June 2, 2004
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Scientists are debating a new study about biological influences on sexual orientation in sheep and what it might mean to humans, as this ScienCentral News video reports.

Innate vs. Choice?

Political debates about same-sex unions are likely to be with us for quite some time, but what does science have to say about sexual orientation? In recent years, some scientists have looked to other species to try to understand whether there may be a biological basis for same-sex preference. The latest such study links same-sex preference to differences in brain structure in male sheep, and its authors and others say it confirms a controversial human brain study published back in 1991.

The new sheep study, published in the February 2004 issue of the journal Endocrinology, resulted from ongoing USDA research seeking to improve breeding productivity.

Sheep breeders have long known that a minority—between six and ten percent—of rams prefer to attempt to mate with other rams instead of ewes. "Research was done on these animals for the last 20 years in terms of their reproductive behaviors and their endocrinology," says Charles Roselli, professor of physiology and pharmacology at Oregon Health and Science University. "There was never a consensus whether the behavior was caused by any aspect of rearing or anything like that, but there were reports that their brains might be different."

Roselli's lab collaborated with Fred Stormshak at Oregon State University and the USDA Sheep Experiment Station in Idaho to study rams that had an exclusive same-sex preference.

They compared the brains of these male-oriented rams to both female-oriented rams and to ewes, and examined a small cluster of brains cells in the hypothalamus that has been shown in many species—including humans—to be about two times larger on average in males than in females. The cluster of cells, or neurons, is sometimes called the "sexually dimorphic nucleus."

"We found that the rams that preferred females had the same cluster of neurons and it was twice as large as the rams that preferred males," Roselli says. "The rams that preferred males, their cluster of cells was equivalent to that of females."

That parallels the findings of a human brain study reported in the journal Science www.sciencemag.org back in 19-91 by Simon Levay.

"The study that we've done is an important cross-species comparison and confirms these results in humans," says Roselli. And he says studying sheep avoids some of the problems of human studies.

"The problems with the human studies in the past are that these brains were obtained post-mortem and so there's no good record of what the human sexual preferences were," he says. "In contrast by working with these sheep, we've characterized them behaviorally, we've worked with them since they were young animals and know just about their entire sexual experience. So we can be more sure about the correlations that we make."

Roselli says the study suggests to him that at least some homosexuality is biologically caused. "This is a very primitive area of the brain and it controls basic autonomic, automatic functions such as the control of feeding, or blood pressure or temperature. It also controls some very complex emotions and behaviors like sexual behavior or sexual desire. It's not an area of the brain that we are conscious of, its perceptions are not perceptions that we are aware of."

What Do Other Experts Think about the Ram Findings?

Neuroscientist Marc Breedlove authored a commentary in the same issue of Endocrinology. "If you can find it in two different species—that there's a part of the brain that's different in males depending on whether they're sexually attracted to males or females—you really have to start thinking that there's some important causal connection between these two things, between this part of the brain and this sexual behavior."

Breedlove says he was an early skeptic of the LeVay study, but his own lab's research over the years has strengthened the idea that prenatal hormones play a key role in shaping our sexuality. Breedlove and others have discovered physical signs of androgen exposure, such as finger-length ratios, that differ on average between males and females, and found that such differences also correlate with sexual orientation.

"What's changed in the last ten years or so, I think, is there's been this preponderance of evidence accumulating that hormones are playing a role in human sexual orientation," he says.

"In the case of the sheep, Roselli and his colleagues in fact looked specifically at how hormones are handled in the part of the brain that's different in 'gay' versus 'straight' rams, and saw that there were differences in the enzymes that are important for making hormones work in that part of the brain," Breedlove explains. "And so that strengthens, to my mind, the idea that hormones are involved in this difference in the brains of these rams; and that it's hormones that are causing the differences in the brain; and it's the differences in the brain that are causing the differences in sexual orientation.

"Once you start weighing all the data, all of which suggests that testosterone exposure early in life has an influence on sexual orientation later in life, you really have to start asking yourself, 'To what extent can you say this person has chosen the hormones they were exposed to before they were born?'" adds Breedlove.

Political Agenda?

But psychiatrist, author and lecturer Jeffrey Satinover says Roselli's and Breedlove's conclusions are incorrect, and harmful.

"It's harmful in two ways," says Satinover. "It lends weight to the false dichotomy 'innate versus choice', because that is not an accurate dichotomy to begin with. And second, it lends weight to the idea that homosexuality is innate, which is incorrect."

Satinover, who serves on the advisory board of the National Association for Research and Therapy of Homosexuality wrote a book about reversing homosexuality, taking issue with the American Psychiatric Association's position that it is not an illness and therefore ought not be treated as one.

He points out that studies of brain differences reveal nothing about causation. He asks, "Do the changes in the brain come first and then the behavior follows, or does the behavior happen first, leading to changes in the brain?"

