The Association Between Fraternal Birth Order and Anal-Erotic Roles of Men Who Have Sex with Men.

The fraternal birth order effect (FBOE) describes the phenomenon that homosexual men tend to have a greater number of older brothers than do heterosexual men. The FBOE is a marker for an innate, biological predisposition for androphilia in genotypic males. The FBOE has been studied since the 1930s and is the most consistent biodemographic correlate of sexual orientation in men. This study sought to determine whether the FBOE applies equally to all men who have sex with men (MSM), or disproportionately to MSM whose anal intercourse behavior is predominantly receptive (Bottoms). Participants included 243 North American adult MSM who responded to advertisements posted on a Web site and other electronic media associated with the GALA festival, a quadrennial gathering of gay and lesbian choruses. Each was asked whether his anal intercourse behavior during the preceding year was predominantly receptive, predominantly penetrative, or about equally receptive and penetrative. Those who indicated their behavior was predominantly receptive were coded "Bottoms"; all others were coded "Not-Bottoms." Participants were also surveyed as to their sibship composition. Bottoms had a significantly greater mean number of older brothers than did Not-Bottoms. There was no significant difference with respect to older or younger sisters or younger brothers. Further, the older sibling sex ratio (OSSR) for the Bottom cohort, but not for the Not-Bottom cohort, was significantly higher than the expected OSSR for the general male population (OSSR = No. older brothers/No. older sisters × 100; expected OSSR for general population = 106). Thus, late fraternal birth order was correlated with receptive anal-erotic behavior among MSM.