Sexuality - Nature or Nurture?
How Much Are Sexual Desires Pre-Determined Before Birth?
Sexuality is one of the most discussed and controversial topics in existence. It seems that virtually everyday, one will read and hear in the media about someone’s beliefs about human sexual relations, feelings and fears.
It is in constant debate whether it is purely genetically ‘intended’ that one grows to become heterosexual, simply to pass on genes and further our families through the generations. Or perhaps it is less simple than this, and heterosexuality is just something individuals must learn to accept as they grow.
Hair Whorls and Sexual Orientation
Evidence towards sexuality arising from nature rather than nurture comes from a study by Klar (2004) investigating hair-whorl rotation.
A hair whorl is defined as a patch of hair which grows in the opposite direction to the majority of the hair. A relatively rare counter clockwise hair whorl rotation was found in 8.2 percent of the heterosexual participants and 29.8 percent of the homosexual participants.
This is very interesting for scientists not only because it may show some kind of genetic influence that may play in a role in sexual orientation, but also hair whorls have been associated the brain development, suggesting the development of the brain may have been influenced by certain genes and influence the sexual orientation of the participants.
The Development of the Brain and Sexuality
Evidence towards brain development influencing sexuality has come in many forms, however some of the most interesting and intriguing evidence comes from investigating the specific area of the brain known as the hypothalamus (Levay, 1991).
Measurements of the volume of four different cells groups in the anterior hypothalamus (referred to as the interstitial nuclei (INAH), 1, 2, 3 and 4). It was found that volumes of the INAH 3 were twice as large in heterosexual male participants than in homosexual males, and female participants. The hypothalamus area of the brain is linked with the regulation of sexual activity. Therefore this evidence directly suggests a biological basis for homosexuality, and therefore supports the claim that sexual orientation emerges through nature and not nurture.
Handedness, which is well known as a behavioural marker of early neurological development (as one brain hemisphere may develop faster than another therefore creating hand preference), has also been associated with sexuality.
A meta-analysis, performed by Lalumiere, Blanchard & Zucker (2000), of studies investigating handedness and sexuality, using a complete sample of 16,423 heterosexual participants and 6,987 homosexual participants found results showing that homosexual participants had 39 percent greater odds of being ‘non-right handed.’ This suggests that the early development of the brain plays a role in later sexuality, therefore suggesting that heterosexuality has a genetic basis.
Sociological and ‘Nurture’ Influences
However, some believe that surely there must be some influence from up-bringing and childhood experience that affects one's sexual orientation.
A study (Bogaert, Blanchard & Crosthwait, 2007). investigating whether the presence of older brothers influenced sexual preference was performed using a sample 302 homosexual men, each individually matched with a heterosexual counterpart born in the same year. The study found that as the number of brothers in family increased, the rate of homosexuality increased. The results go as far to suggest that each additional brother an individual has increases the likelihood of homosexuality by 33 percent.
Although this could be taken to suggest that social upbringing and nurture must have an influence of sexuality, there could well be an opposite explanation.
The researchers suggest this finding could arise through a change in the mother’s immune system. A male foetus provokes a unique internal reaction in the immune system. If this reaction occurs several times, it may influence levels of hormones present and therefore could show evidence for a biological cause for sexuality.
Homosexual Adoptive Parents
There has been yet more controversy about the parental care offered by homosexual couples adopting children. However a study (Golombok & Tasker, 1996) investigated the sexual persuasions of the grown children of 25 lesbian parents. The nurture argument would suggest that they should surely be homosexual themselves, as they have grown up with it being the ‘everyday norm.' However results from this study show that, although the children brought up with lesbian parents are more open to same-sex relationships, the vast majority of children grew up to be heterosexual, with no significant increase from the general population.
This article has only briefly skimmed the surface of the seas of research and reports on sexuality. However even from these few studies it seems that sexuality may not be as deeply influenced by up bringing and nurture as was previously believed.
Perhaps Tennessee William’s Blanche Dubois is correct in A Streetcar Named Desire, with the quote: "What is straight? A line can be straight, or a street, but the human heart; oh no, it's curved like a road through mountains." Perhaps humans are slaves to their own unshakable predisposed desires. Like the famed streetcar, the debate will keep running, seemingly circular.
References
- Bogaert, A.., Blanchard, R., Crosthwait, L. (2007). Interaction of birth order, handedness, and sexual orientation in the kinsey interview data. Behavioural Neuroscience.121. 845-853.
- Golombok, S., Tasker, S. (1996). Do parents influence the sexual orientation of their children? Findings from a longitudinal study of lesbian families. Developmental Psychology. 32. 3-11.
- Klar, A. (2004). Excess of counterclockwise scalp hair-whorl rotation in homosexual men83. 251-255.. Journal of Genetics.
- Lalumiere, M,. Blanchard, R,. Zucker, K,. (2000) Sexual orientation and handedness in men and women: A meta-analysis. Psychological Bulletin. 126. 575-592.
- Levay, S. (1991). A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men. Science. 253. 1034-1037.
Brando in A Street Car Named Desire (1947)
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