HMM: Prenatal Exposure to Progesterone Affects Sexual Orientation in Humans. [archive]

A case–control double-blind design was used drawing on existing data from the US/Denmark Prenatal Development Project. Index cases were exposed to lutocyclin (bioidentical progesterone = C21H30O2; MW: 314.46) and no other hormonal preparation. Controls were matched on 14 physical, medical, and socioeconomic variables. A structured interview conducted by a psychologist and self-administered questionnaires were used to collect data on sexual orientation, self-identification, attraction to the same and other sex, and history of sexual behavior with each sex. Compared to the unexposed, fewer exposed males and females identified as heterosexual and more of them reported histories of same-sex sexual behavior, attraction to the same or both sexes, and scored higher on attraction to males. Measures of heterosexual behavior and scores on attraction to females did not differ significantly by exposure. We conclude that, regardless of sex, exposure appeared to be associated with higher rates of bisexuality. Prenatal progesterone may be an underappreciated epigenetic factor in human sexual and psychosexual development and, in light of the current prevalence of progesterone treatment during pregnancy for a variety of pregnancy complications, warrants further investigation. . . .

In summary, we observed consistent findings across samples of men and women prenatally exposed to exogenous progesterone for a set of variables directly reflective of sexual orientation. Relative to unexposed controls, prenatal exposure to progesterone was significantly associated with: (1) decreased likelihood of self-identification as heterosexual; (2) increased likelihood of having engaged in same-sex sexual behaviors; (3) increased likelihood of reporting attraction to the same or both sexes, and (4) higher scores on the attraction to males scale. Progesterone exposure was not associated with a decrease in measured heterosexual behavior. Among progesterone-exposed cases, non-heterosexual identity was shown to be associated with higher total dosages and longer duration of prenatal exposure to progesterone. . . .

Our groups were carefully matched on 14 highly relevant prenatal, perinatal, and maternal factors which should serve to minimize confounds in comparisons between exposed and control subjects. . . . Follow-up studies would be required to evaluate the levels of same-sex behavior and/or attraction that may have been revealed in subsequent decades. In keeping with the strengths of the current design, we found consistent statistically significant effects across a number of different measures supporting our hypotheses.