Both sexual orientation and sex-typical childhood behaviors, such as toy, playmate and activity preferences, show substantial sex differences, as well as substantial variability within each sex. In other species, behaviors that show sex differences are typically influenced by exposure to gonadal steroids, particularly testosterone and its metabolites, during early development (prenatally or neonatally). This article reviews the evidence regarding prenatal influences of gonadal steroids on human sexual orientation, as well as sex-typed childhood behaviors that predict subsequent sexual orientation. The evidence supports a role for prenatal testosterone exposure in the development of sex-typed interests in childhood, as well as in sexual orientation in later life, at least for some individuals. It appears, however, that other factors, in addition to hormones, play an important role in determining sexual orientation. These factors have not been well-characterized, but possibilities include direct genetic effects, and effects of maternal factors during pregnancy. Although a role for hormones during early development has been established, it also appears that there may be multiple pathways to a given sexual orientation outcome and some of these pathways may not involve hormones.
PRENATAL EXPOSURE TO DES AND SEXUAL ORIENTATION IN MEN
The possibility that exposure to ovarian hormones before birth influences sexual orientation in males also has been investigated. These studies have produced largely negative results. One study compared two groups of men exposed to the synthetic estrogen, DES, prenatally to matched controls. One group included 17 men exposed to DES alone and the second included 21 men exposed to DES along with natural progesterone. The study also included 10 men exposed prenatally to natural progesterone alone and 13 men exposed prenatally to synthetic progestins alone. Each of these groups was compared to matched controls. None of the four groups of hormone-exposed men differed from their respective controls in sexual orientation in fantasy or behavior. In addition, for all four samples combined, non-heterosexual orientation was reported by 8 of the 61 hormone-exposed men (13%), and by 8 of the 60 control men (13%). Two other research teams also have looked at sexual orientation in men exposed to DES prenatally, and have found no evidence of reduced heterosexual orientation. One studied 46 men exposed to DES and 29 unexposed controls. Men exposed to DES had somewhat more heterosexual coital experience than did controls, but did not differ in the number of heterosexual or homosexual coital partners. The second compared 1,342 DES-exposed men to 1,342 unexposed men, and found no difference in the numbers reporting sexual experience with a partner of the same sex, although, as noted above, this study used a relatively insensitive procedure for assessing sexual orientation. Nevertheless, the findings overall suggest that prenatal exposure to estrogen does not feminize sexual orientation in developing males, and this conclusion is consistent with predictions from results of experimental studies in other species, where early exposure of male animals to estrogen does not promote the development of female-typical behavior.
Prenatal exposure to DES and sexual orientation in women
One research team has studied three samples of women exposed prenatally to DES. The first sample included 30 women exposed to DES, 30 unexposed women recruited from the same gynaecological clinic and 12 unexposed sisters of the DES-exposed women. All of the participants had abnormal PAP smear findings. (Although DES rarely, if ever, causes genital virilization, prenatal exposure is often associated with abnormal PAP smears). Sexual orientation was assessed by interview and rated using Kinsey scale scores, and a global rating for lifelong sexual responsiveness (behavior and fantasy combined) was reported for 29 of the DES-exposed women and 30 of the controls. DES exposure was associated with reduced heterosexual orientation. Although 76% of the DES-exposed women were exclusively or almost exclusively heterosexual for lifetime scores, 24% were not. The comparable figure for the matched controls with abnormal PAP smear findings was 0%. The subset of 12 sister pairs showed a similar pattern with 42% of the DES-exposed sisters being not exclusively or almost exclusively heterosexual for their lifetime in terms of fantasy or behavior, compared to 8% of their unexposed sisters. Among the total group of DES-exposed women, five had experienced homosexual activities involving genital contact and two were living with a female partner. The same research team later reported data from this initial study along with data from two more samples of women exposed to DES prenatally. The first additional sample included 30 DES-exposed women, 30 demographically matched controls, with no history of DES-exposure or abnormal PAP smears, and 8 unexposed sisters. In this sample, a global Kinsey rating for lifelong sexual responsiveness was reported for 29 of the DES-exposed women and 30 of the matched controls. For the exposed group, 35% were not exclusively or almost exclusively heterosexual, whereas for the control group the comparable figure was 13%. Among the 20 sister pairs in the first and second samples, 40% of the DES-exposed group, compared to 5% of their sisters, were not exclusively or almost exclusively heterosexual. The second additional sample included 37 DES-exposed women whose mothers’ obstetrical files indicated prescription of at least 1000 mg of DES during the pregnancy, and age-matched daughters of women from the same obstetrical practice, whose mothers’ files showed that no DES was prescribed. For these women, 16% of the DES-exposed group and 5% of the unexposed group were not exclusively or almost exclusively heterosexual. For all three samples combined, 24% of the DES-exposed women, and 6% of the control women were not exclusively or almost exclusively heterosexual.
Prenatal endocrine influences on sexual orientation and on sexually differentiated childhood behavior, National Institutes of Health, Front Neuroendocrinol; 32(2): 170–182, NCBI PubMed PMC3296090, 2011 Apr
A separate investigation of women exposed to DES prenatally concluded that this exposure did not influence their sexual orientation. This study included 3,946 women exposed prenatally to DES and 1,740 women not exposed to DES. The DES-exposed women were somewhat less likely than the unexposed women to have had sex with a female partner. The DES-exposed women also were more likely than the unexposed women to have ever married, and for those who had had sexual intercourse with a man, were less likely to have had sexual intercourse before age 17 or to have had more than one sexual partner. These last differences raise questions about the comparability of the exposed and unexposed groups, and, although the large sample is impressive, the assessment of sexual orientation, in terms of a single question regarding sexual behavior, is relatively insensitive.
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