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Why Do People Have Different Sexual Orientations?

A reader submitted the following question: I'd (selfishly) be interested in hearing more about the current research on the emotional/biological/psychological basis of homosexual relationships.

Dear Reader,

You are not alone in wanting to know more about this subject. Students in my Human Sexuality course ask about it every semester!

Unfortunately, we do not fully understand why variability in sexual orientation exists. What I can tell you is that no sound research has found support for the idea that homosexuality is a choice, that it is socially transmitted or “caught,” or that it is a function of having a dominant mother and an absent father. So what’s responsible for it? Over the past few decades, a variety of studies have found that homosexuality appears to have at least some basis in biology. Specifically, researchers have found a number of genetic and biological characteristics that are associated with homosexuality at a rate much higher than could be accounted for by chance.   

For instance, studies have found significantly higher concordance in sexual orientation among identical compared to non-identical twins.1 What this means is that the more genes two people share in common, the more likely they are to have the same sexual orientation. So, identical twins are more likely to have the same sexuality than non-identical twins.

Other studies have found that gay and lesbian individuals are more likely to be left-handed2 and tend to have different finger-length ratios than their heterosexual counterparts,3 both of which are factors thought to be influenced by exposure to prenatal hormones. Specifically, if you compare the index finger (the 2nd digit) to the ring finger (the 4th digit), heterosexual men tend to have index fingers that are shorter than their ring fingers, while heterosexual women tend to show no difference in length between those two digits. Interestingly, the finger-length ratios of lesbians tend to be more similar to heterosexual men, while gay men (but only gay men with at least two or more older brothers) show an even more exaggerated version of the pattern seen in heterosexual men. Confused yet? You’re not alone. On a side note, these same finger length ratios are not only predictive of sexual orientation, but also romantic jealousy (see here)!

We should note that none of these studies provide evidence of a purely genetic basis for homosexuality. For example, not all identical twins have the same sexual orientation. Likewise, not everyone who is left-handed or shows opposite finger-length ratios is gay. This suggests that there are likely to be many factors contributing to one’s sexuality, with biology being just one of them. In fact, one of the strongest predictors of adult homosexuality is seemingly not biological at all: childhood gender nonconformity (i.e., not conforming to social expectations for one’s gender, such as a girl playing with monster trucks or a boy playing with dolls).4 The more extreme a child's gender non-conformity, the greater the likelihood that he or she will later identify as gay. Thus, there is certainly a lot of room for potential psychological and environmental influences and we are learning more every day about how all of these factors might work together to create different sexualities.

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1Dawood, K., Bailey, J. M., & Martin, N. G. (2009). Genetic and environmental influences on sexual orientation. In Y. Kim (Ed.), Handbook of Behavioral Genetics (pp. 269– 280). New York, NY: Springer.

2Lalumière, M. L., Blanchard, R., & Zucker, K. J. (2000). Sexual orientation and handedness in men and women: A meta-analysis. Psychological Bulletin, 126, 575–92

3Rahman, Q., & Wilson, G. (2003). Sexual orientation and the 2nd to 4th finger length ratio: Evidence for organizing effects of sex hormones or developmental instability? Psychoneuroendocrinology, 28, 288–303.

4Rieger, G., Linsenmeier, J. A., Gygax, L., & Bailey, J. M. (2008). Sexual orientation and childhood gender noncomformity: Evidence from home videos. Developmental Psychology, 44, 46-58.

Dr. Justin Lehmiller - Science of Relationships articles | Website/CV
Dr. Lehmiller's research program focuses on how secrecy and stigmatization impact relationship quality and physical and psychological health. He also conducts research on commitment, sexuality, and safer-sex practices.

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Reader Comments (2)

I would be curious to see if there are finger length differences between heterosexual, homosexual, and bi-sexual people. Bi-sexual folks seem to be this middle ground community of people that don't get as much attention, and they might hold the key to understanding differences, or at least understanding the gray zone between two extremes.

June 18, 2011 | Unregistered CommenterNicole A.

To date, it appears that most studies of finger-length ratios have either excluded bisexuals, or have grouped them together with gays and lesbians. Unfortunately, neither of these approaches is particularly helpful because bisexuality is a unique sexual orientation that merits exploration in its own right. You are absolutely right that studying it further might help us to better understand where sexuality comes from and we can only hope it will receive more attention in future research.

June 23, 2011 | Unregistered CommenterJustin J. Lehmiller
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