"Opposite-Sex Twins and Adolescent Same-Sex Attraction" by Peter S. Bearman and Hannah Brückner, in American Journal of Sociology (Mar. 2002), Univ. of Chicago Press, Journals Division, 1427 E. 60th St., Chicago, Ill. 60637.

It’s commonly supposed these days (and enshrined in many textbooks) that biology plays the main role in determining an individual’s sexual orientation. Sociologists Bearman, of Columbia University, and Brückner, of Yale University, have found some evidence that suggests otherwise.

In a 1994–96 national study, 18,841 middle and high school youths were asked if they had ever had a "romantic attraction" to a person of the same sex; 9.5 percent of the boys and 7.8 percent of the girls said they had. (Far smaller percentages reported having an actual romantic or sexual relationship.)

What caught the authors’ attention was that 16.8 percent of boys with a twin sister reported romantic same-sex feelings, while less than 10 percent of boys with a twin brother did. Genetic influences could hardly explain that seven-percentage-point difference, they say.

Why are boys with a twin sister so much more likely to show signs of a same-sex orientation? Bearman and Brückner suggest that because the twins are so similar, parents and other adults are more inclined to treat them alike—to give them a "less gendered upbringing." Parents in such a situation may tend to be a little more permissive about behavior that might otherwise be branded "sissy." (Boys with a sister who was not a twin were actually less likely than average to report same-sex romantic sentiments.) This may allow a genetic predisposition to a homosexual orientation, if such a predisposition exists, to come to the fore.

What about the girls with twin brothers? Only 5.3 percent of them reported a same-sex attraction. The authors argue that the twins’ "less gendered upbringing" has less impact on girls than on boys because "tomboy" behavior among girls is not normally considered as socially unacceptable as comparably unconventional behavior by boys is.