Prenatal hormones and sexual orientation
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult. Prenatal hormones may be seen as the primary determinant of adult sexual orientation, or a co-factor with genes, biological factors and/or environmental and social conditions.
Sex-typed behavior 
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult. Differences in brain structure that come about from hormones and genes interacting on developing brain cells are believed to be the basis of sex differences in countless behaviors, including sexual orientation. Prenatal factors that affect or interfere with the interaction of these hormones on the developing brain can influence later sex-typed behavior in children. This hypothesis is originated from countless experimental studies in non-human mammals, yet the argument that similar effects can be seen in human neurobehavioral development is a much debated topic among scholars. Recent studies, however, have provided evidence in support of prenatal androgen exposure influencing childhood sex-typed behavior.
Fetal hormones may be seen as either the primary influence upon adult sexual orientation or as a co-factor interacting with genes and/or environmental and social conditions. However, Garcia-Falgueras and Dick Swaab (2010) disagree that social conditions influence sexual orientation to a large degree. As seen in young children as well as in vervet and rhesus monkeys, sexually differentiated behavior in toy preference is differing in males versus females, where females prefer dolls and males prefer toy balls and cars; these preferences can be seen as early as 3–8 months in humans. Further, differences in viewing preference can be seen as early as the first day of life, where females prefer human faces and males prefer mechanical mobiles. Despite this, is it impossible to completely rule out the social environment or the child's cognitive understanding of gender when discussing sex typed play in androgen-exposed girls. Conversely, environmental/social conditions may affect the presentation of sexually differentiated behavior in males more than females, as male-typical play behavior is much more encouraged and female behavior discouraged among male children. Children also tend towards objects which have been labelled for their own sex, or toys that they have seen members of their sex playing with previously.
An endocrinology study by Garcia-Falgueras and Swaab  postulated that "In humans, the main mechanism responsible of [sic] sexual identity and orientation involves a direct effect of testosterone on the developing brain." Further, their study puts forward that intrauterine exposure to hormones is largely determinative. Sketching the argument briefly here, the authors say that sexual organs are differentiated first, and then the brain is sexually differentiated "under the influence, mainly, of sex hormones such as testosterone, estrogen and progesterone on the developing brain cells and under the presence of different genes as well . . . . The changes brought about in this [p. 24:] stage are permanent. . . . [S]exual differentiation of the brain is not caused by hormones alone, even though they are very important for gender identity and sexual orientation."
Organizational aspects 
Fetal gonads develop primarily based on the presence or absence of androgens; production of testosterone and conversion into dihydrotestosterone during weeks 6 to 12 of pregnancy are key factors in the production of a male fetus’ penis, scrotum and prostate. In a female, however, absence of these levels of androgens results in development of typically female genitals. Following this, sexual differentiation of the brain occurs; sex hormones exert organizational effects on the brain that will be activated in puberty. As a result of these two processes occurring separately, the degree of genital masculinization does not necessarily relate to the masculinization of the brain. Sex differences in the brain have been found in many structures, most notably the hypothalamus and the amygdala. Hamman et al. (2003) conducted a study to investigate the role of the amygdala in human sexual behavior. Participants were shown a combination of sexual photos (a heterosexual couple engaging in sexual activity and opposite-sex nudes) and non-sexual photos (a man and woman engaging in non-sexual activity and a fixation cross). Hamman et al. found that males showed an increased level of amygdala activity in response to visually sexual stimuli while females reported a greater level of sexual arousal. Findings suggest that the amygdala may play an important role in mediating sex differences, especially in males, through the neural pathways responsible for emotionally-positive, desirable stimuli. Further studies on the sex differences such as this focused on many different brain structures could provide more solid answers into the masculinization or feminization of the brain.
The study of the organizational theory of prenatal hormones can be difficult, as ethically researchers cannot alter hormones in a developing fetus; instead, scholars must rely on naturally occurring abnormalities of development to provide answers. Most extensively studied in organizational effects of hormones is Congenital adrenal hyperplasia (CAH). The theory being that, if human psychological sex differences are exerted on the fetus by androgens, then females with CAH should be more male-typed and less female-typed than comparison groups of females without CAH. Androgen Insensitivity Syndrome is another abnormality of development useful in determining organizational effects of androgens. If androgen does affect behavior, partial AIS individuals should be less male-typed than typical boys, and more male-typed than typical girls. While effects of androgens have been implicated in sex-typed play behaviors, there seems to be no evidence of a large effect of androgens on gender identity; moderate levels during prenatal development have been shown to increase the probability of a male gender identity, but this is in no way a guarantee.
