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. Fetal hormones may be seen as the primary determiner of adult sexual orientation, or a co-factor with genes and/or environmental and social conditions.

Prenatal hormones and sexuality-linked characteristics

Male homosexuality as hyper-masculine

There is evidence of a correlation between sexual orientation and traits that are determined in utero^[1]. Williams et al. (2000) found that finger length ratio, a characteristic controlled by prenatal hormones, is different in people of distinct sexual orientations.[1] 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 of homosexual men was found by Swaab and Hopffman to be larger in homosexual men than in heterosexual men, [2], the suprachiasmatic nucleus is also known to be larger in men than in women.^[2] Gay men have also been shown to have higher levels of circulating androgens ^[3] and larger penises ^[4], on average, than straight men.

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.

Male homosexuality as hypo-masculine

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—was on average more than twice the size in heterosexual men when contrasted to homosexual men. Initially he could not rule out that this may be due to AIDS since all of his homosexual male subjects had died from it before the autopsies were performed. However in 2003 scientists at Oregon State University announced that they had replicated his findings in sheep.

Female homosexuality

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 ^{[5][6][7][8][9]}. An alternative explanation for this effect is that the fact that girls with this condition are born with masculinized external genitalia leads their parents to raise them in a more masculine manner which then influences 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 the causal factor, not parental influence.

See also

• Biology and sexual orientation
• Environment and sexual orientation
• Handedness and sexual orientation
• Pathogenic hypothesis of homosexuality
• David Reimer

References

1. ^ Wilson, G.D. & Rahman, Q (2005) Born Gay: The Psychobiology of Sex Orientation, Peter Owen, London
2. ^ 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. PMID 7955323. 
3. ^ 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. PMID 4808435. http://ajp.psychiatryonline.org/cgi/pmidlookup?view=long&pmid=4808435. 
4. ^ Bogaert AF, Hershberger S (June 1999). "The relation between sexual orientation and penile size". Arch Sex Behav 28 (3): 213–21. PMID 10410197. http://www.kluweronline.com/art.pdf?issn=0004-0002&volume=28&page=213. 
5. ^ Dittmann, V; Dilling H. (1990 Jun). "Chapter V (F) of ICD-10: mental, behavioural and developmental disorders—introduction and overview.". Pharmacopsychiatry 23 (suppl 4): 137–41. PMID 2197637. 
6. ^ Dittmann, V; von Cranach M, Eckermann G. (1990 jun). "Abnormalities of adult personality and behaviour (section F 6)—results of the ICD-10 field trial.". Pharmacopsychiatry 23 (suppl 4): 170–2. PMID 2197643. 
7. ^ Dittmann, V (1992-08-01). "[ICD-10 in psychiatric diagnosis. The concept and initial practical experiences]" (in German). Versicherungsmedizin 44 (4): 114–9. PMID 1509643. 
8. ^ Zucker, KJ; Bradley SJ, Oliver G, Blake J, Fleming S, Hood J. (1996 Dec). "Psychosexual development of women with congenital adrenal hyperplasia.". Horm Behav 30 (4): 300–18. PMID 9047259. 
9. ^ 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. PMID 15216426. 

• 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. 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. 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. PMID 1438641. http://linkinghub.elsevier.com/retrieve/pii/0306-4530(92)90054-B. 
• 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. http://linkinghub.elsevier.com/retrieve/pii/S0149-7634(05)00032-1. 
• Williams TJ, Pepitone ME, Christensen SE, et al. (March 2000). "Finger-length ratios and sexual orientation". Nature 404 (6777): 455–6. doi:10.1038/3500655510.1038/35006555. PMID 10761903. http://msu.edu/~breedsm/pdf/breedlove2000.pdf. 
• 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. http://linkinghub.elsevier.com/retrieve/pii/S0018-506X(96)90038-0. 

External links

• WEBMD: Pointing the Finger at Androgen as a Cause of Homosexuality
• Boston Globe: "What makes people gay?"