Literature reviews are a common thing in the world of academic research. They often involve esoteric topics confined to a narrow universe of scholarly interest. Few literature reviews, however, have generated as much controversy as the recently published paper “Sexuality and Gender: Findings from the Biological, Psychological and Social Sciences,” by Lawrence Mayer and Paul McHugh.
Among the issues attracting attention in the Mayer-McHugh paper is the claim that the causes of sexual orientation are poorly understood, but—most importantly—that genetics is not the sole determinant of sexual orientation. The hypothesis that those who have lesbian, gay, or bisexual (LGB) orientations were born that way is found to be inconsistent with the data. Mayer and McHugh write:
There is virtually no evidence that anyone, gay or straight, is “born that way” if that means their sexual orientation was genetically determined. But there is some evidence from the twin studies that certain genetic profiles probably increase the likelihood the person later identifies as gay or engages in same-sex sexual behavior.
Mayer and McHugh reviewed research on potential environmental factors, such as abuse, but did not find the evidence convincing for any specific environmental factor. They suggest additional research on potential environmental influences.
A team led by J. Michael Bailey published a literature review on this topic at about the same time that Mayer and McHugh published theirs. At times making different inferences than Mayer and McHugh, particularly when the research evidence is ambiguous, Bailey et al. also conclude that genetics is not the most important causal factor for sexual orientation:
Based on the evidence from twin studies, we believe that we can already provide a qualified answer to the question “Is sexual orientation genetic?” That answer is: “Probably somewhat genetic, but not mostly so.”
Both of these research teams reached the same conclusion about the role of genetics in the development of sexual orientation by reviewing studies of twin siblings. With twin studies we want to know the concordance rate: If one member of a twin pair has a same-sex orientation, what is the probability that the other twin also has a same-sex orientation? Identical (monozygotic) twins share all of their genes in common. If sexual orientation is entirely due to genetics, or perhaps a combination of genetics and prenatal environment, then the concordance rate for identical twins would be one, or quite close to one. Fraternal (dizygotic) twins have the same number of genes in common as any two, non-identical siblings. If the concordance rate for identical twins is greater than that of fraternal twins, then clearly genetics plays a role. When considering studies that used the best sampling methods, Bailey and his colleagues found an average concordance rate for identical twins of .24 and .15 for fraternal twins. Yes, genetics plays a role in the development of sexual orientation, but the environment plays a larger role.
Those who desire a rigorous understanding of the science of sexual orientation should read both of the recent literature reviews. Unlike the data on the role of genetics, the data on environmental factors is often open to more than one interpretation, so considering multiple perspectives is helpful.
Epigenetics helps explain why the born-that-way hypothesis is false even though genes play a role. The epigenome affects the expression of genes. While our genome is relatively static, our epigenome is influenced by the environment. Thus some identical twins come to look different, behave differently, and have differing risk for psychological disorders. Each twin has different environmental experiences. The differing epigenomes result in different expressions of their common genes. Even if a “gay gene” exists, the born-that-way hypothesis would likely be false because interactions with the environment could affect the expression of the gay gene.
The born-that-way hypothesis is important to people on all sides. Members of the LGB community and their allies often believe that a stronger case can be made for societal embrace of the LGB lifestyle if sexual orientation is caused by genetics. Supporters of traditional morality may also feel more comfortable disapproving of behavior when they believe the behavior does not have a biological foundation.
All complex human behavior has a biological foundation. For example, sociobiology suggests that males are hard wired with a propensity to mate with many partners. Even if males are born this way and it is 100 percent due to biology, it does not follow that mating with many partners is moral. The degree to which any behavior is genetically based has no bearing on its morality, nor its benefit to society. We have considerable control over our biological predispositions; civilization would not be possible otherwise.
The nature vs. nurture question has been, in a sense, settled; the answer is (to some degree) a matter of both. Because the answer involves both, those who argue for the virtue, or lack thereof, of any behavior need to always consider legal, societal, or religious claims rather than strictly biological claims.