Since research suggests that hormones alone do not act on sexual orientation and differentiation of the brain, the search for other factors that act upon sexual orientation have led genes such as the SRY and ZRY to be implicated.
Prenatal maternal stress 
Evidence exists in rats that the stress levels of the mother can also have an effect on fetal development. As Ellis & Cole-Harding (2001) outline, the chain of events in prenatal stress affecting brain sex differentiation appears to be that stress causes the mother's adrenal glands to release high levels of stress hormones into the blood system, substantial proportions of which cross the placental barrier and interfere with the fetus' production of sex hormones temporarily, most notably testosterone. These results have been applied to human sexual orientation, although studies of maternal stress often lack generalizability due to small sample sizes and the inherent difficulty of accurately measuring stress levels in pregnancy. Criticisms of this study outline that subprimate mammals are invalid measures of human sexual differentiation, as sex hormones follow a more "on-off" role in sex-typed behavior than is found in primates.
Some studies do suggest that prenatal stress significantly increases the likelihood of homosexuality or bisexuality, although varying evidence exists for which trimester is most important. Evidence does not suggest that nicotine or alcohol consumption during pregnancy has any effect on male sexual orientation. In females, no significant relationship between prenatal stress or sexual orientation has been found. However, evidence has been found that prenatal stress alongside smoking increases the probability of homosexuality in females than just smoking alone, which is in contrast to animal models of nicotine on gendered behaviour. Studies of endocrinology have found implications for amphetamines and thyroid-gland hormones to increase homosexuality in female offspring as well, although it has not been examined in conjunction with prenatal stress levels.
Some have postulated that postnatal (e.g., social and environmental factors) development can play a role in the sexual orientation of an individual, yet solid evidence of this has yet to be discovered. Children born through artificial insemination with donor sperm and consequently raised by lesbian couples have typically been heterosexually oriented. Summed up by Bao and Swaab, "The apparent impossibility of getting someone to change their sexual orientation . . . is a major argument against the importance of the social environment in the emergence of homosexuality, as well as against the idea that homosexuality is a lifestyle choice."
Male homosexuality as hypermasculine 
There is evidence of a correlation between sexual orientation and traits that are determined in utero. Williams et al. (2000) found that finger length ratio, a characteristic controlled by prenatal hormones, is different in lesbians than in straight women. However, they found no difference between gay and straight men.  Another study by McFadden in 1998 found that auditory systems in the brain, another physical trait influenced by prenatal hormones is different in those of differing orientations; likewise the suprachiasmatic nucleus was found by Swaab and Hofman to be larger in homosexual men than in heterosexual men.  The suprachiasmatic nucleus is also known to be larger in men than in women. An analysis of the hypothalamus by Swaab and Hofmann (1990;2007) found that the volume of the suprachiasmatic nucleus (SCN) in homosexual men was 1.7 times larger than a reference group of male subjects, and contained 2.1 times as many cells. During development, the volume of the SCN and the cell counts reach peak value at approximately 13 to 16 months after birth; at this age, the SCN contains the same number of cells as was found in adult male homosexuals, yet in a reference group of heterosexual males the cell numbers begin to decline to the adult value of 35% of the peak value. These results have yet to be replicated, however; there also has yet to be a meaningful interpretation of these results provided in the context of human sexual orientation. Gay men have also been shown to have higher levels of circulating androgens  and larger penises, on average, than hetero men.
Fraternal birth order 
Gay men have more older brothers on average, a phenomenon known as the fraternal birth order effect. It has been suggested that the greater the number of older male siblings the higher the level of androgen fetuses are exposed to. No evidence of birth order effects have been observed in women. The theory holds that FBO is a result of a maternal immune response that is produced towards a factor of male development over several male pregnancies. Bogaert's hypothesis argues that "the target of the immune response may be malespecific molecules on the surface of male fetal brain cells (e.g.,including those in the anterior hypothalamus). Antimale antibodies might bind to these molecules and thus interfere with their role in normal sexual differentiation, leading some later born males to being attracted to men as opposed to women." Garcia-Falgueras and Swaab state that "The . . . . fraternal birth order effect . . . is putatively explained by an immunological response by the mother to a product of the Y chromosome of her sons. The chance of such an immune response to male factors would increase with every pregnancy resulting in the birth of a son."
While direct support has not been found for these hypotheses, evidence that favours this theory exists. Further, while percentages of the likelihood of homosexuality have been estimated to be increased by 33-48% per older brother, these odds really account for only a few percent of the population; thus, this hypothesis cannot be universally applied to the majority of homosexual men.
