The LGBT Health & Development Program

Biological Characteristics Associated with Homosexuality


Posted on December 5th, by Greg in Featured, Youth. No Comments

Researchers have looked at some interesting physical characteristics to differentiate between gay and straight people. This research has uncovered more findings with men and more needs to be done in the future to look at differences between straight women and lesbians.  The existing studies help us to understand the origins of homosexuality.  Let’s take a closer look at this research.

Are gay men more likely to have a counterclockwise hair whorl?

Research on the direction of men’s hair whorl rotation pattern began with a study in 2004 that found that gay men are more likely to have a counterclockwise hair whorl compared to straight men [1]. However, later studies have not been able to replicate this finding [2, 3].

Are gay men more likely to have older male siblings?

Each older brother increases the chance that a man will be gay. Younger brothers do not seem to have an effect and neither do sisters. In fact, siblings don’t seem to be related to a woman’s sexual orientation at all. But among men, each older brother increases the chance of being gay by about 33% [4].

Is sexual orientation related to differences in handedness?

At first research found that gay men were more likely to be non-right handed compared to straight men [5]. More recently, it has been found that handedness and having older male siblings may be related. Gay men with older brothers are more likely to be right-handed [4]. Gay men with no older male siblings are more likely to be non-right handed.

Is sexual orientation genetic?

Identical twins share 100% of their genes.  Fraternal twins only share 50% of their genes. This makes comparing identical and fraternal twins useful for studying genes. Twin research shows that if one identical twin is gay the other twin is also more likely to be gay. Identical twins are significantly more likely to have the same sexual orientation than fraternal twins [6]. This means that genes are important in deciding sexual orientation.  One study found the heritability of sexual orientation to be 62% [7]. This means that 62% of why some people are gay and others are straight is due to genes. This is higher than handedness, which has a heritability of around 25% [8].

What do these differences between gay and straight individuals tell us about homosexuality?

The higher odds of homosexuality for men with older brothers have been hypothesized to be due to the mother’s body treating additional male fetuses as foreign and releasing chemicals that change the child’s brain [4]. If true, this could also account for the minor physical differences found between gay and straight men [9]. Another possibility linked with this theory, is that there may be genes associated with differences in handedness and physical differences in the brain that may also make the developing fetus vulnerable to the chemicals the mother’s body releases. At this point this is just a theory, not fact, but one with increasing evidence of support.

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References

 1. Klar, A. J. S. (2004) Excess of counterclockwise scalp hair-whorl rotation in homosexual men. Journal of Genetics, 83(3), 251-255.

 2. Hatfield, J. S. (2006). The genetic basis of hair whorl, handedness, and other phenotypes. Medical Hypotheses, 66(4), 708-714.

 3. Schwartz, G., Kim, R. M., Kolundzija, A. B., Rieger, G., & Sanders, A. R. (2010). Biodemographic and physical correlates of sexual orientation in men. Archives of Sexual Behavior, 39(1), 93-109.

 4. Blanchard, R. (2008). Review and theory of handedness, birth order, and homosexuality in men. Laterality, 13(1), 51-70.

 5. Lalumiere, M. L., Blanchard, R., & Zucker, K. J. (2000). Sexual orientation and handedness in men and women: A meta-analysis. Psychological Bulletin, 126(4), 575-592.

 6. Mustanski, B. S., Chivers, M. L., & Bailey, J. M. (2002). A critical review of recent biological research on human sexual orientation. Annual Review of Sex Research, 12, 89-140.

 7. Kendler, K. S., Thornton, L. M., Gilman, S. E., & Kessler, R. C. (2000). Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs. The American Journal of Psychiatry, 157, 1843-1846.

 8. Medland, S. E., Duffy, D. L., Wright, M. J., Geffen, G. M., & Martin, N. G. (2006). Handedness in twins: joint analysis of data from 35 samples. Twin Res Hum Genet, 9(1), 46-53.

 9. Bogaert, A. F., & Skorska, M. (2011). Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review. Frontiers in Neuroendocrinology, 32, 247-254.

 





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