This Is Your Brain on Sex (32 page)

These pesky fruit invaders are very, very heterosexual. Right-wing “family” advocates might do well to use these flies as a banner mascot next time they want to mount some sort of protest. Kulbir Gill, a geneticist working at Yale University in the early 1960s, discovered that by mutating a single
Drosophila
gene, he could turn these male bastions of heterosexuality into bisexuals who try to get busy with both females and males equally. In
Gay, Straight and the Reason Why: The Science of Sexual Orientation,
Simon LeVay, a scientist who studies sexual orientation, writes, “When these flies were put together in all-male groups they formed long moving chains resembling conga lines, with each male attempting (unsuccessfully) to mate with the male in front of it.” It is quite a mental image. If you are a Carmen Miranda fan, it might almost allow you to believe scientists decided to nickname this gene “fruity” because of those dancing insects instead of any homophobia on their part. But not quite. Here is an example of how the stereotypes of the day can influence science. In any case,
scientists would later refer to this gene as the slightly less loathsome “fruitless” (
FRU
).
³

Although
FRU
was pronounced the first “gay” gene, it was a bit of a misnomer. Nearly thirty years later, when technology advanced to the point where geneticists could isolate and sequence
FRU,
they found that it did not determine sexual orientation per se. Rather it determined whether or not flies could discriminate between males and females. When scientists manipulated
FRU,
flies with the mutated gene tried to mount males and females indiscriminately. If you can’t tell the difference between a boy and a girl, why wouldn’t you try to mate with whichever fly happened across your path? That’s pretty much what was happening in these mutated fruit flies. One would be hard-pressed to consider this finding a true analogue to homosexual behavior.

Since Gill identified
FRU,
several other “gay” genes have been identified in fruit flies and even in higher-order species, such as mice. Unsurprisingly, many of those genes are implicated in the processing of dopamine and glutamate, key neurotransmitters involved in love and sex behaviors. The discovery of those genes gave credence to the idea that scientists were on the right track—that a homosexual gene was out there, just waiting to be found. Over the next few decades, however, the background changed. With homosexuality removed from the list of disorders in the
DSM,
the scientific focus was no longer on a cure or treatment but on how sexual orientation may develop.

Recent work pinpointed a new possibility, a gene named “genderblind” or
GB,
that expresses a protein that helps transport glutamate from neuron to neuron. In 2007 David Featherstone, a biologist at the University of Illinois at Chicago, mutated
GB
in fruit flies. Like
FRU,
this
GB
mutation changed the behavior of our stalwart hetero flies so that they tried to mate with both boy and girl flies indiscriminately. Again, the flies simply could not process the right sensory information to tell the difference between the two sexes. Featherstone and his colleagues hypothesized that changes to this gene might lead to changes in synaptic strength, and consequently produce an inability to process sexual stimuli.

Sure enough, when Featherstone used drugs or other genetic pathways to strengthen synapses, the flies were once again able to interpret the sensory stimuli and return to heterosexual
mating behaviors. Many everyday Joes read about this study, along with the media brouhaha surrounding it, and deduced that sexual orientation was not biologically hardwired. Rather, it could be altered with drugs or gene therapies, if only scientists found the corresponding gene in humans. Whether or not that was Featherstone’s intention wasn’t the point. This study is still used by some as evidence that homosexuality is an alterable phenomenon.
⁴

Derfner, who doesn’t disagree when his friends describe him as “really, really gay,” believes genes likely have something to say about how sexual orientation develops, just as they might for any other natural variation seen in behavior. But just because something might be alterable does not make it a choice. “I’m surprised that more people weren’t up in arms about [Featherstone’s] study,” he told me. “Even if science develops to the point where we can change sexual orientation, and perhaps that is just a matter of time, I don’t know, it doesn’t address the question of whether we should.”

When I asked him if, at any point in his life, he would have considered a change in sexual orientation had it been available, his answer was an immediate and emphatic “No.”

