Male homosexuality has stumped evolutionary biologists and psychologists for decades. According to evolutionary theory, the highest goal is to pass on one's genes, yet homosexual males will father no direct offspring without the help of surrogates. Moreover, homosexuality seems to be at least partially heritable and is relatively common in developed societies. Why would such an evolutionarily costly trait be so prevalent?

Past studies have suggested that genes related to homosexuality might confer a mating advantage to heterosexuals expressing them, and that female relatives of homosexual males may receive a boost to fecundity. Evidence supporting those notions remains sparse, however.

A study published in August to PLoS ONE provides support for the former contention. Lead author Julien Barthes, along with his counterparts Pierre-André Crochet and Michel Raymond, suggest that a sexually-transmitted gene on the X chromosome may yield higher femininity or attractiveness in women, and thus signal increased levels of fertility. But, as a side effect, when this gene is expressed in a male on his single X chromosome, it can increase the odds that he will be homosexual. The researchers further suggest that social stratification in societies exacerbates the expression of this gene, as women expressing it may be able to rise to a higher social class, possibly by attracting powerful or affluent mates.

To test their hypothesis, the authors conducted an exhaustive review of the archaeological record for evidence of homosexuality in human prehistory. They also perused anthropological accounts to determine how widespread homosexuality is across societies worldwide. They then assessed levels of social stratification in 92 societies (shown below) across the globe and examined how that stratification correlated to the probability that male homosexuality is present.

The researchers could find no conclusive evidence that homosexual behavior was present in human prehistory, before the rise of organized societies. Moreover, their anthropological search revealed that homosexuality does not appear to be a universal trait. In some societies, particularly those that are smaller and more isolated, homosexuality is entirely absent. For example, the Aka people of the Central African Republic don't even have a word to describe same-sex sexual interaction. It is literally unheard of. Finally, the authors did indeed find a link between social stratification and homosexuality. The more highly stratified a society was -- by rank, wealth, or power, for example -- the greater the chance that male homosexuality was present.

The authors' hypothesis is an intriguing one, and would seem to suggest that wealth inequality, which is a key driver of social stratification, may indirectly contribute to male homosexuality. The theory is a fairly novel notion, however, and will need much more evidential support before it can be taken too seriously.

To that end, Barthes and his co-authors invite others to analyze their work.

"Each step toward a better understanding of the evolution and spread of male homosexual preference among humans would contribute to a constructive social debate."

Source: Barthes J, Crochet P-A, Raymond M (2015) Male Homosexual Preference: Where, When, Why? PLoS ONE 10(8): e0134817. doi:10.1371/journal.pone.0134817

(Image: AP)