I’ve said it before, but it’s probably time to say it again. The most likely explanation for human homosexuality is that it is caused by some pathogen. It’s too common to be mutational pressure (and we don’t see syndromic versions, as we would in that case), it’s not new, identical twins are usually discordant (~75% of the time), and it’s hell on reproductive fitness. There is no way it is adaptive: the helpful gay uncle notion, group selection, compensating advantage in females, etc: these range from impossible to bloody unlikely. It doesn’t exist in most hunter-gatherers: you have to explain what it is you’re even talking about when you ask them. Presumably with diagrams.

As for Freudian explanations, exotic-becomes-erotic, etc: just reading the social-science literature on the subject is enough to make you wonder if the human brain really does exist to cool the blood.

A fair number of the smarter people interested in the subject agree with me. Not that they think it proven, but they agree that it is the only theory out there that makes any evolutionary sense. Bill Hamilton thought it made sense. So does Alan Grafen. Mike Bailey thinks it more likely than any other explanation tendered thus far.

My model – not the only possible model based on a pathogen, but reasonable – leans on a couple of natural examples. One is narcolepsy. We now know that narcolepsy happens when a particular kind of neuron, concentrated in a little region in the hypothalamus, somehow gets zapped. 99% of narcolepsy cases happen in the 25% of the population that has a particular HLA type – which suggests that something, probably a virus, triggers an overenthusiastic immune response that zaps a neuron subpopulation that produce a particular neurotransmitter (called hypocretin or orexin) that regulates appetite and sleep patterns. And it doesn’t do anything else: narcoleptics aren’t stupid. You can compare narcolepsy to type I diabetes or Parkinson’s disease. Suppose there’s a neuron subpopulation that performs a key function in male sexual desire: wipe out that subpopulation, and Bob’s your uncle.

Another is toxoplasma, which we now know changes mouse behavior in ways that increase a mouse’s chance of being devoured by a cat, the definitive host for toxo. Infected mice are attracted to cat urine, while uninfected mice avoid it. In fact, in infected mice, cat urine apparently triggers activity in neural pathways involved in sexual arousal. Microorganisms can reprogram sexual attraction in mammals.

I have had people complain that I’m neglecting the social aspects of homosexuality, what it means, how people think of it. Let me tell you a story. In certain parts of west Africa, boys are expected to start menstruating around age 14. And they do, sort of: you start seeing blood in their urine. When that happens, there’s a big ceremony, everyone says ‘today you are a man’. Whatever. The thing is, that’s about the time they put the boys into the flooded rice fields, where they’re exposed to schistosoma haematobium, which causes urinary schistosomiasis. It’s bad for you: it can impair growth and cognitive development in children, reduces productivity, and is a potent cause of bladder cancer over the long term.

Our explanation of male menstruation as urinary schistosomiasis must undermine these people’s traditional culture. Eliminating schistosomiasis would undermine it even further, just as the rubella vaccine dealt a heavy blow to deaf culture by cutting the number of congenitally deaf children in half. .

Isn’t that just too damn bad.

P.S. The title comes from the coolest response to this hypothesis I’ve ever received, from some inspired fool in New Zealand: “And descending into the most disturbing depths of madness and the most depraved abuse of science yet conceived, we will even be told that homosexuality is caused by a virus or a bacterium. “