Roughly one in seven gay men may owe his sexual orientation to the fact he has older brothers, say U of T researchers. Earlier studies have shown that each additional older brother increases the odds of homosexuality in a younger brother. "This phenomenon, known as the fraternal birth order effect, was first shown by Professor Ray Blanchard [of U of T's Department of Psychiatry and the Centre for Addiction and Mental Health] and has since been confirmed by other scientists," says CAMH post-doctoral fellow James Cantor. Cantor is co-author of a new study to determine the proportion of gay men whose sexual orientation is due to this effect. "This latest study demonstrates just how important that link is," says Cantor.
In their study, Cantor and colleagues from U of T and CAMH applied statistical analysis to data collected from a sample of 302 gay men and 302 heterosexual men about the number of siblings each had.
According to the researchers, the sexual orientation of about 15 per cent of gay men in the sample could be attributed to the older brother effect. Their analysis also suggests that, in the theoretical case of a boy with two-and-a-half older brothers, he would be twice as likely to be gay as a boy with no older brothers.
The study does not determine a cause of the correlation between homosexuality and having older brothers. However, there is growing evidence in other research that it may be pre-natal in nature, based on findings that gay men with older brothers tend to weigh less at birth than straight men with older brothers.