In conjunction with fraternal birth order, handedness provides further evidence of prenatal effects on sexual orientation, because handedness is regarded by many as a marker of early neurodevelopment. Other correlates to handedness (e.g., cerebral laterality, prenatal hormonal profiles, spatial ability) have been linked to sexual orientation, either empirically and/or theoretically. In right-handed individuals, the number of older brothers increased the odds of homosexual orientation, but this effect was not seen in left-handed individuals.
Implicated genes in fraternal birth order 
A gene of the Rh system has been discussed as a possible candidate for affecting fraternal birth order, as it has been linked to both handedness and immune system functioning. Gene variants in the Rh system are implicated in a maternal response to what is known as Hemolytic disease of the newborn. Rh is a factor in blood, and in cases where the mother is absent of this (Rh-) while carrying an Rh+ fetus, an immune response may develop with deleterious effects. The Rh gene hypothesis is a strong candidate because not only does it involve the maternal immune response, but it has been implicated in handedness as well.
Variants of the androgen receptor (AR) gene have also been discussed, in that non-right-handedness in men has been linked with fewer repeats of the AR gene, which in turn is associated with lower testosterone. A theory that high prenatal testosterone leads to neuronal and axonal loss in the corpus callosum is supported by this hypothesis.
Male homosexuality as hypomasculine 
In a 1991 study, Simon LeVay demonstrated that a tiny clump of neurons of the anterior hypothalamus—which is believed to control sexual behavior and linked to prenatal hormones— known as the interstitial nuclei of the anterior was, on average, more than twice the size in heterosexual men when contrasted to homosexual men. Due to this area also being nearly twice the size in heterosexual men than in heterosexual women, the implication is that the sexual differentiation of the hypothalamus in homosexuals is in a female direction. In 2003 scientists at Oregon State University announced that they had replicated his findings in sheep. Later studies in humans, however, have yet to confirm this finding.
Female homosexuality 
Most empirical or theoretical research into women's sexual orientation has, historically, been guided by the idea of lesbians as essentially masculine and heterosexual women as essentially feminine. Typically, this belief is traced to the early "inversion theory" of sex researchers who state that homosexuality is a result of biological abnormalities that "invert" sexual attraction and personality. Handedness research has provided implications; because more men than women present a preference for their left hand, the higher proportion of non-right handedness that has been discovered among lesbians when compared to heterosexual women demonstrates a possible link of prenatal masculinization and sexual orientation. Backing this up are reports that lesbians display more masculinized 2D;4D digit ratios than heterosexual women, based on data gathered from at least six different laboratories. This effect has not yet been observed between homosexual and heterosexual males. However, the validity of this measure of digit ratios remains controversial as a predictor of prenatal androgen, as many other prenatal factors may play roles in bone growth in prenatal stages of development. While many studies have found results confirming this hypothesis, others have failed to replicate these findings, leaving the validity of this measure unconfirmed.
Diethylstilbestrol (DES), a drug that has been in the past prescribed to prevent miscarriages, has also been studied in relation to women's sexual orientation. It has been observed to exert a masculinizing/defeminizing effect on the developing brain of the fetus. When compared to controls, higher percentages of DES-exposed women (17% vs 0%) reported that they had engaged in same-sex relations; however, the great majority of DES women stated an exclusively heterosexual orientation.
Girls with congenital adrenal hyperplasia (an autosomal recessive condition which results in high androgen levels during fetal development) have more masculinized sex role identities and are more likely to have a homosexual sexual orientation as adults than controls. An alternative explanation for this effect is the fact that girls with this condition are born with masculinized external genitalia, which leads their parents to raise them in a more masculine manner, thus influencing their sexual orientation as adults. However, the degree to which the girls' genitals are masculinized does not correlate with their sexual orientation, suggesting that prenatal hormones are a stronger causal factor, not parental influence.
Together with Congenital Adrenal Hyperplasia, DES studies have provided little support of the prenatal hormone theory of sexual orientation; they do, however, provide the framework for possible pathways to a homosexual orientation for a small number of women.
Gender identity disorder 
In individuals with GID, prenatal exposure to testosterone has been hypothesized to have an effect on gender identity differentiation. The 2D;4D finger ratio, or relative lengths of the 2nd "index" and 4th "ring" fingers, has become a popular measure of prenatal androgen because of accumulated evidence suggesting the 2D;4D ratios are related to prenatal exposure to testosterone. Many children with GID differentiate a homosexual orientation during adolescence, but not all of them; adults with "early onset", or a childhood history of cross-gender behavior, often have a homosexual orientation. Adults with "late onset", or those without a childhood history of said behavior, are more likely to have a non-homosexual orientation.
