What determines sexual orientation?

Earlier this year, a project backed by the Wellcome Trust set out to find “the most important question about life that science needs to answer”. After much consultation with scientists and the public, the panel selected the question: “is sexuality genetic?”

Indeed, the puzzle of what causes homosexuality is one which scientists are used to being probed about. However, as Dr Alan Sanders, a researcher who specialises in the determinants of sexual orientation, describes; “you can think of [the factors] contributing to heterosexuality as much as you can think of [them] contributing to homosexuality.”

His research, published in February of this year, claimed the discovery of genes linked to male sexual orientation, and caused much controversy.

This is understandable – the causes of sexual orientation are not only contentious within the scientific community, but they are also important politically. The dangers of such research is clear; The Daily Mail once reported on a similar study with the headline “Abortion hope after ‘gay genes’ findings”, while The New York Times suggested that people are either “Straight, Gay or Lying” when reporting on a study which failed to find evidence for the existence of bisexuality.

Of course, nobody should need to rely on scientific evidence in order to justify their most intimate relationships, and the determinants of sexual orientation should be irrelevant to liberation and equality. But what is the scientific consensus on the causes of diversity in sexual orientation? And can a “gay gene” even exist – does homosexual behaviour not contradict the very concept of evolution via natural selection?

Current scientific opinion suggests not – one notable “gay uncle” hypothesis proposes that individuals who don’t have children aid the survival of other individuals in their family. Hence, the presence of genes linked to homosexuality within a family’s DNA could potentially increase the likelihood of their offspring surviving and passing on this DNA. Alternatively, it’s been suggested that genes linked with homosexuality in men may act also to make women more fertile, explaining the conservation of this genetic material throughout evolution.

Indeed, the genetic material associated with male homosexuality is located on the X chromosome which is passed from men to women, suggesting that such an explanation is plausible. Nonetheless, the data from this single study only accounts for 40 per cent of the chance of a man of being homosexual. In fact, any genetic indicator is a lot more complex than a single “gay gene” – this particular segment of DNA was neither sufficient nor necessary to make men gay.

Similarly, it is possible that exposure to hormones in the womb influences an individual’s sexual orientation. Although the development of genitalia has clear genetic causes (the “sex-determining region” of DNA on the Y chromosome), the development of male and female brains is thought to be determined by exposure to sex hormones such as testosterone. Moreover, the determination of the genitalia and brain development occur at different intervals during pregnancy, suggesting that they could be influenced independently, and result in diversity in sexual orientation.

However, many of these suggestions lack evidence, and it’s possible that estimations about biological determinants of sexual orientation have been exaggerated. For instance, the claim that over 400 species of animals exhibit homosexuality is perhaps overstated – very few show long-term preference for members of the same sex.

It’s also important to consider how social effects might influence sexual orientation. This is perhaps best exemplified by the Papua New Guinean Sambia tribe, who believe that all men transition through an age of homosexuality before becoming heterosexual later in life. Indeed, researchers found that individuals’ sexual orientation did change with age according to commonly accepted metrics such as the Kinsey scale. Nonetheless, any specific social causes are likely to be extremely complex and difficult to identify.

Furthermore, given the taboo that has traditionally surrounded homosexuality, studying the social aspects of sexual orientation is inherently problematic. Indeed, the idea of a person being homosexual is a relatively modern belief – homosexuality only entered common usage as a term to describe an individual (rather than a behaviour) in the late 19th century, when the medical and judicial communities started using the description to diagnose and convict those involved in homosexual behaviour.

Like all of science, we should approach the debate about the causes of sexual orientation with scepticism and open-mindedness. Most important, however, is that we don’t allow science to be manipulated for political reasons; sexual orientation is not a problem to be corrected. Rather, it’s a healthy aspect of human nature which scientists are interested to understand.

This article was originally published as a Bang! Science Magazine feature.

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