There's a new peer-reviewed research paper out this week that purports to show a link between an animal's levels of serotonin—a brain chemical probably best known for its association with happy feelings—and who that animal chooses to be its sex partners.
You're going to see this study in the news, and you're going to hear about it anecdotally for years to come. I can almost guarantee it. Why? Because, in mice, serotonin appears to affect how bisexual a male mouse is willing to be. Specifically, the male mice in this study demonstrated more bisexual tendencies when their serotonin levels were low.
The other thing I can almost guarantee: The results of this study are going to be widely misinterpreted and misused. Luckily, we have Ed Yong—science blogger extraordinaire. On his site, Not Exactly Rocket Science, Ed offers a clear, understandable, and fascinating explanation of what this study is really about, what it proves and doesn't prove, and why the results of this study can't be automatically extrapolated to apply to humans.
[In the new paper] Liu and Jiang cite a study by Milton Wainberg, which they say showed that "SSRIs inhibited compulsive sexual behaviours in homosexual and bisexual men". But Wainberg isn't happy with this description.
His trial tested the effects of SSRIs in gay and bisexual men with compulsive sexual behaviours. The drugs did lower their libido, as well as reducing the frequency of solo sex acts like masturbation. But contrary to what Liu and Jiang write, all of the volunteers, whether they took SSRIs or placebo, showed less compulsive sexual behaviour. More importantly, even though their serotonin levels had gone up, none of the trial's volunteers started having more heterosexual sex.
Results like this make it clear that we must be cautious before extrapolating the results of rodent studies into humans. Serotonin may be a common player in animal nervous systems but its effects can vary from species to species. For example, drugs that affect serotonin levels have very different effects on the sexual behaviour of rabbits and rats.
At most, the results in these studies can tell us something about the biology of sexual preference. In that regard, there does seem to be something in Liu and Jiang's results, and certainly intriguing hints that are worth following up. Problems will only arise if (or perhaps, when) people try to apply the results to cultural debates.