Satinover wonders if some scientists have a political agenda. "People are interested in the biology of sexual orientation for a very clear reason, and as a matter of fact scientific studies have been done specifically to prove that. If a person is told...that sexual orientation is a matter of biology as opposed to choice," he says, "they instantly develop a more positive attitude towards it. And therefore unscrupulous scientists have been running around leaving the impression that science has demonstrated that it's quote 'biological,' because that has a direct political impact—whether it's true or not."

He argues that just about every behavior that can be studied is found to have both a biological and an environmental component, so the search for biological causes is meaningless. "The idea it's either innate or it's a choice is ludicrous, it's a straw man," he says. "And if you can show that it's not innate, that does not demonstrate that it's a choice, or if you can show that it's not a choice, that does not prove that it's innate."

Repeating the Human Study

William Byne, who directs the Neuroanatomy Laboratory at the Bronx VA Medical Center and is an associate professor of Psychiatry at Mount Sinai School of Medicine, was a vocal critic of the 1991 LeVay paper. In a study published in 2001, he tried to duplicate it. As in LeVay's study, gay male brains were obtained from autopsies of gay men who died of AIDS, while there was no way to confirm the orientation of the presumed heterosexual brains. But like LeVay, Byne took pains to control for this.

While Breedlove is convinced that Byne's study confirmed Levay's, Byne himself calls it "inconclusive." Like LeVay, "we found a trend for the nucleus to be smaller in the homosexual men," he says. But unlike LeVay, "in our study that difference didn't reach statistical significance."

Also unlike LeVay, Byne's lab tried to account for the size difference by counting the number of neurons that made up the nucleus. "When we counted cells, there was no difference in the number of cells in this nucleus between the two groups of men," he says, "and I believe our estimates of cell number are more reliable than our estimates of the volume of this nucleus. However, I should add that we also measured the volumes of three other nucleii in the region of this cell group and none of the other three areas showed variation in volume with sexual orientation, so that suggests that the trend that we noted applied only to this cell group that Simon LeVay measured."

Byne says one of his criticisms of the LeVay study has changed since the early '90s: More evidence has accumulated about structural differences between the brains of men and women. "If the sex difference doesn't exist, then it wouldn't even be logical to expect homosexual individuals to exhibit opposite sex brains," Byne says. "Now that we can point to a particular region of the brain as exhibiting a sex difference, it makes more sense to ask the question, 'Is this part of the brain sexually atypical in homosexual individuals?'"

image: Oregon State University
Byne is cautious on the question of whether rams can be considered an animal model of homosexuality. "I want to emphasize that...it's not at all clear how we can extrapolate from findings in sheep to sexual orientation in humans," he says. "At this point, in light of evidence from a number of fields and also in light of my own studies, I think that the hypothesis that this nucleus has something to do with sexual orientation is more plausible. But I think that a lot of research remains to be done."

Nature vs. Nurture?

Satinover argues that most people associate "biological" or "nature" or "innate" with "can't be changed"; and they associate "environmental" "nurture" or "social" with "changeable". But has the genomics revolution begun to change those meanings? These days, finding a biological cause for a certain characteristic tends to imply that it could be altered through science.

In fact, Roselli's next series of sheep experiments will manipulate masculine hormones in the womb and see if there is an effect on adult sexual oriention. "We're blocking the normal action that androgens would have during fetal life," Roselli explains. "If we can show that, then we've shown that there is definitely a cause and effect relationship between fetal hormone exposure and adult partner preferences. These studies will probably take several years to know the outcome of because the animals have to grow up and they have to be tested repeatedly to see that their preferences are exclusive one way or the other."

While Roselli is interested in getting at cause and effect, that experiment could also raise the question of whether sexual orientation could ever be controlled in people.

To wonder along the lines of Breedlove's question, could somebody else—say, sheep breeders or human parents—choose what hormones individuals are exposed to before they are born? "The way the data look to me, I think the most you could hope for is that you could slightly change the probability of being straight or gay. I don't think there's ever going to be any treatment in sheep that will guarantee you that all the rams will be straight when they grow up," Breedlove says. "Likewise, even if we are seeing evidence that testosterone exposure early in life is affecting human sexual orientation, I'm very skeptical that it will ever be possible for someone to offer any treatment that will guarantee to parents that the offspring, when they grow up, will be straight. I don't think that's ever going to be feasible. And by the way...I don't see any need for those sorts of treatments."

In fact, it seems the one thing all these scientists can agree on is that both biological and social factors likely interact in ways that might never be teased apart by science.

"Twin and family studies have provided strong evidence for a genetic contribution," Byne says, "but those same studies have provided very compelling evidence for a social contribution... So I think the issue is not: Is it biological or is it social? But the important issue is: What biological factors are involved, what social factors are involved, and how do these factors interact in shaping sexual orientation?"

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