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Sex on the brain: The biology of sexual orientation

Michelle Wirth | Thursday, April 8, 2010

Editors note: This is the second installment in a series of columns by Notre Dame faculty members exploring current scholarly research in sexuality concentrating on sexual orientation and related issues.
What causes sexual orientation? What causes us to be attracted to and fall in love with the people that we do?
Almost all our behaviors and traits are a product of both nature and nurture. The “Nature/Nurture Debate” actually does not make much sense, because genes and environment have a constant interplay throughout the lifespan. However, whether through genes or learning, there is no doubt that sexual orientation is manifested somehow in our brains. This is because the brain is responsible for all our thoughts, behaviors, personality characteristics — everything that we are. The brain is not a fixed entity — learning changes your brain every day, whether very fleeting changes, like a phone number you forget immediately, or long-lasting behavior patterns, like being shy or outgoing — and, perhaps, like sexual orientation.
What is it that changes our brains to be straight or gay? There are probably many influences, but one may be the level of hormones that your brain was exposed to while developing in utero. One of the clues that androgens (testosterone-like hormones) influence sexual orientation comes from a disorder called Congenital Adrenal Hyperplasia (CAH). In this disorder, the adrenal glands’ hormone production goes haywire so that they make too much androgen. This doesn’t seem to impact developing male fetuses, since the androgen levels are not much different than what their bodies would normally make. But female fetuses with CAH find themselves swimming in much higher levels of androgens than they otherwise would. The disorder can be easily corrected with drugs once it’s discovered at birth, and androgen levels for CAH girls are normal from there on out. However, was there an effect of the extra androgens on the brains of these girls as they were developing in utero?
It seems that there is. When CAH girls grow up, 33 percent of them describe themselves as homosexual or bisexual, compared to six to 10 percent of the non-CAH population. So, androgens must play a role in sexual orientation, but they can’t explain it completely. After all, 67 percent of CAH women identified as heterosexual. And most homosexual and bisexual women do not have CAH. So androgens in the developing brain play a role in sexual orientation (at least in women), but they’re not the whole story.
Our next clue comes from differences between straight and gay people in tiny regions of the brain involved in sex and reproduction, found in post-mortem brains by Simon LeVay and colleagues. A region of the hypothalamus called INAH3 turned out to differ in size between straight men and women — men’s INAH3 is about twice as large as women’s. Gay men, however, had a smaller, straight-female-sized INAH3. Could INAH3 be the “sexual orientation area” of the brain?
It’s possible. The hypothalamus is certainly an important area for sexual behavior, although what exactly INAH3 does is unknown. But another consideration is the chicken-and-egg problem: What causes what? One possibility is that the size of INAH3 causes sexual orientation: a larger INAH3 means you will be attracted to women, and smaller INAH3 means you will be attracted to men. But another possibility is that being attracted to women for years and years causes the size of INAH3 to increase. Remember that your brain is changing and responding all the time in response to your experiences. And the brains being studied were adult brains, after the individuals had died.
We can find out more by looking at sheep, a species which shares with us some reproductive traits. Interestingly, eight percent of rams (male sheep) have a sexual preference for other rams, rather than ewes (female sheep). It turns out that sheep also have an INAH3, and it follows exactly the same pattern as in humans: rams that prefer ewes have an INAH3 twice as big as the INAH3 of ewes, but rams that prefer rams have the smaller, ewe-sized INAH3. We still have the chicken-and-egg problem: Does being attracted to ewes or rams change the size of INAH3? Or does the INAH3 size control the attractions of the sheep? We don’t know. But we do know that human cultural experience cannot explain the INAH3 results. Whatever the connection between this brain region and sexual orientation, it is something we share with other species.
These are just two studies among an ongoing body of research seeking to find out more about the biology of sexual orientation in humans. So far scientists think that, like most human behavior, sexual attraction is the result of a complex orchestration between genes, early hormone exposures and other environmental factors. There is probably no one thing that determines sexual orientation. But each clue gives us a little more information about human sexuality and how we each come to be who we are.

Michelle Wirth is a professor of Psychology. She can be contacted at mwirth@nd.edu
The views expressed in this column are those of the author and not necessarily those of The Observer.