One of the most recent “gay” gene discoveries occurred during a study of the regulation of sugar uptake, an enzyme called
fucose mutarotase
. Chankyu Park and his team of researchers at the Korea Advanced Institute of Science and Technology originally called this gene
FucU,
until a kind journal editor suggested
FucM
might be more appropriate. When the group knocked out
FucM
(or, as I like to call it, the gene formerly known as
FucU
) in mice, they found that it changed the sexual behaviors of females. The best part of this story is that the discovery was a complete surprise to the researchers. There was no original hypothesis that mating behaviors would be altered with the removal of the gene.
⁵
“It was an unexpected result, but it’s very difficult to expect what the phenotype of a gene will be, even when you think you know its function,” Park said to me. “We speculated that knocking it out might result in immune system issues, but even that was a guess.”

Instead Park and his colleagues discovered peculiarities in mating behaviors in female knockouts. The male mice acted normally, getting busy whenever they were able. But the females,
housed in cages with both male and other female animals, avoided the males at all costs. They did not assume a lordotic position during estrus and did not respond to or even sniff the male’s urine. What’s more, these knockout females tried to mount the other girls in a very male-like fashion.

“It’s the first, to my knowledge, demonstration of a homosexual gene in females,” Park said. He hypothesizes that
FucM
is influencing the proteins involved in sexually dimorphic brain development, with the result that those female knockouts have brain areas involved in sexual behavior that look more like the boys’ sexually activated brain areas, particularly the preoptic area.

So scientists have found so-called gay genes, of several types, in fruit flies and mice. In some cases they have been able to reverse homosexual behaviors with drugs or the mutation of other related genes. A genetic correlate in the human genome, however, has not been found, and we cannot ignore the fact that the so-called homosexual behavior observed in these flies and mice is not strictly analogous to homosexual behavior in human beings.

It is a critical distinction. Gay human beings generally show a strict preference for one sex, their own, over the other. They can discriminate between the sexes; they just prefer their own. This occurs in both genders. Park’s model may better mimic the behavior of homosexual women than previous genetic models, but the alterations had no effect on the males. Despite the hope that genes would shed some light on the differences between heterosexuals and homosexuals, there is no clear answer in the current research. The only conclusion that can be drawn from the current body of work is that there are some, as yet unknown underlying genetic correlates to homosexuality. And that may vary between males and females.

Looking to the Animal Kingdom

It might be easier to pinpoint which genes are involved in homosexual behavior (as opposed to finding them accidentally) if we could observe analogous behavior in other species. With so many overlapping behaviors between humans and our closest evolutionary relatives, primates, you would think that
there’d be at least one monkey species that shows homosexual behavior. Although same-sex sexual behavior is observed widely in primate species—monkeys are happy to sexually cavort with members of the same sex—these animals never show a preferential choice for their own sex. And that preference for one sex over the other is what defines sexual orientation: gay men are attracted to other men, lesbian women are attracted to other women. Primates simply don’t behave that way.
⁶
“This shows a difficulty in extrapolating animal models to humans,” Kim Wallen said to me. “The requirement is that they consistently share features. When you look at same-sex behavior, the behavior you see in monkeys doesn’t share a key hallmark with that you see in humans. The surface similarity doesn’t fit with the substance of the phenomenon you are trying to study.”

There is one exception in the animal kingdom: sheep. As it so happens, approximately 8 percent of rams, or male sheep, are same-sex-oriented. They still exhibit male-like mounting behaviors; they just happen to work that magic on other rams. That preference does not change even when these rams are castrated (though castration does slow their roll a little by reducing overall mounting attempts).
⁷
There don’t appear to be lesbian sheep, however, so it is hard to draw exact parallels. A gay sheep gene has not been discovered, either—no analogue of the
FRU, FucM,
or
GB
gene has been demonstrated in this species.

Given the uniqueness of homosexual behavior in humans, many neuroscientists have opted to forgo animal models altogether and go straight to the source: humans. Alas, the work that has been done in human models has not yielded the desired results. “There is evidence that the X chromosome plays a role in sexual orientation,” said Dick Swaab, a Dutch neuroscientist who has been leading the study of sexual orientation for decades. Genome-wide association studies of a genetic marker on the X chromosome called XQ28, by a lab at the National Institutes of Health led by Dean Hamer, suggest there may be a link to male homosexuality, but the findings have yet to be replicated.
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Despite this possible genetic marker, Swaab offered a word of caution: he doubts that there is a single gene at work in these behaviors and proposes that any genes associated with sexual orientation are most likely related to brain development processes occurring while an individual is still in the womb. Remember those his and her brains from
chapter 6? Perhaps the development of those sexually dimorphic areas, while the fetus is still in utero, is altered or disrupted in some way, leading not only to different probabilities of disorders like schizophrenia and anorexia nervosa, but also to different sexual orientations.

