The cause may also be
a pathogen that alters its host’s sexual orientation in order to enhance its chances of spreading to other hosts. This is the “gay germ” theory proposed by Greg Cochran (Cochran et al., 2000). It’s interesting, and there are certainly precedents for this kind of psychological manipulation … from
zombie ants to
rats losing their fear of cats. 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.
Greg Cochran’s “Gay Germ” Hypothesis – An Exercise in the Power of Germs
Depths of Madness - The first post in the series. Here Cochran lays out the basic case, and notes key facts that point to the pathogen (e.g., high rate of discordance between identical twins, the evolutionary maladaptiveness of obligate male homosexuality and the paradox of how it could have become so common, how pathogens can affect brain function)
Paternal Age and Homosexuality – Explains why genetic load (burden of accumulated deleterious mutation) can’t explain the existence of male homosexuality, especially at its relatively high prevalence. Notes that unlike afflictions that likely are caused by genetic load, there doesn’t seem to be increased incidence of homosexuality in the children of older fathers.
Group Selection (and homosexuality) – Why group selection (multi-level selection) of the type proposed by E.O. Wilson doesn’t work in general and why it certainly can’t explain male homosexuality. Debunks the “gay uncle” hypothesis.
Homosexuality, epigenetics, and zebras – Why developmental noise, including in-utero epigenetic modifications, cannot explain male homosexuality (in short, natural selection has a strong incentive to prevent low-fitness phenotypes from manifesting, so such congenital defects are all rare).
Heads exploding – The introspective post. Appropriately titled, Cochran invites discussion on the social and scientific
consequences of nailing down the biological cause of homosexuality – any cause, regardless if that turned out to be the pathogen as he suspects. (As we’ve seen, public discussion of any biological determinant of homosexuality will have consequences, and as I’ve discussed, not necessarily good ones.)
Not Final! – Key post where Cochran reviews the case for the gay germ, demonstrating the unworkability of all the alternatives. Shows how processes of elimination (essentially, the
reductio ad absurdum) can sometime be a useful method of getting at the truth. Excludes ideas such heterozygote advantage (requires very strong selective pressure for advantage – but nonetheless ruled out by GWAS), sexual antagonistic selection (i.e., benefit to females but costly to males – also ruled out by GWAS), group selection (impossible, and no evidence anyway). Explains the how immune complexes generated by molecular mimicry can cause damage to specific tissues not infected by the pathogen. Notes the ubiquitous impact of genes, but the often highly indirect nature of this impact, which may explain the low but non-zero heritability of homosexuality. And notes that pathogens are often responsible for common fitness-reducing syndromes.