Why are some people gay?

Human behavior is very complex, especially human sexuality. Recently I wrote about the "gay paradox," that is, how could supposed "gay genes" remain in a population given that homosexuals have far fewer children than heterosexuals. In that story, an Italian research team came up with a hypothetical model that might explain it. Today, in Science's online news service, ScienceNOW, I report on a new study suggesting that environmental factors might be somewhat more important than genetics in determining who is gay and who is straight (and who is in between.) The link is free for four weeks from publication, and then requires online access to Science, so I am reproducing the text below.

The authors of this study in the Archives of Sexual Behavior stress that environmental factors include not just social and familial influences, but also the biological environment, such as that in the womb. And since these results are based on a study of twins (the largest twin study of sexual orientation ever undertaken), I very briefly discuss the methodological problems and controversies that surround such studies. Since these news items are very short I was not able to go into that very important issue in any depth, but it is also critical to understanding claims for differences in IQ among population groups and other findings about the supposed genetic basis of differences among groups. I hope to return to this subject again soon.

One additional remark: Readers will note that the twin sample the researchers analyzed was considerably smaller than the entire Swedish twin registry. The response rate to the survey was actually quite high: About 59% for men and 65% for women. But not all twins answered the questions about sexual partners, and then the team could only include in their sample those twins who both answered the questions. That whittled the sample down to more than 7000 individuals, still much larger than any other study.

Gay Is Not All in the Genes

By Michael Balter
ScienceNOW Daily News
30 June 2008

Why are some people gay? Most researchers who study sexual orientation think that both genetic and environmental factors play a role, but the relative contributions of each remain unclear. A new study of Swedish twins reinforces earlier findings that environmental influences--including the environment in the womb--may play a greater role than genes.

Scientists studying complex human behaviors often turn to twin studies. Researchers look at both identical and fraternal twins to see how often they share a trait--a parameter called concordance. The greater the concordance among genetically identical twins compared with fraternal twins--who share only half of their genes--the more likely that genetic factors are involved.

Earlier twin studies of sexual orientation have suggested varying degrees of genetic and environmental influences. But they have suffered from the limitations typical of all twin studies. These include small sample sizes and assumptions that identical and fraternal twins both have the same family environments; if identical twins are treated more similarly by their parents than fraternal twins, for example, this could be mistaken for a genetic influence. Recruitment biases are also an issue: Some studies have enlisted participants who openly identify themselves as gay, who may not be typical of the entire homosexual population.

To try to get around these problems, a team led by Niklas Langström, a psychiatrist at the Karolinska Institute in Stockholm, recruited subjects from the Swedish Twin Registry, the world's largest. All 43,808 twins born in Sweden between 1959 and 1985 were invited to participate in a Web-based survey that comprised a wide range of questions about personal behaviors and experiences. The team ended up with a sample of 3826 twin pairs, of which 2320 were identical and 1506 fraternal. Of that sample, roughly 5% of men and 8% of women reported sexual activity with a member of the same sex at least once during their lifetimes. Then they plugged the survey responses into a standard mathematical model for comparing identical and fraternal twins.

The results, published online this month in the Archives of Sexual Behavior, confirm earlier findings that identical twins are more concordant for same-sex behaviors than fraternal twins are but only modestly so: In men, genetic effects appeared to explain 34% to 39% of the differences between the two twin groups, whereas in women, genetics accounted for only about 18% to 19% of the difference--a finding consistent with other research showing that sexual orientation in women is not as rigidly determined as it is in men.

As for what environmental factors might be at play, the authors point out that these might not be entirely social but could also be biological. For example, some studies have suggested that exposure to prenatal hormones or even the mother's immune system could influence the sexual development of a fetus.

J. Michael Bailey, a psychologist at Northwestern University in Evanston, Illinois, who led earlier twin studies of sexual orientation, calls the new study "good, important, and one unlikely to be bettered in the near future." But Jonathan Beckwith, a geneticist at Harvard Medical School in Boston, says that the new work fails to overcome a number of problems faced by previous twin studies. He notes that the final sample included only 12% of the males in the Swedish registry, leaving open the possibility of recruitment bias. And Beckwith says that the failure to control for family environment could inflate estimates of genetic influence.

More science: Boston University's Astronomy Department opens its rooftop observatory to visitors most Wednesday evenings. For more information click here.

Still more science: My colleague Greg Miller reports in ScienceNOW, in a story called "A very Memorable Trip", on a study about the lasting effects of hallucinogenic drugs. After subjects take them in a carefully controlled study, "most people rate the experience among the most personally meaningful and spiritually significant of their lives, researchers report online today in the Journal of Psychopharmacology. Such findings are helping to renew interest in research with hallucinogens, a field whose reputation long suffered from the psychedelic excesses of the 1960s." The link to the story is free for 4 weeks beginning July 1.

News Update: We have news today of an outrageous appeals court decision in the case of Maher Arar, the Syrian-born Canadian who was detained at JFK airport in 2002, then sent to Jordan and on to Syria where he claims he was tortured. The U.S. court of appeals for the Second Circuit ruled that he had no standing to sue in federal court because he was never "technically" in the United States. It seems there is no limit to the extent to which some judges will go to kowtow to the Bush administration's attempts to strip all rights from anyone it chooses under the banner of the "war on terror." According to the New York Times report I link to above: "In an occasionally scathing dissent, one judge, Robert D. Sack, said Mr. Arar’s suit should have been able to proceed because the argument that he was never really in the United States was 'a legal fiction.' ” That's putting it mildly. Such a ruling means that U.S. officials can send anyone trying to enter the United States anywhere in the world they feel like, with no redress. The U.S. Supreme Court has already ruled against the Bush administration's claim that the naval base at Guantanamo is outside U.S. jurisdiction, so this might be one to take all the way to the top.

More update: Arianna Huffington's post today at the Huffington Post is entitled, "Memo to Obama: Moving to the Middle is for Losers." Love the title, the political point should go without saying--or have we forgotten the 2000 and 2004 elections already?