David J. Linden is a Professor in the Department of Neuroscience at the Johns Hopkins University School of Medicine. He is the author of Unique: The New Science of Human Individuality, The Accidental Mind: How Brain Evolution Has Given Us Love, Memory, Dreams, and God, The Compass of Pleasure: How Our Brains Make Fatty Foods, Orgasm, Exercise, Marijuana, Generosity, Vodka, Learning, and Gambling Feel So Good, and Touch: The Science of the Hand, Heart, and Mind. His laboratory has worked for many years on the cellular substrates of memory storage, recovery of function following brain injury and a few other topics.

"So there is a small genetic but significant genetic component to sexual orientation. And it's slightly different in males and females. And interestingly, it's not general. So for example, if I were to have a gay brother, then the chance of me being gay would become higher. But if I have a lesbian sister, that does not change the chance of me being gay or vice versa.

If a woman has a lesbian sister, then the chance of her being attracted to women is higher. And if she has a gay brother, it doesn't make any difference at all. So it's not like gayness or straightness is what heritability is acting on. It's attraction to males or attraction to females. And that's a subtle distinction, but I think it's very important.

The other thing that is really interesting and fascinating is that there is, from a big meta-analysis that was done by the American Psychological Association, there is really no evidence whatsoever that links events in the family to your probability of being gay or straight or bi. So, well, that's a mystery.

If it's not how you were raised by your family and it's only a little bit genetic, what is it? Well, you know, I think you had a hint of some of it when you're talking about hormones. There is some evidence that hormonal exposure in utero matters. So, for example, if biologically female fetuses are exposed to what we call androgens, the class of male hormones, that includes testosterone, that increases the probability that the child who is born and then grows up will be attracted to women when they grow up. Even if that child is biologically female. Likewise, there seems to be something similar for gay men and exposure to estrogen and female sex hormones. That said, there's of course mystery. We're far from understanding in totality how the trait of sexual orientation arrives. And we also know that there are enormous cultural influences. There are societies that have sort of a revered place for homosexual behavior in the Pantheon and others where it is really looked down upon, and that seems to have influence on how this trait develops."

www.davidlinden.org

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