Updated, 10/17/15. See below!
In this post, I will review Gregory Cochran’s “gay germ” hypothesis. I wanted to make an index of Cochran’s posts from his and Henry Harpending’s blog West Hunter that discuss it. These posts don’t seem to all show up under the “Homosexuality” category there, and I wanted links to them to be all in one place. So here are the key posts, with a brief synopsis of each post’s major points. In reviewing this hypothesis, I cite an earlier paper co-authored by Cochran on the matter, one which discusses the role pathogens may play in all manner of human diseases and behaviors.
The “gay germ” hypothesis is very much Cochran’s, not mine (I haven’t contributed anything to it, except, perhaps, the name). I just believe that it is very likely correct, and promote it. Here is the list:
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.)
Math is Hard – Rips on E.O. Wilson, and innumerate social scientists in general, who are largely unable to quantitatively evaluate their variously kooky hypotheses (as I’ve said elsewhere, “everything, and I mean everything, can be quantified”). Dishes on Wilson’s “gay uncle” hypothesis as example of these, one of many other nonsensical notions in social science.
Hamilton Rules OK! – Reviews the mathematics of kin altruism (Hamilton’s rule), and again notes why “gay uncle” type altruism doesn’t work.
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.
Biological Determinism – Cochran references yours truly as he talks about the pervasiveness of heredity in all things (i.e., All Human Behavioral Traits are Heritable), with illustrative anecdotes about twins. Notes the role that genes play in infectious diseases, illustrating that they are often a necessary but not sufficient component of the outcome in question – i.e., you must invoke pathogens to explain something like tuberculosis. Such is the case with male homosexuality.
Evolution of Virulence – An in-depth discussion of the evolutionary pressures that lead to the insidiousness of pathogenic organisms. In particularly, notes that since pathogenic organisms’ populations are immensely large and their generation times are so short, they evolve far faster than their hosts could ever hope to. Discusses Trojan Horse pathogens, those that try to hide from their hosts’ defenses by mimicking host molecules, and why these selection on these types of infectious agents tend to favor late-appearing illnesses.
Edit, 10/17/15: [Not Even Yong - Starts off clearly stating that the latest tiny twin study claiming epigenetics was behind homosexuality is bunk. Goes into ways that would be informative in finding the cause of homosexuality. Particularly, looking at the brains of discordant identical twins, not the genomes. Notes that such a study is impossible in current political climate. ***End Edit***]
The case for the gay germ is somewhat indirect, but very strong. Critics often level the charge that there is “no evidence” for Cochran’s hypothesis – i.e., that the offending pathogen has yet to be identified. But the claim that there is “no evidence” isn’t really true; there is in fact plenty of evidence. The facts are certainly consistent with a pathogenic explanation, even if we don’t have the pathogen itself nailed down. But, the most compelling evidence comes in the form of ruling out potential alternative explanations. This itself is a form of evidence. The Sherlock Holmes quote, “when you have eliminated the impossible, whatever remains, however improbable, must be the truth,” is an excellent guiding principle, and is certainly valid here. As I’ve said before, there is something to be said for explanations, that, while maybe not conclusively proven, have the virtue in that they lack meaningful competition. This perhaps one of the clearest examples.
(Republished from JayMan's Blog
by permission of author or representative)