Reiner, a psychiatrist and urologist with the University of Oklahoma, has evaluated more than a hundred of these cases. For decades, the standard medical response to boys born with severely inadequate penises (or none at all) was to castrate the boy and have his parents raise him as a girl. But Reiner has found that nurture - even when it involves surgery soon after birth - cannot trump nature. Of the boys with inadequate penises who were raised as girls, he says, "I haven't found one who is sexually attracted to males." The majority of them have transitioned back to being males and report being attracted to females.
During fetal development, sexual identity is set before the sexual organs are formed, Reiner says. Perhaps it's the same for sexual orientation. In his research, of all the babies with X and Y chromosomes who were raised as girls, the only ones he has found who report having female identities and being attracted to males are those who did not have "receptors" to let the male sex hormones do their masculinizing in the womb.
What does this all mean? "Exposure to male hormones in utero dramatically raises the chances of being sexually attracted to females," Reiner says. "We can infer that the absence of male hormone exposure may have something to do with attraction to males."
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