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.
The hormonal theory of sexuality and gender identity holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation and or gender identity that emerges later in the adult. Differences in brain structure that come about from chemical messengers and genes interacting on developing brain cells are believed to be the basis of sex differences in countless behaviors, including sexual orientation.:25 Prenatal factors that affect or interfere with the interaction of these hormones on the developing brain can influence later sex-typed behavior in children.:24 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.[page needed] However, Garcia-Falgueras and Dick Swaab 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. It is 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, children 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.":25 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 stage are permanent. ... Sexual differentiation of the brain is not caused by hormones alone, even though they are very important for gender identity and sexual orientation.":24
Fetal gonads develop primarily based on the presence or absence of androgen hormones, mainly testosterone, dihydrotestosterone (DHT) and androstenedione; production of testosterone and conversion into dihydrotestosterone during weeks 6 to 12 of pregnancy are key factors in the production of a male fetus's penis, scrotum and prostate. In a female, on the other hand, 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.:24 Sex differences in the brain have been found in many structures, most notably the hypothalamus and the amygdala. However, few of these have been related to behavioral sex differences, and scientists are still working to establish firm links between early hormones, brain development and behavior. 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). CAH is a genetic disease that results in exposure to high levels of androgens beginning early in gestation. Girls with CAH are born with masculinized genitalia, which is corrected surgically as soon as possible. CAH provides the opportunity for natural experiments, as people with CAH can be compared to people without it. However, "CAH is not a perfect experiment", since, "social responses to masculinized genitalia or factors related to the disease itself" can confound results. Nonetheless several studies have shown that CAH has a clear but not determining influence on sexual orientation; women with CAH are less likely to be exclusively heterosexual than are other women.
Since hormones alone do not determine 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 ZFY to be implicated.
Prenatal maternal stress
As of 2006 results from studies in humans had found conflicting evidence regarding the effect of prenatal exposure to hormones and psychosexual outcomes; Gooren noted in 2006 that studies in 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.:24 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.:24
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.:24 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."
Fraternal birth order
According to a multitude of studies over several decades, 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 the fraternal birth order effect 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 male specific molecules on the surface of male fetal brain cells (e.g., including those in the anterior hypothalamus). Anti-male 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.":24
Maternal antibodies against Y-chromosone neuroligin have been implicated in this effect, among other evidence that favours this theory. Further, while percentages of the likelihood of homosexuality have been estimated to be increased by 15–48% 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. Most, but not all, studies have been able to reproduce the fraternal birth order effect. Some did not find any statistically significant difference in either the sibling composition or rate of older brothers of gay and straight men, including large, nationally representative studies in the US and Denmark.
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. As with other purported marks indicating higher incidence of homosexuality, however, the link with handedness remains ambiguous and several studies have been unable to replicate it.
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 greater CAG repeats in 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 hypermasculine
There is evidence of a correlation between sexual orientation and traits that are determined in utero.[page needed] A 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 not been replicated, however; there also has yet to be a meaningful interpretation of these results provided in the context of human sexual orientation. Some highly disputed studies suggest gay men have also been shown to have higher levels of circulating androgens and larger penises, on average, than heterosexual men.
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.
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.
In individuals with gender dysphoria, previously known as gender identity disorder (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.:24 Only 23% of childhood gender problems will result in 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 ... .":26 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 Gooren 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.
Endocrine disrupting chemicals (EDCs) are chemicals that, at certain doses, can interfere with the endocrine system in mammals. Work on possible neurotoxic effects of endocrine disruptors, and their possible effects on sexual orientation when a fetus is exposed to them, is in its infancy: "we mostly know about the relationship between EDC exposure and neurobehavioral function through an examination of outcomes within a limited sphere of questions." While studies have found that xenoestrogens and xenoandrogens can alter the brain's sexual differentiation in a number of species used as animal models, from the data in hand to date, it is "misleading ...to expect EDCs to produce profiles of effects, such as sexually dimorphic behaviors, as literal copies of those produced by native hormones. Such agents are not hormones. They should not be expected to act precisely as hormones."
- Biology and sexual orientation
- Environment and sexual orientation
- Fraternal birth order and male sexual orientation
- Handedness and sexual orientation
- Genetics of gender
- David Reimer
- Epigenetic theories of homosexuality
- Garcia-Falgueras A, Swaab DF. Sexual hormones and the brain: an essential alliance for sexual identity and sexual orientation. Endocr Dev. 2010;17:22-35. doi:10.1159/000262525 PMID 19955753
- Hines, Melissa (October 2010). "Sex-related variation in human behavior and the brain". Trends in Cognitive Sciences. 14 (10): 448–456. doi:10.1016/j.tics.2010.07.005. PMC 2951011. PMID 20724210.
- Wilson, G., & Q. Rahman, Born Gay: The Psychobiology of Human Sex Orientation, Peter Owen Publishers; 2nd edition (May 1, 2008) ISBN 9780720613094
- Bao, Ai-Min; Dick F. Swaab (18 February 2011). "Sexual differentiation of the human brain: Relation to gender identity, sexual orientation and neuropsychiatric disorders". Frontiers in Neuroendocrinology. 32 (2): 214–226. doi:10.1016/j.yfrne.2011.02.007. PMID 21334362.
