August 10, 2006 - A study in the American Journal of Sociology by Columbia/Yale researchers Bearman and Bruekner contradicts a recent study by researcher Bogaert of Canada and declares that at least one social factor is important in the development of male SSA.

The study shows that males in opposite-sex twin pairs are twice as likely to have SSA as expected -- which is almost certainly a social effect. Parallel calculations for the extent of a genetic effect on SSA gave a result of zero for both males and females, which is similar to, but lower than, two other recent studies.

The study entitled "Opposite-sex twins and adolescent same-sex attraction" appeared in 2002, but has received no media attention, partly because its abstract did not show the interesting details contained in the paper. When contacted by Dr. Warren Throckmorton, the chief author Peter Bearman, said that no other researchers had contacted him about the report.

A problem with SSA studies has been that they have been based on volunteers rather than being really representative of the population. In contrast, an excellent sample was used in the Bearman/Bruekner study. The National Longitudinal Study of Adolescent Health (Add Health) was started in 1994 and surveyed tens of thousands of children who were truly nationally representative. In this survey, 3,139 sibling pairs were specially selected from among this group, including many twin pairs for the genetic part of this study. Although refusal to participate was allowed, 80% agreed, making it probably the highest acceptance rate in any genetic study, and least subject to bias.

Included in the study were opposite sex twins, identical twins, fraternal twins, sibling pairs and even adopted pairs. There were generally several hundred individuals in each category. Although the males in opposite twin pairs were twice as likely to develop SSA, statistical examination by Dr. Throckmorton and his colleagues showed that the result was only about as significant as the study by Bogaert in which he claimed development of SSA in adopted males was correlated with number of elder brothers in the original family but not the adopted family.

Bearman and Bruecker explained their opposite twin effect by parental influence. They say that demands for unisex equal treatment by opposite-sex twins lead to less masculine influence on the males and greater masculine influence on the females. However, this is far more critical for males and leads to greater SSA.

They also did a traditional twin study among their subjects and found the genetic contribution was zero, and chance was a very important factor. In identical male twins, if one twin had SSA there was only a 7% chance the co-twin had SSA, and the corresponding result for females was about 5%. These results, known as the concordance rates, have steadily decreased with time as studies have become more representative, suggesting previous volunteer-related bias.

The study joins two previous ones (Bailey, et al. 2000, Hershberger 1997) to give a low genetic contribution for same sex attraction (which might not be exactly the same as sexual orientation). Averaging the three studies, the genetic contributions for male and female SSA are now 10% and 25% respectively. The only literature figure for genetic contribution to OSA (Opposite Sex Attraction) or heterosexuality, is 18% for both sexes combined (Hershberger, 1997).

Bearman and Brueckner could find no trace of a fraternal birth order on the males in their opposite-sex twins, although the effect is generally rather well established by other research. Bogaert, contacted by Throckmorton, thinks that the study is a rogue result, mainly a chance finding. He emphasizes it is for unstable adolescents and may not be comparable with studies on adults. However, if this once again points to the instability of SSA in adolescents, it also points to the fact that social factors must play a large part and that adolescents, among others, could seriously consider change of sexual orientation

References

Bailey, JM; Dunne, MP; Martin, NG (2000): Genetic and Environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology 78, 524-536.

Bearman, PS; Bruckner, H (2002): Opposite-sex twins and adolescent same-sex attraction. American Journal of Sociology 107, 1179-1205.

Hershberger, SL (1997): A twin registry study of male and female sexual orientation. Journal of Sex Research 34, 212-222.