The trouble with genetic studies on homosexuality

Over at Nature, there's a story about a study of the genetic profile of a large number of people which tentatively finds:

....genetic patterns that could be associated with homosexual behaviour, and showed how these might also help people to find different-sex mates, and reproduce.

But the limitations of the study are really more interesting that their results.  I'll extract some parts, first explaining what they did:

Evolutionary geneticist Brendan Zietsch at the University of Queensland in Brisbane, Australia, and his colleagues used data from the UK Biobank, the US National Longitudinal Study of Adolescent to Adult Health and the company 23andMe, based in Sunnyvale, California, which sequence genomes and use questionnaires to collect information from their participants. The team analysed the genomes of 477,522 people who said they had had sex at least once with someone of the same sex, then compared these genomes with those of 358,426 people who said they’d only had heterosexual sex.  ....

Zietsch and his team decided to test whether these genetic patterns might provide an evolutionary edge by increasing a person’s number of sexual partners. They sorted the participants who had only had heterosexual sex by the number of partners they said they had had, and found that those with numerous partners tended to share some of the markers that the team had found in people who had had a same-sex partner.

The researchers also found that people who’d had same-sex encounters shared genetic markers with people who described themselves as risk-taking and open to new experiences. And there was a small overlap between heterosexual people who had genes linked to same-sex behaviour and those whom interviewers rated as physically attractive. Zietsch suggests that traits such as charisma and sex drive could also share genes that overlap with same-sex behaviour, but he says that those traits were not included in the data, so “we’re just guessing”.

Doesn't take much to think of problems with this study:

All of the participants lived in the United Kingdom or United States, and were of European descent. And the databases’ questionnaires asked about sexual behaviour, not sexual attraction. Most of the participants were born during a time when homosexuality was either illegal or culturally taboo in their countries, so many people who were attracted to others of the same sex might never have actually acted on their attraction, and could therefore have ended up in the wrong group in the study.....

Julia Monk, an ecologist and evolutionary biologist at Yale University in New Haven, Connecticut, thinks that these caveats are so important that the paper can’t draw any real conclusions about genetics and sexual orientation. Sexual behaviour and reproduction, she says, occupy a different place in modern societies than they did for human ancestors, so it’s difficult to infer their role in our evolution. For instance, people might engage with more sexual partners now that sexually transmitted diseases can be cured. And the existence of birth control and fertility treatments negates many of the reproductive advantages that genes might provide. “It’s clear that people’s behaviour when it comes to sex and reproduction is highly culturally informed, and maybe digging into genetics is next to impossible,” Monk says....

 Dean Hamer, a retired geneticist in Haleiwa, Hawaii, who published some of the first studies on the genetics of sexual orientation, is disappointed with the study. Defining sexual orientation on the basis of a single same-sex encounter is not a useful way of categorizing people, he says, because many people who identify as heterosexual have experimented with a same-sex partner. “You’re not even asking the right people the right question,” Hamer says. Instead, he thinks the researchers have found genetic markers associated with openness to new experiences, which could explain the overlap between people who have had a homosexual partner and heterosexual people who have had many partners.

 Yes, I can see how hugely complicated it must be to draw any firm conclusions from such studies.