This Is Your Brain on Sex (33 page)

Swaab hypothesizes that it all comes down to testosterone. Between six and twelve weeks along in a pregnancy, the Y chromosome, if present, starts a testosterone storm in the womb. The appearance of this extra testosterone results in the development of the penis and accompanying boy goodies. In its absence the female genitals will develop. Testosterone is not just for gonads. Along with estrogen and progesterone, it also has developmental and organizing effects in the brain. Sometime after the whole penis/vagina question is decided, these hormones, along with a cornucopia of other proteins and chemicals, influence the brain’s development, resulting in sexually dimorphic areas—you know, those his and her brains. Puberty’s accompanying rush of hormones seals the deal, activating those sex-specific brain circuits originally organized during this secondary process in the womb.

A glitch in that process (or perhaps even more than one) leads to differences in sexual orientation as well as sexual identity. Transsexuality, or transgenderedness (the term preferred by some in the lesbian, gay, bisexual, and transgendered community), is a condition in which a person with the outdoor plumbing of one sex strongly identifies with the opposite sex. These individuals often say that they were born in the wrong body, that their outsides do not reflect their true gender. It is an extreme condition—and one that can cause immense suffering for both the individual and his or her family. Like homosexuals, transgendered individuals also show sex-atypical brain differences, specifically in the hypothalamus and the bed nucleus of the stria terminalis, both linked to sexual behavior in rodents. In these areas transgendered individuals show patterns in volume and number of brain cells more like the gender they identify with than whatever gender they inhabit. This particular condition is still considered
a disorder in the
DSM,
though many in the LGBT community believe it should not be. Swaab has maintained that although transsexuality should not carry a stigma, it is still a medical condition. Changing the body to match one’s internal gender involves invasive sexual reassignment surgery, hormone therapy, and a heck of a lot of counseling. “You can live without problem as a homosexual—there is no reason to consider it a disorder,” Swaab told me. “Transsexuality is different. It requires one to adapt to a body that can’t adapt to the brain. That’s a big problem.”

What might account for brain development that results in homosexuality rather than transsexuality? The answer is unknown. These brain differences are not apparent until adulthood, so there is no way to detect them early in life. Scientists have suggested that mutations to genes that are responsible for a type of estrogen receptor, an androgen receptor, and aromatase, an enzyme that converts testosterone to estrogen, may all lead to abnormal hormone levels in utero, and, as such, are potential factors underlying transsexuality. However, any causal relationships between any of these and transsexuality cannot be determined. Something is happening in the brain’s development; the making of that precise hormonal and chemical cocktail that results in matching gonads and brains is somehow off. What may be behind it all has yet to be determined.

Even leaving transsexuality out of the equation, there do not seem to be any easy answers to the development of homosexuality. It is unlikely that the glitch is the same in homosexual men and women. While studies suggest that excessive testosterone in the womb may lead to homosexuality in women, it is probably not working in isolation. Moreover, the situation in men is not quite as straightforward. Some studies suggest a lack of testosterone; others suggest that too much may lead to sexual orientation differences in men. Other studies indicate that testosterone isn’t involved at all. Ray Blanchard, a professor of psychiatry at the University of Toronto, has proposed a different mechanism for homosexuality in men, one that involves an immune response in the womb.
¹¹

Want to know a fun fact about homosexual men? The estimated odds of being gay go up over 30 percent with each older brother you have. It’s called the fraternal birth order (FBO) effect. Gay men generally have more older brothers than their straight
contemporaries. It is a very robust finding and one limited to males. Having older sisters doesn’t change your odds of being gay, nor is there any type of sibling effect seen in gay women. Here’s another interesting tidbit: homosexual men with older brothers tend to have lower birth weight than heterosexuals. The combination of these findings led Blanchard to suggest that, over multiple male births, the maternal immune system starts producing antibodies against the fetus’s antigens. This progressive immunization may result in variations in brain development and thus sexual orientation.
¹²
There is quite a bit of evidence to support this theory, but once again, it is difficult to prove either way. And since the FBO effect can account for only an estimated one in seven gay men, there may be a handful of different ways brain organization leading to homosexuality may be altered in the womb.

Smell You Later

What is it that attracts you to someone? I have met few people who are attracted only to a penis or a vagina, regardless of sexual orientation. They get there eventually, sure, but the attraction starts elsewhere. But where?

“The big question we still haven’t answered is what sexual orientation is actually toward, what is it orienting to,” Rahman said. “The intuitive response is gender or sex. But that could mean any number of things. It could be the shape of a body, a type of face, movement, something in the voice, or a combination of all these things. What’s more, we don’t know to what extent individual differences play a role in this orienting response.”

There is a high probability that heterosexuals and homosexuals are simply processing sensory information—sights, smells, sounds, and tastes—a little bit differently. Take pheromones, for example. As I discussed in an earlier chapter, these bodily compounds are controversial, but they may tell us something about the different ways the heterosexual and homosexual brains process chemical messengers. Savic and her colleagues looked at brain activation using positron emission tomography in homosexual men, heterosexual men, and heterosexual women who had been exposed to androstadienone
(AND), a compound found primarily in male sweat, an estrogen-like steroid (EST) that is found in female urine, and more common odors like lavender and cedar. Savic knew from previous studies that AND and EST activate the anterior hypothalamus in a gender-specific manner. What she did not know was if there would be differential effects based on sexual orientation.