Beyond the Bedroom

We have all seen the homosexual stereotypes. Gay men are too often portrayed as skinny, effeminate creatures who are into fashion and little dogs, while lesbian women get the thick and butch treatment. If you have gay friends or family members, you know these are oversimplified caricatures. Just as with heterosexuals, there is great variability in the gay community. Sure, you have your queens and your butches, but you also have folks you might not even know played for the other team if they hadn’t told you. Despite this immense diversity, epidemiological and neuropsychological studies have pinpointed some interesting differences between heterosexual and homosexual populations.

On the cognitive front, homosexual men consistently demonstrate more difficulty with mental rotation and spatial perception tasks than heterosexual men; they are about on par with heterosexual women. They make up for this lack with better spatial location memory. Also like heterosexual women, they have good recall of spatial landmarks during navigation. They tend to do better on several language indices too. Rahman believes these cognitive differences suggest variation in the brain. “Mental rotation, for example, we know is dependent on the parietal lobe. Gay men perform on these tasks just like heterosexual women do,” Rahman asserted. “What’s interesting is that lesbian women don’t differ on these tasks. That suggests the parietal areas may be organized differently.”

Rahman argues that sexual orientation is not just about whom you are attracted to; it is a package deal, with a variety of other traits involved. These include differences in problem solving, spatial navigation, language, and social cognition. By testing those differences we may better understand the brain areas involved, as well as how these systems have evolved to grow together and complement each other.

This makes me think about Alexander
Ophir and Steve Phelps’s resident and wanderer prairie voles—their finding that the number of vasopressin receptors on certain spatial areas of the brain are more predictive of monogamy than the number of receptors on the reward circuitry. Reproductive behaviors may not involve just sex and bonding; spatial processing may somehow help the selection of successful reproductive strategies over time. That idea likely applies to homosexual behaviors too. It is certainly something to consider.

Neuroanatomical Differences

With neuroimaging techniques now more accessible to research labs, scientists can do more than just assume there are differences in the brain; they can test the hypothesis directly. The basic assumption in neuroscience is that all behaviors have an underlying neural correlate. If gene-based hypotheses about changes in normal, sexually dimorphic brain development hold any water, then researchers should see distinct differences between heterosexual and homosexual brains—and, as it turns out, they do.

Several structural and functional brain differences have been found between heterosexual and homosexual men, many of them in Swaab’s lab. In 1992 Roger Gorski and Laura Allen at the University of California, Los Angeles, found that homosexual men have a larger anterior commissure, an area of the brain thought to be involved in language ability, than their heterosexual peers. Remarkably, the size of this area was more similar to that seen in heterosexual female brains. Ivanka Savic, pheromone researcher extraordinaire, noted sex-atypical asymmetries in homosexual brains, meaning that the homosexual brain looks a lot more like that of a heterosexual member of the opposite sex than like the brain of someone of the same gender. For example, straight men have asymmetric brains with larger right hemispheres; so do lesbians. The amygdala, the area responsible for emotional salience, shows similar connections in gay men and straight women, yet a different pattern of connectivity is shared by straight men and gay women. Other sexually dimorphic areas, such as the anterior hypothalamus, implicated in sexual behavior, and the suprachiasmatic nucleus, or biological clock, also
show sex-atypical differences in homosexual individuals. Studies of sexual arousal suggest that separate brain circuits are activated in homosexual and heterosexual participants.
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There is ample evidence that heterosexual and homosexual brains simply develop differently, resulting in dissimilar circuitry, cognitive skills, and behavior.
¹⁰
“We don’t know if these differences are there because the connections in the brain are different or there is some effect of sexual practice or experience on brain organization,” Savic told me. “But whatever it is, it’s there and it’s different.”