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.