Homosexuality refers the quality or characteristic of being romantically or sexually attracted to the members of the same sex. Rahman and Wilson (2003) reported that anywhere between 2-5% men label themselves as exclusively homosexual, whereas 1-2% of women label themselves as exclusively homosexual. This means homosexuality represents an albeit small but a significant minority phenotype in humans. Bailey, Dunne and Martin (2000) showed sexual orientation being more bimodal in men, meaning men are more likely to be exclusively heterosexual or homosexual, whereas women are more variable and have higher degrees of bisexuality. Homosexual men have also reported that they can recall themselves being more feminine during childhood whereas homosexual women have reported to be more masculine during childhood (Zucker and Bradley, 1995). There are a number of causes to homosexuality such as the genetic and heritability explanation, evolutionary explanation, neurodevelopmental explanation and to a lesser extent, a psychosocial explanation.
The heritability explanation shows that sexual orientation aggregates in families. For example, Pillard and Weinrich (1986) suggested that homosexual men are more likely to have homosexual brothers than heterosexual men. Twin studies showed that there is a 50% of identical twins to both be gay and 19% chance for non-identical twins to both be gay. With respect to gay genes, there is no one gay gene that can cause homosexuality, but multiple genes. Majority of the literature in this area point towards the gene marker for homosexuality on an X-linked Xq28 gene. This in turn affects the development and function of gonadal steroid receptors in the brain (Saifi & Chandra, 1999) Since this is a sex-linked X gene, which comes from the mother in gay men, these men are more likely to have maternal gay uncles and gay cousins from the mother’s side (Turner, 1995).
There are many evolutionary theories that suggest that homosexuality was favored in the past somehow, which is why it still persists in today’s world. One such theory is proposed by Rahman and Wilson (2003).
There was evidence that early hominids (our common ancestors) would engage in aggressive and fatal inter-group sex which led to reduced individual survival. Then through mutation, ancestral males that were more feminine in behavior and bisexual in sexual preference were produced. These increased and maintained same-sex alliance. Ancestral females were attracted to these more feminine hominids who displayed more loyalty, kindness, and reduced aggression. When these bisexual male hominids had sexual intercourse with females, they produced offsprings who were also similar in behaviour, i.e., bisexual males with more feminine traits (and thus more feminizing alleles). This strategy allowed “exclusively gay alleles” to invade the population over time. Similar pattern is observed in homosexual female hominids who had more masculinizing alleles, which maintained intra-female alliances and increased female aggression resulting in greater care and protection for the young.
Additionally, there exists evidence which shows how the neuroanatomy of the brain in heterosexuals versus homosexuals differ. For example, Swaab and Hofman (1990) showed that the supra-chiasmatic nucleus (SCN), a region involved in circadian rhythms and sexual behaviour is larger and more elongated in homosexual men than in heterosexual men. This shape is similar to that of women. LeVay (1991) reported another region in the area of hypothalamus, the third interstitial nucleus of the anterior hypothalamus (INAH-3), was smaller in homosexual men than in heterosexual men, similar pattern is observed in women. Finally, Allen and Gorski (1992) reported the midsagittal plane of the anterior commissure (AC) to be larger in homosexual men than in heterosexual men, again a similar pattern is observed in women. Whereas the neuropsychological evidence shows that males perform better in mental rotation tasks while females perform better in verbal fluency tasks. However, homosexual men perform better in verbal fluency tasks and poorly on mental rotation tasks (Willmot & Brierly, 1984). The same goes with visuomotor targeting tasks, where homosexual men and heterosexual women display similar accuracy levels when conducting this test, which were lower than heterosexual men. On the other hand, homosexual women and heterosexual men showed higher and similar accuracy levels suggesting similarities between heterosexual men and homosexual women.
The psychosocial explanation of homosexuality, i.e., evidence for parental influences on sexual orientation is minimal. For example, Bailey et al (1995) showed that over 90% of sons of gay fathers are heterosexual and Golombok and Taskers (1996) reported similar rates of adults raised in lesbian households. In both studies, the amount of time spent living with homosexual parents did not correlate with sexual orientation, thus environmental transmission is supported. However, one theory which has received some support is by Bem (1996) which is that because young boys who are gay are often gender non-conforming, they would spend more time with girls. This might make them see other boys in a more “exotic” sense and this can develop into eroticism during puberty. Nevertheless, this theory is very simplistic and does not incorporate individual differences or why young boys are gender non-conforming in the first place.
Allen, L. S., & Gorski, R. A. (1992). Sexual orientation and the size of the anterior commissure in the human brain. Proceedings of the National Academy of Sciences, 89, 7199-7202.
Bailey, J. M., Dunne, M. P., & Martin, N. G. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology, 78, 524-536.
Bailey, J. M., & Zucker, K. J. (1995). Childhood sex-typed behaviour and sexual orientation: a conceptual analysis and quantitative review. Developmental Psychology, 31, 43-55.
Bem, D. J. (1996). Exotic becomes erotic: a developmental theory of sexual orientation: Psychological Review, 103, 320-335.
Golombok, S., & Tasker, F. (1996). Do parents influence the sexual orientation of their children? Findings from a longitudinal study of lesbian families. Developmental Psychology, 32, 3-11.
LeVay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253, 1034-1037.
Pillard, R. C., & Weinrich, J. D. (1986). Evidence of familial nature of male homosexuality. Archives of General Psychiatry, 43, 801-812.
Rahman, Q., Wilson, G. D. (2003). Born gay? The psychobiology of human sexual orientation. Personality and Individual Differences, 34, 1337-1382.
Saifi, G. M., & Chandra, H. S. (1999). An apparent excess of sex and reproduction related genes on the human X chromosome. Proceedings of the Royal Society of London, Series B: Biological Sciences, 266, 203-209.
Swaab, D. F., & Hofman, M. A. (1995). Sexual differentiation of the human hypothalamus in relation to gender and sexual orientation. Trends in Neurosciences, 18, 264-270.
Turner, W. J. (1995). Homosexuality, Type 1: In Xq28 phenomenon. Archives of sexual behavior, 24, 109-134.
Willmott, M., & Brierley, H. (1984). Cognitive characteristics and homosexuality. Archives of sexual behavior, 13, 311-319.
Zucker, K. J., & Bradley, S. J. (1995). Gender identity disorder and psychosexual problems in children and adolescents. New York: Guildford Press.