It may not be much of a surprise, but that is exactly what she found. Homosexual men and heterosexual women showed hypothalamic activation in response to the AND, with maximum effect in the medial preoptic area and anterior hypothalamus. There was no significant difference in brain activation when the groups smelled the common odors.
¹³

When Savic attempted the same study in lesbian women, she found the expected corollary. Lesbians and heterosexual men showed activation in the anterior hypothalamus in response to the EST; heterosexual women did not. Savic argues that this pair of studies provides strong evidence that sexual orientation shows differential processing of sex-related stimuli.
¹⁴

What about transgendered individuals? Do they also show differences in their processing of hormones? Savic thought testing AND and EST in nonhomosexual male-to-female transsexuals was the next logical step. In case you are confused, that’s a person in a male body who identifies as a female and has only ever had sex with women. The jargon can get tricky; intuitively you might think a male-to-female transsexual would have sex only with men. I did at first. But in this study Savic and her colleagues chose to define sexual orientation based on birth genitalia rather than any other factor.

When compared to heterosexual female controls, this group of male-to-female transsexuals showed very similar brain activation, but with an interesting twist. That is, the AND activated the hypothalamus in a similar cluster of neurons seen in women, while there was a small effect of EST activation in a different area of the hypothalamus similar to that seen in men. The latter effect was limited, but present. The data suggest, Savic argues, sex-atypical brain circuitry in the hypothalamus in these transgendered males.
¹⁵

The same caveats apply in these studies as in any other pheromone study. Savic used a high
level of pheromones, hundreds of times greater than you would find in actual sweat. Yet despite this, the differential patterns of activation observed in these different groups suggest there are important differences in the way the brain processes chemical messengers based on both sexual orientation and sexual identity.

Love Is Love

Much of the neuroimaging work concerning sexual orientation has focused on the ways the brains of homosexuals differ from those of heterosexuals in response to olfactory and visual stimuli. This is a good place to start because different things turn them on. But that’s not to say there aren’t some key similarities between the two groups. What happens if we focus just on feelings of love and ignore who happens to put what where during sex (which, frankly, probably differs as much within these groups as between them)? Remember Semir Zeki, the professor of neuroaesthetics at University College London who was one of the first researchers to try to map the neural correlates of love? He noted that hundreds of years of poetry and narratives describe love in similar ways. It did not escape his attention that those depictions remained similar regardless of whether the author was discussing love of the same sex or the opposite sex. Thus, he hypothesized, love should look the same across all brains, regardless of sexual orientation.

Zeki and his colleague John Paul Romaya scanned twenty-four people, twelve men and twelve women. All of the study participants admitted to being passionately in love, as well as in committed sexual relationships; half of both sexes just happened to be committed to members of the same sex. As in Zeki’s first study, he and Romaya measured cerebral blood flow as individuals viewed photos of their partner and a familiar acquaintance of the same sex and approximate age.

Once again Zeki and Romaya found activation of the hypothalamus, ventral tegmental area, caudate nucleus, putamen, insula, hippocampus, and anterior cingulate cortex. And again there was extensive deactivation across the cortex. These results replicated Zeki’s original study, as well as the idea that love is both rewarding and blind. The two could find no differences between the heterosexual and homosexual participants. As far as the brain is
concerned, passionate love is passionate love, no matter what gender you or your intended happen to be.
¹⁶

What about Learned Behaviors?

With such a strong focus on epigenetics, I’m sure you are thinking there must be some kind of nurture component involved in the development of sexual orientation. It seems a logical guess. Perhaps certain learned behaviors or sexual experiences somehow further shape that neural circuitry laid down in the womb. After all, how often have you heard the old story that domineering mothers influence sexual orientation? But Swaab laughed at this notion. “When I give lectures at the medical school, I always ask the students who did
not
have a dominant mother to raise their hand. No one raises a finger,” he said, continuing to chuckle. “I think having a dominant mother is far too prevalent to influence sexual orientation.”

Just as there is no neurobiological evidence to suggest there is a choice when it comes to sexual orientation or sexual identity, there is nothing that indicates experience might have the ability to change it. “People always wanted to talk about choice. It’s nonsense,” Swaab claimed. “The truth is, the choice was made for you in the womb.”

Based on his own personal experience, that of friends, and his research for his book, Derfner agrees that there was never a choice. He believes research into the genes involved in brain development sounds promising. “It makes sense to me, it feels true,” he told me. “Obviously, though, I feel like it’s probably all the factors people are talking about plus twenty-three more that haven’t been discovered yet.”

There is no single answer on sexual orientation or sexual identity, no clear-cut process in the womb that results in this behavioral variability. It’s possible, if not probable, that a variety of different processes result in these behavioral phenotypes. No amount of domineering parents, show tunes, buzz cuts, or Harley Davidsons is likely to have the power to change them.

“It’s not like every gay man wakes up one day and says, ‘Gee, that Barbra Streisand is fantastic!’ or ‘I need to go see a musical and learn to knit,’” Derfner affirmed. “There is a lot of variation. And if we can leave politics aside, though
I wonder if it’s possible to do such a thing in America today, perhaps these variations can one day be explained biologically.”

What’s the Point?

At the neuroscience conference in San Diego, Steve Wiltgen told me he was asked one question, time and time again, while presenting his poster on the history of neurobiological research on homosexuality: “Should we be doing this kind of research?” He admitted he was hard-pressed to form a reply. “The truth is, I really don’t know how to answer the question. When I started my own research, I wondered why this wasn’t a more active line of study. There’s so much to learn. Then I thought about the focus on cures and wondered what this information would tell us. If we did find a gay gene, what would society want to do with it? Since I can’t answer that, I go back and forth about whether we should continue.”

Like Wiltgen, Derfner is somewhat ambivalent yet hopes more knowledge may bring greater understanding. “The Ivory Tower geek in me says, ‘Absolutely, study this.’ But we’re so far from knowing what’s really involved. If we get closer in, say, one hundred or two hundred years, my answer might change. I might say no.”