The Genetics of Same-Sex Sexual Orientation
What we know so far
A recent paper by Thomas Felesina and Brendan Zietsch summarizes what we’ve learned to date about the evolutionary genetics of same-sex sexual orientation. Here are the five main conclusions of the paper.
Same-sex sexual orientation is around 30% heritable. This makes it among the least heritable psychological traits. Note, though, that the environmental component is almost certainly not the social environment or nurture in the traditional sense. More likely, it’s biological factors such as atypical prenatal hormonal exposure.
The heritable component of same-sex sexual orientation is the product of many, many gene variants, each of which has only a tiny effect - consistent with the Fourth Law of Behavior Genetics.
Gay men and lesbians have fewer offspring than their straight counterparts, as shown in the following graph.
This raises an interesting and difficult question: How do gene variants predisposing people to same-sex sexual orientation persist in the human gene pool? The short answer is that we don’t know. One possibility, however, is that when these variants are found in straight people, they confer a reproductive advantage - an advantage that matches or outweighs the reproductive disadvantage associated with same-sex sexual orientation.
Same-sex sexual behavior is fairly common in other animals. Exclusive same-sex sexual orientation, however, is rare or entirely absent.
I’m planning a longer post on this topic, so stay tuned for that.
In the meantime, you can access Felesina and Zietsch’s paper here or request a free copy here.
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Related Reading From the Archive
Birth Order and Same-Sex Sexual Orientation
The fraternal birth order effect refers to the observation that the more older brothers a man has, the more likely he is to be gay. The original claim was that the effect is found only in men, and that older sisters have no impact on sexual orientation. The leading explanation for the phenomenon is the
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“How do gene variants predisposing people to same-sex sexual orientation persist in the human gene pool?” I’ll posit that a tragically large proportion of same-sex attracted females were/are still compelled to enter heterosexual relationships.
What happened to the plausible theory that genes predisposing to same-sex sexual orientation persist in humans because gay and lesbian individuals effectively "dote" on close relatives, raising their fitness?
https://www.sciencedirect.com/science/article/abs/pii/S1090513801000745