This old question has been in the news again recently because of the publication of a massive study on the subject that’s just been published, written by two Johns Hopkins affiliated professors, Laurence Mayer and Paul McHugh, that says there’s little to no scientific evidence that people are born with those traits.
If you want to read the entire article—it’s long—the whole thing can be found online here
A few points I’ll make at the outset:
(1)) Anyone who’s followed research in the field should already know that the evidence for an absolute biological genetic cause for either of the phenomena is murky, but that there has been strong evidence of a genetic contribution that is not trifling. That evidence has come from twin concordance studies (particularly concerning homosexuality), which I’ve already written about at some length in the addendum of this post.
So I fail to see how this Johns Hopkins article is news, but I suppose it is news in the sense that there are a lot of people politically devoted to a less nuanced point of view on either side, and the ones who are into strict heritability of the traits will be up in arms.
(2) McHugh is a psychiatrist at Hopkins who has previously written similar articles, particularly about the treatment of transgendered people. I wrote about one of these articles before here. Hopkins was a pioneer in sex-reassignment surgery and has since backed off from doing it because of problems with poor outcomes, and McHugh is nothing if not a controversial figure as a result.
One powerful research design for assessing whether biological or psychological traits have a genetic basis is the study of identical twins. If the probability is high that both members in a pair of identical twins, who share the same genome, exhibit a trait when one of them does — this is known as the concordance rate — then one can infer that genetic factors are likely to be involved in the trait. If, however, the concordance rate for identical twins is no higher than the concordance rate of the same trait in fraternal twins, who share (on average) only half their genes, this indicates that the shared environment may be a more important factor than shared genes…
…[W]ell-designed twin studies examining the genetics of homosexuality indicate that genetic factors likely play some role in determining sexual orientation. For example, in 2000, psychologist J. Michael Bailey and colleagues conducted a major study of sexual orientation using twins in the Australian National Health and Medical Research Council Twin Registry, a large probability sample, which was therefore more likely to be representative of the general population than Kallmann’s. The study employed the Kinsey scale to operationalize sexual orientation and estimated concordance rates for being homosexual of 20% for men and 24% for women in identical (maternal, monozygotic) twins, compared to 0% for men and 10% for women in non-identical (fraternal, dizygotic) twins. The difference in the estimated concordance rates was statistically significant for men but not for women. On the basis of these findings, the researchers estimated that the heritability of homosexuality for men was 0.45 with a wide 95% confidence interval of 0.00–0.71; for women, it was 0.08 with a similarly wide confidence interval of 0.00–0.67. These estimates suggest that for males 45% of the differences between certain sexual orientations (homosexual versus heterosexuals as measured by the Kinsey scale) could be attributed to differences in genes…
…a heritability estimate of 0.45 does not mean that 45% of sexuality is determined by genes. Rather, it means that 45% of the variation between individuals in the population studied can be attributed in some way to genetic factors, as opposed to environmental factors…
…[In another study, the] values indicate that, while the genetic component of homosexual behavior is far from negligible, non-shared environmental factors play a critical, perhaps preponderant, role. The authors conclude that sexual orientation arises from both heritable and environmental influences unique to the individual, stating that “the present results support the notion that the individual-specific environment does indeed influence sexual preference.”
I could go on—the article certainly does—but I’ve not read most of it yet and so I’ll stop there. Suffice to say that I fully expect the answer to be some variation on the theme “it’s nature and nurture, and we don’t know the exact combination of each.” Which would make these things not so very different from many many other things in life, and somewhat of a mystery, which is where I’m at on it.
That makes me a non-militant on the subject. But I find it rather fascinating, and also somewhat irrelevant in that I think the issue of the rights of each group is separate from the issue of the traits’ heritability. That last bit puts me quite in the minority, I think, because a lot of people are invested in the question of heritability because they believe that the question of rights depends on it.
[NOTE II: There’s a great deal more research on twin concordance in gay people than in transgendered people. I believe that’s because the first subject has been studied for a longer time, but more importantly because it is far more common in the population and therefore it is easier to find experimental subjects. The McHugh article cites only one twin study on transgenderism, as far as I can tell, and that one is a case study involving two cases, which can tell us little about anything but those cases.
I was unable to find good studies on the subject; in a quick Google search, this was about it, and I’m not sure how reputable a study that is, although the results seem to fall into a similar camp to those of the gay studies on twin corcordance. At any rate, we already know that identical twin concordance for transgenderism is far from perfect, as evidence by the photos in this post of mine.]