HOW did human same-sex attraction come to be? At first glance it seems to be an evolutionary paradox. For a trait to evolve, it has to be passed on to children to whom it confers some sort of advantage. But as gay sex, of itself, cannot yield offspring, we should expect
same-sex attraction to go extinct.
Evolutionary biologists have long struggled with this paradox, but my colleagues and I believe that if you come to the puzzle from a different angle, the apparent contradiction disappears. The trick is to recognise the complexity of human sexual activity and sexuality.
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Firstly, same-sex attraction only looks like a paradox if we consider human populations to be made up of two distinct groups: people who are exclusively gay and people who are exclusively straight. But human sexuality isn’t like this.
Every study since the pioneering work of Alfred Kinsey and his colleagues in the 1940s and 50s has backed up the idea that sexuality varies continuously from a majority of people who identify as exclusively straight to a minority of people who identify as exclusively gay. In the middle are a range of people, including those who identify as bisexual, mostly straight or mostly gay.
Acknowledging this spectrum radically changes the evolutionary question. It means that we should be asking how variation in sexuality evolved, not just how same-sex attraction has evolved.
Secondly, the majority of sex, be it gay or straight, isn’t for reproduction. For humans and our chimpanzee and bonobo relatives, sex has a range of social functions that include play, social bonding, affiliation and even barter, conflict resolution, dominance and appeasement. Thinking about the evolution of sex has to consider these social functions as well.
My colleague Brian Hare at Duke University and I proposed a new hypothesis for the evolution of same-sex attraction in a recent paper published in ().
An important aspect of recent human evolution has been selection to be proactively social: prosociality. If survival and reproduction depend on being part of a functional social group (as was the case for early humans and our primate ancestors), individuals that are highly prosocial can rapidly integrate into a group, operate better within a group and have greater mobility between groups. Selection for prosociality has resulted in a whole range of traits for greater social awareness and tolerance, better social communication and reduced aggression. This includes an expanded role for sex in social contexts that include adult social bonding, adult play and conflict resolution. That applies to both gay and straight sex.
Consider bonobos, our closest primate relative. They diverged from chimpanzees about 2 million years ago and as a species are extremely prosocial. They are also, famously, highly sexual, using sex (gay and straight) for a range of social functions. We think something similar occurred in recent human evolution. Selection for increased prosociality increased the frequency and diversity of expression of social sex and increased same-sex attractions.
A recent study showed human sexuality could be influenced by hundreds of genes and there is no single gene for same-sex attraction. Instead, many genes influence variation in sexuality, each acting in small and different ways, which may explain the variation we see in people’s sexuality.
Variation is expected for complex traits involving hundreds of genes: consider human IQ or height, for example. But we don’t feel a need for special evolutionary arguments to explain the existence of very smart or very tall people, they are just one end of spectra of variation.
