Gay times
If there is a biological basis for same-sex attraction, there should be genetic factors influencing sexuality. But if there is a genetic component to homosexuality, it would seem to conflict with evolutionary theory. Genes favouring homosexuality should reduce the likelihood of mating, and so they should become less common in the next generation, eventually being lost altogether.
But homosexuality has been a feature of human society for many centuries, and it may well have a biological basis. How can this paradox be solved?
Evolutionary angles
There are several ways in which genes favouring homosexuality might be passed on because they provide some kind of selective advantage.
For example, suppose there is a version of a gene that makes someone more likely to be homosexual. If a person inherits two copies of this gene, he or she may be highly likely to be homosexual and unlikely to reproduce and pass on genes. But having one copy of this gene might be an advantage – perhaps making someone more attractive to a wider range of potential mates, or a more nurturing individual whose children are more likely to survive. This would promote the gene's survival, and balance the effect seen in people with two copies of the gene.
This mechanism is seen in nature. For example, genes that cause inherited diseases such as sickle cell disease have been maintained in this way – two copies of the sickle cell gene cause serious disease but one copy protects against malaria.
In another model, benefits are obtained not by the person with the gene(s) affecting sexuality but by close relatives. Non-reproducing members of a family might make it more likely that a family group will prosper, because they have extra help with childcare or bring in extra food. The genes in the group as a whole will prosper, including those favouring homosexuality.
This does not mean, however, that there is a 'gay gene'. Almost certainly, an aspect of human behaviour as complex as sexuality will be influenced by many factors, some genetic, some environmental.
A study in the 1990s famously identified a region of a chromosome that seemed to contain a gene favouring homosexuality. But these findings have never been repeated and no specific gene linked to homosexuality has ever been identified in humans.
Biological differences
Some other biological differences have been seen between homosexual and heterosexual people, but the full extent of these, and their significance, is uncertain. In general, the differences are very small and are not seen in all studies.
For example, there are suggestions that relative finger lengths are an indicator of sexual orientation, but the evidence is contradictory. The finger length relationship has been put down to the influence of male hormones on the baby growing in the womb. There is evidence that this is important: for example, women with congenital adrenal hyperplasia, who are exposed to high levels of male hormones before birth, are less likely than other women to be exclusively heterosexual.
Another correlation has been proposed between sexual orientation and both the direction of hair whorl rotation (clockwise or anticlockwise) and handedness (the two traits are actually defined by the same biological mechanism).
There is also evidence that having older brothers increases the likelihood that a boy will be homosexual. This could be due to changes in the womb environment caused by the earlier pregnancies.
One point to emphasise about such correlations is that they are statistical associations and not predictive: being left-handed, or having elder brothers, does not mean that someone will automatically be homosexual – just slightly more likely.
There is some evidence that the brains of homosexual males may differ from those of heterosexual males, but this is far from certain. Perhaps more crucially, even if differences do exist, nothing is known about their origins – whether they are genetically defined, reflect someone's early life history, or are actually a response to homosexuality rather than its cause.
The animal world
Although comparisons between humans and animals should be made with caution, they can provide insight into possible biological mechanisms.
What is certain is that homosexuality is not restricted to humans. Same-sex relationships have been seen in many different species – both long-term bonding and short-term sexual encounters. Many male penguins, for example, bond for life – sometimes with other males, turning their beaks up at the chance to mate with receptive females.
Rams that direct their sexual attentions towards other rams have been particularly well studied. There are suggestions that the behaviour of these animals, which account for about 1 in 10 male sheep, is linked to a particular area of the brain, which is much smaller in female sheep than rams. In male-oriented rams, though, it is the same size as in ewes.
In addition, the levels of an enzyme involved in sex hormone metabolism (aromatase) are different in areas of the brain known to be important in sexual behaviours, providing a possible link between hormones and brain and behaviour.
Fly sexuality
Some of the most striking recent work on sexual orientation has been carried out on the fruit fly, Drosophila.
Fruit flies have an elaborate courtship ritual, in which males dance and pursue females in order to persuade them to mate. Remarkably, these male- and female-specific roles depend on one gene, known as 'fruitless' (male flies lacking this gene show no interest in females, do not mate and so do not have offspring – hence 'fruitless').
There are actually two forms of the 'fruitless' protein, a female form and a male form. They are produced from the same gene but its messenger RNA is processed differently in males and females, so different proteins are made in the different sexes. If a female fly is genetically engineered to make the male form, its behaviour is transformed – it courts and tries to mate with females.
Very recently, it has been discovered that particular brain structures are dependent on the form of fruitless protein (male or female) that a fly makes. So, in flies at least, there is a 'male brain' and a 'female brain' controlling sexual behaviour.
Does it matter?
Homosexuality is a highly charged issue. In the UK, it is now illegal in most circumstances to discriminate against someone on the basis of their sexuality, but homosexual acts were against the law until 1967 (and remain so in a number of countries). Many religious leaders hold strong views about homosexuality, and homosexual people face considerable negative attitudes, discrimination and even violence.
The argument used to be that homosexuality was 'learned' or an active choice, and so could be 'unlearned' (or should be punished). Pro-homosexual campaigners supported biological explanations, which implied that homosexuality was something people were 'born with' and was therefore part of the normal spectrum of human behaviour.
Scientific research can shed light on this innate versus 'lifestyle' choice argument. Although the exact mechanisms are far from clear, the evidence suggests that homosexual behaviour has a grounding in someone's biology and is not a simple matter of choice. Its widespread existence in nature argues against the view that it is 'unnatural'.
Ultimately, does it matter anyway? Attitudes can accommodate either explanation. Someone against homosexuality on principle, for example, would liken a genetic influence to a genetic disorder, a deviation from a supposed ideal.
Some people have argued that homosexuality is such a contentious issue that studies aimed at improving its biological understanding should not be carried out. Yet sexuality is an important aspect of human biology, so is surely a legitimate area of study. Attitudes may not be changed much by better biological understanding, but at the very least scientific findings can be used to dismantle damaging myths or counter inaccurate information.