People cannot choose their sexual orientation (Image: EPA/CameraPress)

The government of Uganda has moved to further criminalise homosexual behaviour in that country and has challenged US scientists to provide evidence that sexual orientation is determined before birth. Only then will Ugandan authorities consider a rethink, they claim.

The new law, signed this week by president Yoweri Museveni, includes life sentences for repeat offenders found guilty of “aggravated homosexuality”, such as serial offences.

It has attracted global condemnation from leaders including those in the US and UK, and could threaten aid flowing into the African nation. In response to Museveni’s call for the evidence on sexual orientation, J. Michael Bailey, a professor of psychology at Northwestern University in Evanston, Illinois, gives his take on the current relevant science.


What causes sexual orientation?

We don’t know. At least not at the level of detail that any scientist could acknowledge is satisfying and complete. But that does not mean we know nothing.

Do people choose their sexual orientation?

No. As German philosopher Arthur Schopenhauer understood 200 years ago: “A man can indeed do what he wants, but he cannot want what he wants.” That is, we choose our behaviour, not our desires. Sexual orientation is a pattern of desire, regardless of associated behaviour, and so it is not chosen.

So sexual orientation is not learned?

This seems to be the question the Ugandan leadership is most interested in. They say they worry that homosexuality is socially contagious, that older homosexual individuals recruit younger people. There is considerable evidence against this. Most homosexual people recall that they had homosexual feelings before they had any sexual experience.

Childhood gender nonconformity – femininity in males, masculinity in females – strongly predicts adult homosexuality, and such behaviour is often evident very early, long before children are sexually aware.

The most persuasive evidence comes from rare cases in which children were born male but were surgically reassigned as female for medical reasons. Prenatally they are male; postnatally and socially, they are female. Consistent with their prenatal biology but not their social rearing, as adults they are attracted to women rather than men. If one cannot make a boy attracted to other males by “turning him into a girl”, how likely is any hypothesis that social forces shape male sexual orientation?

Is sexual orientation genetic or environmental?

Studies of identical and fraternal twins raised together – those separated at birth are too rare to have been studied enough – can provide a sense of how much genes and environment matter. Studies have usually shown that identical twins are more similar than fraternal twins in their sexual orientations, which supports the idea of genetic influence.

But the studies with the best sampling techniques – and most trustworthy numbers – have shown that if one twin is homosexual the other usually is heterosexual, even if they are genetically identical. This shows that environment also matters, perhaps even more than genetics. But to scientists the meaning of “environment” is simply “anything not encoded in DNA at birth”. “Environmental” should not be equated with “socially caused”. A trait can have environmental causes and yet be completely inborn.

Have we discovered genes for sexual orientation?

There are no replicated findings of particular genes affecting sexual orientation, but there have been findings of genetic linkage for male sexual orientation. Genetic linkage studies cannot identify particular genes. Rather, they look for segments of chromosomes that contain genes that affect a trait; but the chromosomal segments contain many genes. When two relatives with a given trait – such as pairs of gay brothers – tend to share the same chromosomal segments more than they would be expected to by chance alone, this suggests that one or more genes for the trait are in the chromosomal segments.

Dean Hamer controversially reported in 1993 that there was a segment of the X chromosome (Xq28) that showed linkage to male sexual orientation. There have been failures to replicate this finding. But in a much larger study than any previously published, involving about 400 pairs of gay brothers, our research team, headed by Alan Sanders, a professor of psychiatry at North Shore University Health System in Illinois, has now replicated Hamer’s finding.

In addition, we replicated another finding suggesting that genes on chromosome 8 may also influence male sexual orientation. An article with these findings is currently being considered for publication in a scientific journal.

Are the causes of male and female sexual orientation similar?

We’re not sure. Female and male sexual orientation differ in some ways that they are expressed. Women are more likely than men to experience sexual desire for both sexes, for example. There are also some similarities, such as the childhood gender-nonconformity issue.

Can sexual orientation be changed?

Homosexual behaviour can probably be discouraged by penalising it, especially if the penalties are severe. There is no good evidence that orientation can be changed, however. And remember that changing boys into girls did not change their sexual orientations. So how likely is it that the threat of prison might?

Should we make decisions on the acceptability of homosexuality based on the science of sexual orientation causation?

No. The acceptability of human acts should depend on their consequences rather than their causes. Regardless of what causes sexual orientation, homosexual people cause no more harm than heterosexual people do. It will be quite some time before scientists unite behind a shared understanding of the causes of sexual orientation. But scientists and non-scientists alike can unite today behind a shared vision of human rights that includes equal rights for all people regardless of their sexuality.

J. Michael Bailey is a professor of psychology at Northwestern University in Evanston, Illinois