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Contrary to what you may have heard, scientists have yet to discover a “gay gene.” But there is scientific evidence that homosexuality has a hereditary component and usually runs in families.

“There’s no genetic evidence for homosexuality. But there’s no gene for race determination either,” explained Dr. David Wagner, Associate Professor of Medicine at the University of Colorado, Denver. “It’s the way genes are managed and controlled. It’s their epigenetics that make a difference.”

Epigenetics, or epi-marks, are chemical compounds that bind to DNA and affect the way genes function by switching certain ones on or off. They affect how cells in the body take up different functions – why, for example, a liver cell is different from brain cell, which is different from a skin cell, though all those cells carry an individual’s entire genetic code. Epigenetics can also influence traits in individuals as a whole. A relatively new field in biology, epigenetics may hold the key in unlocking the mystery behind how sexual orientation is determined in the womb.

The science behind sexual orientation began in part with a 1991 study by Dr. Simon LeVay, then a neurobiologist at the Salk Institute for Biological Studies in La Jolla, California. LeVay measured the size of a group of cells called the nucleus within the anterior hypothalamus, a region of the brain previously linked to the regulation of sexual behavior in other animals.

This region is usually smaller in females, and is directly correlated to testosterone levels during early brain development. LeVay’s findings indicated that, on average, this portion of the brain was smaller in gay men when compared to straight men.

“The discovery that a nucleus differs in size between heterosexual and homosexual men illustrates that sexual orientation in humans is amenable to study at the biological level,” LeVay concluded in his 1991 paper.

Similar studies followed, including a 2008 Swedish paper published in the Proceedings of the National Academy of Science. The brains of 90 healthy adults (50 heterosexuals and 40 homosexuals) were imaged with an MRI scanner. The results showed a similar asymmetrical hemisphere structure for lesbians and straight men, while gay men and straight women shared a similar symmetrical hemisphere structure.

This evidence would suggest that there are indeed real structural differences in the brains of homosexuals when compared to heterosexuals of their biological sex – making the gay male brain a bit more like a straight woman’s brain, and a lesbian’s brain a bit more like straight man’s brain. But how do these correlations translate into a biological cause for sexual orientation?

A December 2012 study conducted by a team of evolutionary geneticists at the National Institute for Mathematical and Biological Synthesis (NIMBioS) published a paper theorizing that epigenetics, not genes, were a possible factor in determining an individual’s sexual orientation.

“This paper is trying to address causation,” said Wagner. “Take the [LeVay’s] correlative study and try to look for specific epi-marks in those individuals so that we can then marry causation to correlation.”

To understand how this works, it’s important to first have a clear understanding of epigenetics. Evolutionary Biologist Dr. Jeremy Byard Yoder of the University of Minnesota wrote about the NIMBioS paper in his aptly named blog, Nothing in Biology Make Sense! 

Yoder outlines this helpful analogy: “You could think of epi-marks as annotations to the genetic code, like notes in the margins of a book that help a reader remember what passages to return to, or how different parts of the text connect to each other.”

So how do epi-marks tie into sexual orientation? Depending on the sex, each embryo in the womb has a specific sensitivity to androgens – hormones, such as testosterone, which influence the development of male genitals and other characteristics.

A male embryo is more sensitive to the levels of androgens than a female embryo. The NIMBioS study hypothesizes that epi-marks could potentially regulate the genetic code, which determines the embryo’s responsiveness to those androgen levels.

Normally those epi-marks are erased from genes shortly after the embryo begins to develop. However, sometimes these epi-marks are not entirely deleted. “Epi-marks that escape erasure can be passed from parent to offspring, like marginal notes on a photocopied page,” wrote Yoder.

This means the epi-marks of a female embryo that made it resistant to androgens could potentially be passed on to the male embryo – from mother to biological son. The opposite would be true for father to daughter. These changes in responsiveness to androgens in the womb could then lead to changes in genital formation, sexual identity and sexual orientation.

“The possibility here would be that some epigenetic programming that made my mother a particularly fit and fertile human female,” Yoder told Out Front, “those markers could have been incompletely erased in the egg that became me, and so that contributed to the possibility that I developed as a gay man.”

It’s important to emphasize that the NIMBioS study is theoretical, using a mathematical model based on the data of previous studies. It does not claim to offer concrete empirical evidence.

But there is, however, evidence from other areas of study in biology that epigenetic markers can be passed from one generation to the next.

“We have really good data on this happening in plants,” said Yoder. “There’s a lab right down the hall from mine that does work on specific kinds of epigenetic markers in corn. We do know this happens in humans, but we have a much less complete understanding of how often it happens and what kinds of things might be affected by it.”

Wagner remarked the paper’s logic in linking epi-marks to sexual orientation was sound, but there were conceptual problems.

The study “seemed to imply that a female who would be homosexual, it’s because she got too large of an androgen level at some point of development,” he said. “And they imply that a male got too little of androgen. I don’t think there’s any evidence for that, directly.”

But Wagner added the study could potentially explain the wide diversity of human sexual expression – not limited to determining sexual orientation as distinct categories.

“This would be hypothetical, but maybe one person has 80 percent of a certain epi-mark, and that makes her a lesbian or a guy a gay guy. But if they had 50 percent, maybe it can create bisexual status.”

