GUEST OPINION: Homosexuality is biological, not just genetic

BY PETER ’06

WHETHER HOMOSEXUALITY is innate, whether it can be changed, and whether it is immoral are three distinct questions that, for various psychological reasons, are often treated as one. And because many people – including policymakers and judges – will inevitably conflate them despite their logical independence, the question of what causes homosexuality remains relevant. Over the last decade, there has been an avalanche of evidence suggesting the existence of a biological substrate to sexual orientation.

First, it is difficult to dispute that genes play some role in the development of homosexuality. Recent studies using twin registries show that the concordance of homosexuality in identical twins is consistently higher than that between fraternal twins. This evidence, combined with molecular genetic studies and research showing that homosexuality runs in families, suggest that genes exert at least some influence on some forms of homosexuality.

But the media’s focus on genes has obscured the dominant biological theory of homosexuality – a theory that doesn’t directly rely on the genetic evidence. Let’s begin with a simple observation: most men are attracted to women and most women are attracted to men. Whatever causes this sex difference is also likely to bear on why a non-negligible portion of the population deviates from it. According to the dominant biological theory, this difference in who we’re attracted to is caused by a corresponding difference in the regions of the brain that control sexual attraction. The blueprint for how this region develops is prenatally determined in the womb by hormones, whose upstream cause is not wholly dependent upon genes. Thus, differences in exposure to prenatal hormones cause differences in sexual attraction. Supporting evidence comes from numerous studies demonstrating differences between gay men and lesbians and their heterosexual counterparts on a laundry list of traits linked to prenatal hormones (e.g. specific cognitive abilities, certain brain structures, and auditory systems).

Two examples will illustrate why genes don’t capture the full story. First, there are non-genetic biological causes of traits. For example, despite the higher concordance of homosexuality in identical versus fraternal twins, there are identical twins who do not share the same sexual orientation. Because they share 100% of their genes, any differences between them are attributable to environment. But “environment” doesn’t equal “social.” For instance, each fetus of an identical twin pair may experience subtly different prenatal hormone environments. One trait that scientists have linked to the prenatal environment (e.g. testosterone) is the ratio of the index finger to ring finger. In identical twin pairs with one lesbian and one heterosexual twin, the lesbian was found to have a ratio indicative of greater exposure to prenatal testosterone than the heterosexual. Identical genes, different prenatal environments, different sexual orientations.

Second, because parental genes randomly combine upon each conception, there is no reason why a couple’s third son, for instance, would independently be more likely to be gay than their first – and yet this is precisely what we find. With each successive son that a mother bears, the greater the chances her next son will be gay (keeping in mind a baseline of 2-3%). This is the fraternal birth order effect. Higher numbers of older brothers translate into a higher probability of male homosexuality. The proposed theory is that when male fetal cells enter a mother’s circulation, her immune system recognizes them as foreign; this triggers antibodies, which enter the fetus’ brain and shift development away from a male-typical pathway (e.g. sexual attraction to females). The strength of this immunization increases with each male fetus, explaining why the chances of male homosexuality increase with each older brother.

Two caveats are instructive. First, the fraternal birth order effect is limited to males: the chances of homosexuality only increase with each older brother, not sister, and there is no comparable effect for lesbians. Second, and quite obviously, not all gay men have older brothers. It is estimated that fraternal birth order accounts for the sexual orientation of 15-29% of gay men.

Both these facts underscore a crucial point: there are different causes of sexual orientation for different people. The shibboleth of equal parts nature and nurture doesn’t always hold true. If homosexuality in fact has multiple etiologies, then the sexual orientation of at least some people may be largely, if not entirely, biological. Moreover, it’s unlikely that we can divine the causes of our own sexual orientation through introspection; no one can close their eyes and remember what the prenatal hormone environment was like. Rather than personal anecdotes, we need empirical research.

While we should logically separate the question of causation from the question of morality, it is plain that many people are unable or unwilling to do so. Perceptions about what cause homosexuality are often divorced from science and married to morality. But to whatever extent possible, our opinions about this must be guided by scientific research.

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