A recent paper discussed the results of a GWAS study of same -sex behavior, based on data from the UK Biobank. The data wasn’t ideal – it was based on self-reporting, and asked whether the respondent had ever had a same-sex experience, rather than trying to detect orientation… But that’s life.
They found two SNPs that influenced both male and female homosexuality, two that affected males only, one that affected females only. All had very small but statistically significant effects.
If we had lots of SNPs, we could look for trends ( are they predominantly expressed on certain tissues or processes ?) but with only 4 in males and 3 in females, not really possible. It does look as if the genetic architecture of homosexuality only partially overlaps between the sexes ( 0.63) – usually, say for things like height or iq, the overlap is close to 100%. So the genetics, like every observation, suggests that male and female homosexuality are qualitatively different. Yet the degree of shared genetic influence is also interesting – I don’t think it would be predicted by most strategy models.
The fraction of the variance influenced by these few SNPs is low, less than 1%. The contribution of all common SNPs is larger, estimated to be between 8% and 25%. Still small compared to traits like height and IQ, but then we knew that the heritability of homosexuality is not terribly high, from twin studies and such – political views are more heritable.
So gene influence homosexuality, but then they influence everything. Does it look as if the key causal link ( assuming that there is one) is genetic? No, but then we knew that already, from high discordance for homosexuality in MZ twins.
Most interesting to me were the genetic correlations between same-sex behavior and various other traits.
The genes correlated with male homosexuality are also correlated ( at a statistically significant level) with risk-taking, cannabis use, schizophrenia, ADHD, major depressive disorder, loneliness, and number of sex partners. For female homosexuals, risk-taking, smoking, cannabis use, subjective well-being (-), schizophrenia, bipolar disorder, ADHD, major depressive disorder, loneliness, openness to experience, and number of sex partners.
Generally, the traits genetically correlated with homosexuality are bad things. As far as I can see, they look like noise, rather than any kind of genetic strategy. Mostly, they accord with what we already knew about male and female homosexuals: both are significantly more likely to have psychiatric disorders, far more likely to use drugs. The mental-illness association maybe looks stronger in lesbians. The moderately-shared genetic architecture seems compatible with a noise model.
The strategy ideas never made sense ( in terms of a workable kin-selection, sexually antagonistic, or other genetic model) , but I think this study makes that clearer.
Find that homosexuality was genetically correlated with various kinds of unpleasantness was apparently an issue in the preparation and publication of this paper. The authors were at some pains to avoid hurting the feelings of the gay community, since avoiding hurting feelings is the royal road to Truth, as shown by Galileo and Darwin.