Tommy Bullen explores the evolutionary puzzle of homosexuality
‘Is someone born gay?’ is a long disputed, perhaps overly-speculated question, and one which I often find people have a ‘belief’ about as though it were a religious or moral issue. People repeatedly give opinions such as ‘of course people are born gay’, ‘it’s down to the parenting’ or even the outdated view ‘people simply choose to be gay’ with very little knowledge of the scientific research surrounding the dispute.
Genetic studies can be applied to homosexuality just as they are to the heritability of intelligence or the predisposition to certain diseases. As it turns out, the question isn’t quite as simple as ‘are we born gay?’ Research suggests sexuality isn’t solely affected by genes or environment but a combination of the two, much like height or weight. There’s certainly no single genetic factor, or a ‘gene for homosexuality’, that’s been found.
To further complicate matters, like most human characteristics, homosexuality isn’t a binary condition. Despite this, the labels ‘straight’ and ‘gay’ are commonly used and research into bisexuality, asexuality, transgenderism and even lesbianism, is lacking relative to male homosexuality.
Twin studies are widely used to determine genetic influences. One such report on gay males found that in 52% of identical twins where one sibling is gay, the other is too. For non-identical twins, the figure is only 22%. From this and other similar studies one can infer that there is a strong genetic factor involved (explaining the higher concordance for identical twins), but that the environment also plays a part (otherwise the figure for identical twins would be 100%).
Research suggests sexuality isn’t solely affected by genes or environment but a combination of the two
Other approaches have looked for specific gene variants associated with homosexuality: several studies carried out in the 1990s found that a region of the X chromosome, Xq28, has a variant that occurs in much higher rates in homosexual men. Although this was the first evidence of specific genes that influence sexual orientation, it wasn’t the elusive ‘gay gene’: it doesn’t determine sexuality, but simply increases one’s likelihood of being gay.
As for the environmental factors, aside from obvious cultural influences, the conditions in the mother’s womb during pregnancy have proven to be very important. In particular, the number of older brothers the mother has carried previously appears to increase the likelihood that the next son is gay – one study showing by 33% with each older brother. It’s thought this is because a mother has an immune response to the foetus’s male proteins, reducing the ‘maleness’ of the foetus, with the effect becoming stronger with each son.
An important question is how homosexuality has withstood evolution. Evolution is driven by reproductive success so one would expect homosexuality to be so disadvantageous that it shouldn’t exist. One theory states that genetic factors increasing feminine behavioural traits in men make them more attractive to women, and so in low levels these genes are advantageous and are maintained by evolution. However, occasionally an individual may receive a particularly large ‘dose’ of these genes by chance, affecting their sexuality.
Another leading theory known as ‘kin-selection’ suggests that in our evolutionary past gay family members made up for the lack of their own children by boosting the reproductive success of their relatives. This could be by providing food to relatives who also carry homosexual genetic factors at low levels, so those genes are indirectly passed on.
The nature vs. nurture argument is evidently an oversimplification of the matter; there are undeniably both genetic and environmental factors involved – and each is probably responsible to different degrees in different people. Perhaps some people are born gay, whereas some people are born with a predisposition to become gay. The picture is even less clear for lesbians and other members of the LGBT+ spectrum, but hopefully future research with ever-advancing genetic technology will continue to shed light on this issue.
Featured image credit: Ludovic Bertron