Though the NIMBioS study offers new insight into how sexual orientation may be determined in the womb, some argue that the complexity of human sexuality reaches well beyond the scope of biological research.

“The whole search for a genetic connection (to sexual orientation) is one that I’m suspicious of,” said Professor Tamara Williams Van Horn, who teaches a course on the social construction of sexuality at the University of Colorado Boulder.

“I think it comes from a philosophy of science that is very positivistic and not necessarily a good use of our political dollars right now, nor is it a really good use of science in some ways because usually those kinds of studies attempt to essentialize what most social scientists agree is a least cultural, if not fluid along the life course.”

Van Horn did not discount the importance of scientific research. She said there is an important interplay between biological and sociological factors. “I certainly don’t want to say all biology is bunk and that all of that genetic research is for nothing. I think that it is important that the disciplines stay in conversation with one another.”

But the university course Van Horn teaches works to answer the question of sexual orientation through the lens of social science, examining roles that governments, religious institutions, political forces and families play in the construction of sexual identity.

“Instead of starting with the individual and their body, we’re looking at how other things that we do as a society reflect our understandings of gender and sexuality,” said Van Horn. “Rather than looking for the gay gene, for example, we really look at how over time we’ve organized ourselves in different societies and cultures, creating categories like homosexual, for example.”

Language is a key component in social construction, often used as a tool to divide groups of people within cultures, Van Horn said. For instance, the word homosexual is a relatively new term in human history. “It is actually a remnant of the 19th century, and heterosexuality was actually the term that developed first, used by psychoanalysts to figure out why certain people were having too much sex. It was really meant to describe a deviant form of sexuality.”

GayBrainPullquote1Anti-LGBT groups often associate the word homosexual with deviant behavior and attribute homosexuality to a lifestyle choice, without a biological cause, which they believe is a factor in supporting policies that are increasingly recognized as discrimination. That’s one reason many LGBT people hope to explain sexual orientation or same-sex attraction through biological causes, answering the question often brought up in political debates: “is homosexuality a choice?”

Dr. John Corvino, Chair of the Philosophy Department at Wayne State University in Michigan and author of the book, What’s Wrong with Homosexuality? said the question itself is problematic. Corvino described a dilemma in framing a question to have only two possible answers: Homosexuality as a choice, or homosexuality as hardwired.

“There are two separate issues here,” said Corvino. “One is the question of how we come to have the sexual orientation that we have. The other question is can we choose to change that in some way.”

Corvino writes in his book that he neither knows nor cares if he was “born” homosexual. “I don’t remember the way the world was when I was born, and I can’t discern my genetic makeup by simple introspection. All I know is that I’ve had these feelings for a long time, they’re a deep fact about me, and they’re not something that I can readily change, even if I wanted to.”

Corvino jokes that it’s almost heretical for a gay man to claim he doesn’t know if he was “born this way.”

“I think a lot of gay people, in going through the coming out process, face the idea (particularly from their parents) that this is somehow an act of defiance,” said Corvino. “So they feel like, in order to establish that ‘no, this is a real and deep part of me,’ they have to show that they were somehow born with it.”

But Corvino points out that for a trait to be a real and deep characteristic of someone’s life, it doesn’t need to have a biological origin – like religion, for example. Furthermore, there are many biologically–driven human characteristics that can be changed, such as hair color. That’s not to imply one can change sexual orientation.

Being born with a trait, and whether you can change that trait, are two completely different issues, Corvino says.

“In some sense everything about us is biological. We are flesh–and–blood human beings,” said Corvino. “So, I think a better question is, what are the different biological pathways that then interact with different environments to affect the sexual feelings and dispositions that we have?”

Corvino believes science can provide essential insight into who we are as human beings, but he voiced several concerns about the NIMBioS paper’s attempt to explain homosexuality.

“Notice how discussions of the study keep focusing on the cause of homosexuality, rather than sexual orientation more generally. It’s as if being heterosexual is the ‘default setting,’ and then we have to figure out ‘what went wrong’ when people turn out gay. But evolutionary theory doesn’t posit that we should expect each individual to replicate his or her genes; it posits that we should expect species to reproduce themselves.”

GayBrainPullquote2Corvino was also concerned by the study’s implication of gender inversion. “Early theories of sexual orientation suggested that gay men were kind of like women in men’s bodies or that lesbians were men in women’s bodies. And it seems to me that that also imports a lot of assumptions and prejudices into the analysis of the data.”

So, does epigenetics point to a definitive biological origin to homosexuality? For Wagner, the question is both imperative and irrelevant. “Do I think I was born gay? Yes, I do. Do I think it was a certain series of epi-marks in my brain that caused this to happen? Yes, I do. I also think, what the hell does it matter? If it were a choice – and I’m saying as a gay medical scientist that it is not a choice – but if it were, and this is an absolutely free country, then it doesn’t matter.”

It’s clear that science has a long way to go in unraveling the question behind how human sexuality is determined. Furthermore, sociological and environmental factors, along with additional biological evidence, must be taken into account without simplistic answers.

“I think that we can focus less on how we came to have the feelings that we have and focus more on how to express those feelings in a productive and healthy and responsible manner,” Corvino said. “And I think that that’s a challenge not just for gay people, but for all people.”