- Berenbaum, Sheri A.; Adriene M. Beltz (April 2011). "Sexual differentiation of human behavior: Effects of prenatal and pubertal organizational hormones". Frontiers in Endocrinology. 32 (2): 183–200. doi:10.1016/j.yfrne.2011.03.001. PMID 21397624.
- Hines, Melissa (2011). "Prenatal endocrine influences on sexual orientation and on sexually differentiated childhood behavior". Frontiers in Neuroendocrinology. 32 (2): 170–182. doi:10.1016/j.yfrne.2011.02.006. PMC 3296090. PMID 21333673.
- Ngun, Tuck C.; Negar Ghahramani; Francisco J. Sanchez; Sven Bocklandt; Eric Vilain (October 2010). "The genetics of sex differences in brain and behavior". Frontiers in Neuroendocrinology. 32 (2): 227–246. doi:10.1016/j.yfrne.2010.10.001. PMC 3030621. PMID 20951723.
- Gooren, Louis (November 2006). "The biology of human psychosexual differentiation". Hormones and Behavior. 50 (4): 589–601. doi:10.1016/j.yhbeh.2006.06.011. PMID 16870186.
- Bogaert, Anthony; Ray Blanchard; Lesley Crothswait (October 2007). "Interaction of Birth Order, Handedness, and Sexual Orientation in the Kinsey Interview Data". Behavioral Neuroscience. 121 (5): 845–853. doi:10.1037/0735-7044.121.5.845. PMID 17907817.
- McCarthy, Margaret M. (2019). "Sex differences in neuroimmunity as an inherent risk factor". Neuropsychopharmacology. 44 (1): 38–44. doi:10.1038/s41386-018-0138-1. ISSN 0893-133X. PMC 6235925. PMID 29977075.
- Blanchard, Ray; Richard Lippa (April 2007). "Birth Order, Sibling Sex Ratio, Handedness, and Sexual Orientation of Male and Female Participants in a BBC Internet Research Project". Archives of Sexual Behavior. 36 (2): 163–176. doi:10.1007/s10508-006-9159-7. PMID 17345165.
- Ray Blanchard; Richard Lippa (2007). "Birth Order, Sibling Sex Ratio, Handedness, and Sexual Orientation of Male and Female Participants in a BBC Internet Research Project". Archives of Sexual Behavior. 36 (2): 163–76. doi:10.1007/s10508-006-9159-7. PMID 17345165.
- Blanchard R (12 Jun 2017). "Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years". Arch. Sex. Behav. 47 (1): 1–15. doi:10.1007/s10508-017-1007-4. PMID 28608293.
- B. P. Zietsch; et al. (2012). "Do shared etiological factors contribute to the relationship between sexual orientation and depression?". Psychological Medicine. 42 (3): 521–532. doi:10.1017/S0033291711001577. PMC 3594769. PMID 21867592.
- Mariana Kishida; Qazi Rahman (2015). "Fraternal Birth Order and Extreme Right-Handedness as Predictors of Sexual Orientation and Gender Nonconformity in Men". Archives of Sexual Behavior. 44 (5): 1493–1501. doi:10.1007/s10508-014-0474-0. PMID 25663238.
- Francis AM (2008). "Family and sexual orientation: the family-demographic correlates of homosexuality in men and women". J. Sex Res. 45 (4): 371–7. doi:10.1080/00224490802398357. PMID 18937128.
- Frisch M; Hviid A (2006). "Childhood family correlates of heterosexual and homosexual marriages: a national cohort study of two million Danes". Archives of Sexual Behavior. 35 (5): 533–47. doi:10.1007/s10508-006-9062-2. PMID 17039403.
- Mustanski, B. S.; Bailey, J. M.; Kaspar, S. (2002). "Dermatoglyphics, handedness, sex, and sexual orientation". Archives of Sexual Behavior. 31 (1): 113–122. doi:10.1023/A:1014039403752. PMID 11910784.
- Terrance J. Williams; Michelle E. Pepitone; Scott E. Christensen; Bradley M. Cooke; Andrew D. Huberman; Nicholas J. Breedlove; Tessa J. Breedlove; Cynthia L. Jordan; S. Marc Breedlove (2000). "Finger-length ratios and sexual orientation" (PDF). Nature. 404 (6777): 455–456. doi:10.1038/35006555. PMID 10761903.
- Qazi Rahman; Kenneth Clarke; Tirma Morera (2009). "Hair Whorl Direction and Sexual Orientation in Human Males". Behavioral Neuroscience. 123 (2): 252–256. doi:10.1037/a0014816. PMID 19331448.
- Witelson, Sandra; R. S. Nowakowski (1991). "Left out axons make men right: A hypothesis for the origin of handedness and functional asymmetry". Neuropsychologia. 29 (4): 327–333. doi:10.1016/0028-3932(91)90046-B. PMID 1857504.
