Bisexuality is quite common among men and women whose mothers received additional doses of the sex hormone progesterone while they were pregnant.
This is one of the findings of a study led by June Reinisch, Director Emerita of The Kinsey Institute in the US, published in Springer's journal Archives of Sexual Behavior. The study tracked the sexual development of 34 Danes whose mothers were treated with the hormone to prevent miscarriage.
According to the research team, progesterone appears to be an underappreciated factor influencing the normal development of variations in human sexuality and psychosexuality. The findings warrant further investigation given that little is known about the effects on offspring of natural variations in levels of maternal progesterone and that progesterone is widely used to treat pregnancy complications.
The 34 participants in the study were drawn from the Copenhagen Perinatal Cohort, which comprises information collected from virtually all children born between 1959 and 1961 at the university hospital in Copenhagen, Denmark. The 17 men and 17 women were selected because their mothers exclusively received the progesterone lutocyclin to prevent a miscarriage. These men and women were compared with a carefully selected control group who were not exposed prenatally to lutocyclin or any other hormone medication, but who otherwise matched the study participants based on 14 relevant physical, medical, and socioeconomic factors. The participants were all in their mid-20s when asked about their sexual orientation, self-identification, attraction to each sex, and sexual history using questionnaires and a structured interview with a psychologist.
It was found that men and women whose mothers were treated with progesterone were significantly less likely to describe themselves as heterosexual. One in every five (20.6 percent) of the progesterone- exposed participants labeled themselves as other than heterosexual. Compared to the untreated group, the chances were greater that by their mid-20s they had already engaged in some form of same-sex sexual behavior (in up to 24.2 percent of cases), and that they were attracted to the same (29.4 percent) or to both sexes (17.6 percent). Both exposed males and females also had higher scores related to attraction to men.
"Progesterone exposure was found to be related to increased non-heterosexual self-identification, attraction to the same or both sexes, and same-sex sexual behavior," says Reinisch. "The findings highlight the likelihood that prenatal exposure to progesterone may have a long-term influence on behavior related to sexuality in humans."
The research team believes further studies on the offspring of women medically treated with progesterone and other progestogens during their pregnancies as well as studies examining the effects of natural variation in prenatal progesterone levels are warranted to provide more insight into the role that this hormone plays in the development of human behavior.