Bryan Caplan  

Nature, Nurture, and Orientation: The Latest Evidence

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The evidence on nature, nurture, and sexual orientation just got a lot better.  (For earlier discussion, see here, here, and here).  Gene Expression summarizes the latest evidence:

Three recent twin studies have largely overcome previous methodological issues, demonstrate clear genetic influences on sexual orientation... These studies all used large, population-based samples - that is, the subjects were not recruited to the study based on sexual orientation...

[...]

Estimates of genetic influences were high across all three studies. The Australian study found heritability of 48% for sexual orientation across males and females together. The Finnish study estimated genetic influences on sexual orientation of 45% and 50% for men and women, respectively. Neither study found any evidence of an effect of shared environment. The Swedish study gave somewhat different results - the heritability for male heterosexuality was quite high, 39%, with no effect of a shared environment. However, the estimated heritability for female heterosexuality was lower in this study, around 18-19%, and a significant contribution from the shared environment was found for females in this study (16-17%).

I'm a little worried by the studies' high estimates of the prevalence of homosexuality; it suggests that gays were more likely to respond.*  And if gays were more likely to respond, then gay pairs of twins are especially overrepresented.  By itself, however, such a response bias would actually depress estimates of genetic effects and inflate estimates of shared environmental effects. 

Not clear why?  Suppose that people only responded if both were gay.  Then the concordance difference between identical and fraternal twins would be zero, indicating no genetic effect, even though siblings' concordance rate would be far above the population average, indicating a massive nurture effect.

* I haven't read the original studies yet, so I apologize if any of the researchers somehow avoided this problem.


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COMMENTS (9 to date)
Tracy W writes:

Generally I'm fine intellectually with mental characteristics being significantly influenced by genes.
Inherited homosexuality though, I just can't get my head around. Talk about a set of genes doomed for natural deselection.
(Morally I think homosexuality is fine, in and of itself. I just don't understand how it can be genetic).

david writes:

@Tracy W

"J. Goard" on the gnxp blog commented this:

It doesn’t seem like that big of a riddle to me. It is fairly well-established that non-heterosexuality correlates with increased masculinity in women and increased femininity in men. Other research (from Zietch and colleagues, for example) shows significantly greater number of sexual partners for both heterosexual men with more feminine traits, and heterosexual women with more masculine traits. In short, it seems that typical patterns of sexual attraction serve to keep gender dimorphism from “going off the rails”, which means that many of the same traits which predispose someone to homosexuality are advantageous when they turn out heterosexual.
Mike Hammock writes:

Can someone restate the paragraph about concordance differences versus rates? I don't think I followed it.

Tracy W writes:

David, ah, so that's it, Sounds much more plausible than that helping-nephews-and-nieces hypothesis.

Gene writes:

I would want to see more evidence that homosexuality actually resulted in reduced fertility before assuming that it would deselect out of the population.

I would expect that most homosexuals throughout history got married to opposite sex partners and had children.

Also, I would be curious if the study has any impact on in utero theories of homosexual origins -- some or possibly all women are more likely to turn out homosexual children due to conditions during pregnancy.

Justin Ross writes:

Growing up, we had a neighboring family with 5 kids, 3 biological and 2 adopted. At least two of the biologicals turned out gay, and the two adopted were not. No twins, but I hope they end up as data points somewhere.

ziel writes:

I would expect that most homosexuals throughout history got married to opposite sex partners and had children.

They almost certainly would have had less children. And even if it's only very little less, that would be enough to eliminate it from the population or keep it from ever reaching the prevalence it's at today (3 to 5%).

So what's needed is a countervailing (balancing) effect to keep it at that level. I agree with Tracy W. that the "gay uncle" thesis is rather farfetched. I don't find the "more feminine guys are super babe magnets" effect much more plausible.

Matt Flipago writes:

Gene
Not only would they have less, but almost every society had some job or group of people who didn't need have children. So even a fairly small amount of people having less then the average should cause their population to diminish, even at 10% less, which I would assume is an estimate too close to the average family. If infant mortality is 50% and you want five alive children, then you need to get her pregnant 10 times, ad that's not easy for most women.

David Etlund writes:

To revisit the "helpful uncle" concept, it is interesting to consider the fraternal birth order effect: in males, having older biological brothers increases the likelihood of homosexuality in the adult, which would suggest the mother's biology at work. I prefer not to think I was born to be my brother's babysitter, but oh well.

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