- Panzica, GC; Aste, N; Viglietti-Panzica, C; Ottinger, MA (March 1995). "Structural sex differences in the brain: influence of gonadal steroids and behavioral correlates". Journal of Endocrinological Investigation. 18 (3): 232–52. doi:10.1007/BF03347808. PMID 7615911.
- Swaab DF, Zhou JN, Ehlhart T, Hofman MA (June 1994). "Development of vasoactive intestinal polypeptide neurons in the human suprachiasmatic nucleus in relation to birth and sex". Brain Res. Dev. Brain Res. 79 (2): 249–59. doi:10.1016/0165-3806(94)90129-5. PMID 7955323.
- Swaab, Dick; M.A. Hofman (December 1990). "An enlarged suprachiasmatic nucleus in homosexual men". Brain Research. 537 (1–2): 141–148. doi:10.1016/0006-8993(90)90350-K. PMID 2085769.
- Brodie HK, Gartrell N, Doering C, Rhue T (January 1974). "Plasma testosterone levels in heterosexual and homosexual men". Am J Psychiatry. 131 (1): 82–3. doi:10.1176/ajp.131.1.82. PMID 4808435.
- Peter Doerr; et al. (1976). "Further Studies on Sex Hormones in Male Homosexuals". Arch Gen Psychiatry. 33 (5): 611–614. doi:10.1001/archpsyc.1976.01770050063010. PMID 1267577.
- Nick Neave; Meyrav Menaged; David R. Weightman (December 1999). "Sex Differences in Cognition: The Role of Testosterone and Sexual Orientation". Brain and Cognition. 41 (3): 245–262. doi:10.1006/brcg.1999.1125. PMID 10585237.
- Bogaert AF, Hershberger S (June 1999). "The relation between sexual orientation and penile size" (PDF). Arch Sex Behav. 28 (3): 213–21. doi:10.1023/A:1018780108597. PMID 10410197.
- Peplau, Letitia; Mark Huppin (October 2008). "Masculinity, Femininity and the Development of Sexual Orientation in Women". Journal of Gay & Lesbian Mental Health. 12 (1–2): 145–165. doi:10.1300/J529v12n01_09 (inactive 2020-01-22).
- Gobrogge, Kyle L; S. Marc Breedlove; Kelly L. Klump (February 2008). "Genetic and Environmental Influences on 2D;4D Finger Length Ratios: A Study of Monozygotic and Dizygotic Male and Female Twins". Archives of Sexual Behavior. 37 (1): 112–118. doi:10.1007/s10508-007-9272-2. PMID 18074216.
- Dittmann, V; Dilling H. (June 1990). "Chapter V (F) of ICD-10: mental, behavioural and developmental disorders—introduction and overview". Pharmacopsychiatry. 23 (suppl 4): 137–41. doi:10.1055/s-2007-1014552. PMID 2197637.
- Dittmann V, von Cranach M, Eckermann G (1990). "Abnormalities of adult personality and behaviour (section F 6)--results of the ICD-10 field trial". Pharmacopsychiatry. 23 Suppl 4: 170–2. doi:10.1055/s-2007-1014559. PMID 2197643.
- Dittmann, V (1992-08-01). "[ICD-10 in psychiatric diagnosis. The concept and initial practical experiences]". Versicherungsmedizin (in German). 44 (4): 114–9. PMID 1509643.
- 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.
- Hines M, Brook C, Conway GS (February 2004). "Androgen and psychosexual development: core gender identity, sexual orientation and recalled childhood gender role behavior in women and men with congenital adrenal hyperplasia (CAH)". J Sex Res. 41 (1): 75–81. doi:10.1080/00224490409552215. PMID 15216426.
- Wallien, Madeleine; Kenneth J. Zucker; Thomas D. Steensma; Peggy T. Cohen-Kettenis (August 2008). "2D:4D finger-length ratios in children and adults with gender identity disorder". Hormones and Behavior. 54 (3): 450–454. doi:10.1016/j.yhbeh.2008.05.002. PMID 18585715.
- Meyer-Bahlburg, Heino (28 August 2005). "gender identity outcome in female-raised 46,XY persons with penile agenesis, cloacal exstrophy of the bladder, or penile albation". Archives of Sexual Behavior. 34 (4): 423–438. doi:10.1007/s10508-005-4342-9. PMID 16010465.
- Staff, NIEHS. Last Reviewed, June 05, 2013. Endocrine Disruptors
- Weiss, Bernard (2013-12-01). "The Intersection of Neurotoxicology and Endocrine Disruption". Neurotoxicology. 33 (6): 1410–1419. doi:10.1016/j.neuro.2012.05.014. PMC 3458140. PMID 22659293.
- Roselli, C.E.; Stormshak, F. (2009). "Prenatal Programming of Sexual Partner Preference". J. Neuroendocrinol. 21 (4): 359–364. doi:10.1111/j.1365-2826.2009.01828.x. PMC 2668810. PMID 19207819.
- Green, R. (1987). The "sissy boy syndrome" and the development of homosexuality. New Haven, CT: Yale University Press.
- 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" (PDF). 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