Prenatal androgen exposure has been associated with an increased chance of patient-initiated gender reassignment to male after being initially raised as female in early childhood or infancy. Gooren found that organizational effects of prenatal androgens are more prevalent in gender role behavior than in gender identity, and that there are preliminary findings that suggest evidence of a male gender identity being more frequent in patients with fully male-typical prenatal androgenization.
Individuals with complete Androgen Insensitivity Syndrome are almost always brought up as females, and the differentiation of gender identity/role is feminine. This example is important in demonstrating that chromosomes and gonads alone do not dictate gender identity and role.
Because organ differentiation and brain differentiation occur at different times, in rare cases transsexualism can result. Only 23% of childhood gender problems will result on transsexuality in adulthood.
Drawing on some transsexualism cases, Garcia-Falgueras and Swaab state that "[f]rom these examples it appears that the direct action of testosterone on the developing brain in boys and the lack of such action on the developing brain in girls are crucial factors in the development of male and female gender identity and sexual orientation . . . ." Countless studies have been run on peripheral levels of sex steroids in male and female homosexuals, a considerable number of which claimed to find "less ‘male hormone’ and/or more ‘female hormone’ in male homosexuals and vice versa in female homosexuals". However, these findings have been reviewed and have subsequently been dismissed by scholars as suffering from faulty design and interpretation.
Factors implicated in the development of transsexuality include chromosomal abnormalities, polymorphisms of certain genes, and variations in aromatase (cytochrome P450 CYP19) and CYP17. Girls with Congenital Adrenal Hyperplasia show an increase in probability of transsexuality later in life; however, this risk is still only 1-3% in CAH. Although historically abnormal sexual differentiation has pointed to androgens as a causal factor, there are codeterminants of gender identity and sexual orientation with overriding effects of androgens on the brain, in male transsexuals or homosexuals, or making androgen effects on the brain redundant, as in female transsexuals or homosexuals. These factors are currently unknown, and thus no clear cut answer for the cause of transsexualism and homosexuality exists.
Due to relatively small population sizes, generalizability of studies on transsexuality cannot be assumed.
See also 
- Biology and sexual orientation
- Environment and sexual orientation
- Handedness and sexual orientation
- Genetics of gender
- David Reimer
- Mental roots of sexual orientation
- Epigenetic theories of homosexuality
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Further reading 
- Green, R. (1987). The "sissy boy syndrome" and the development of homosexuality. New Haven, CT: Yale University Press.
- Dailey, T. and Sprigg , P. (2001). Getting It Straight — What the Research Shows about Homosexuality, Family Research Council, Washington D.C.
- Dittmann RW, Kappes MH, Kappes ME, et al. (1990). "Congenital adrenal hyperplasia. I: Gender-related behavior and attitudes in female patients and sisters". Psychoneuroendocrinology 15 (5–6): 401–20. doi:10.1016/0306-4530(90)90065-H. PMID 2101963.
- Dittmann RW, Kappes MH, Kappes ME, et al. (1990). "Congenital adrenal hyperplasia. II: Gender-related behavior and attitudes in female salt-wasting and simple-virilizing patients". Psychoneuroendocrinology 15 (5–6): 421–34. doi:10.1016/0306-4530(90)90066-I. PMID 2101964.
- Dittmann RW, Kappes ME, Kappes MH (1992). "Sexual behavior in adolescent and adult females with congenital adrenal hyperplasia". Psychoneuroendocrinology 17 (2–3): 153–70. doi:10.1016/0306-4530(92)90054-B. PMID 1438641.
- Rahman Q (2005). "The neurodevelopment of human sexual orientation". Neurosci Biobehav Rev 29 (7): 1057–66. doi:10.1016/j.neubiorev.2005.03.002. PMID 16143171.
- Williams TJ, Pepitone ME, Christensen SE, et al. (March 2000). "Finger-length ratios and sexual orientation". Nature 404 (6777): 455–6. doi:10.1038/35006555. PMID 10761903.
- Zucker KJ, Bradley SJ, Oliver G, Blake J, Fleming S, Hood J (December 1996). "Psychosexual development of women with congenital adrenal hyperplasia". Horm Behav 30 (4): 300–18. doi:10.1006/hbeh.1996.0038. PMID 9047259.
- WEBMD: Pointing the Finger at Androgen as a Cause of Homosexuality
- Boston Globe: "What makes people gay?"
- Homosexuality: Nature or Nurture?
- Homosexuality and Biology - Atlantic Magazine
- CCN Health: Male hormone levels in womb may affect sexual orientation
- Can hormones affect sexual orientation?
- The Influence of Prenatal Hormone Exposure on Sex, Gender, Gender Identity and Sexual Orientation