Genetics Versus Gay Germ Theory

What can GWAS tell us about gay germ theory?

The last time I talked about the causes of homosexuality, I discussed cultural theories of acquired homosexuality. In line with the theory’s prediction, we found that countries that were more pro-LGBT also had more people watching gay porn. The correlation was .55 and had Google Trends not rounded the data to the nearest percent, the true correlation would have likely been much higher! 

I ended that blog post by noting that the causation could go the other way. Nations with many gay people could become more tolerant. But that begged the question, what other cause could there be for differing proportions of gay people? If it is not a cultural virus, why couldn’t the cause be an actual virus?

Many find the theory offensive (here’s the SPLC crying about it). If discovered that homosexuality was caused by a germ, then it may be accurate to say that homosexuality is a disease. But if we are interested in the truth, we can’t discount ideas because they make us uncomfortable.

What I find most enticing about the theory is that from an evolutionary perspective, gay germs should be frequent in nature. 

Many germs have evolved to manipulate the behaviour of animals to their own advantage. In 1859, Alfred Russell Wallace discovered the zombie-ant-fungus. The parasite controls the ant's mind, making it climb to the top of grass near the nest to spray spores, giving the fungus a sniper’s nest to shoot ffom. Or take rabies. Animals with the disease become aggressive and are more likely to bite other animals spreading the disease. Rabies also causes hydrophobia - fear of water - possibly preventing it from becoming diluted. There are many ways germs can affect behaviour subtly to their advantage. The role of gut bacteria in determining human behaviour is a major mainstream topic of study. 

The parasite Toxoplasma seems to alter attitudes to sexual masochism and submissiveness. Yes - Respectable Science™ is claiming that germs can make you kinky. #TrustTheScience

There would be enormous benefits for germs which can make their hosts gay. If there’s anything we have learnt from Monkeypox and HIV, it’s that homosexuals make great vectors of disease. They very frequently engage in close contact with many people allowing diseases to run riot through the gay community. More generally any germ that encourages increased sexual interaction, wins a natural selection jackpot. Diana Fleischman has gone out on a limb to predict such germs should exist. 

But the strongest arguments for gay germ theory were given by its discoverer - Gregory Cochran. Cochran notes that to survive natural selection, genetic disorders must have massive advantages to outweigh their disadvantages. This problem makes them rare - even in severely malaria-infected regions, sickle anaemia can only reach up to 4% of the population at. More common genetic-related diseases hit in old age after we have produced. By contrast, frequent diseases hitting early in life, and creating large fitness costs are almost certainly caused by environmental factors - recent lifestyle changes (drugs, cigarettes, fatty food) that we have not evolved for, or because of germs that can evolve faster than us.

Homosexuality clearly looks like the latter group of diseases. To the extent that it is genetic, gay genes must provide incredible benefits. Under a gay uncle theory, the disease has to create 2 nephews/nieces for every child not born due to homosexuality. More broadly homosexuality in men is only around 35% heritable. If in a pair of identical twins, one has engaged in same-sex behaviour, the other only has a ~20% chance of engaging in it. The heritability is even lower in women. It is less heritable than political views and less heritable than homophobia. If 65% of the variance is explained by the environment, there is plenty of space for environmental factors. 

I don’t like merely summarising old ground - other people can do it better. so if you are interested in the theory I recommend taking it straight from the horse’s mouth here, here and here. My friend homoheritikus has a great summary here.

Gay germ theory is just over 20 years old. What I want to discuss is what the recent GWAS studies can tell us about the theory. In 2019, Ganna et al. looked for the genes associated with homosexuality. Being of low heritability, they could only find 5 SNPs that were significant. One of their most reported findings was that there was a big genetic correlation between having same-sex relationships and having more opposite-sex partners. 

In a follow-up paper, the authors seized this information to argue that homosexuality was caused by antagonistic pleiotropy. The genes that predispose you to become gay also make you better with the opposite sex. They run a simulation of natural selection, calibrated with the results of their GWAS. Lo and behold their simulations suggested antagonistic pleiotropy really should allow for the substantial prevalence of homosexuality. 

I’m not sure how good their simulation is, but their result certainly seems to make sense. From what I can see, antagonistic pleiotropy tends to be the preferred explanation of the field. 

On the other hand, wouldn’t the gay germ theory lead to the same results? If certain genes predisposed you to ‘catch the disease’, then those genes could either directly increase sexual contact or do so indirectly by giving you the disease. The genetic correlations and simulations are certainly not a nail in the coffin for gay germ theory. Moreover, if gay genes just predispose you to be slutty, it’s not necessary that it would be an evolutionary advantage. Sometimes men can and should have standards. 

But there’s one aspect of the GWAS that has been completely missed - the enrichment analysis. When we find genes associated with a trait, say intelligence, we can then look at which tissues and organs express the genes associated with intelligence. As you’d expect, brain tissues express the intelligence genes, giving us a rather obvious hint regarding where the causal pathways lie. 

In my opinion, gay germ theory makes a clear prediction for enrichment analysis. ‘Gay genes’ should be expressed in areas of the body associated with the immune system, as that determines whether you catch the disease. So what does the enrichment analysis show? Are gay genes expressed in the bone marrow, the spleen and the lymph nodes?

The GWAS could not find any tissue where gay genes were significantly expressed. So the results were only presented in the appendix. However, the spleen and ‘skin sun exposed (lower leg)’ seem to have somewhat low p values. The authors noted that the “top 10% of the genes ranked by t-statistics were considered as brain-tissue specific genes”. Overall, this looks like antagonistic pleiotropy affecting rather than gay germs. 

One thing did shock me though - the lowest p-value was for ‘Cells EBV-transformed lymphocytes’. That refers to white blood cells infected by the Epstein-Barr virus (EBV), a virus Cochran has explicitly named as a suspect. And it is thought that EBV functions by changing gene expression in white blood cells. On the other hand, homosexuals are just far more likely to catch EBV which can also explain this result.

With no significant hits for immune system tissues, the GWAS seems to make gay germ theory less likely. At the moment, antagonistic pleiotropy really does look like the most promising theory. More generally, if homosexuality was caused by a germ we would expect homosexuals to come from poor and low-IQ family environments, which we don’t see. We’d also expect homosexuality to be very common in the poorest, dirtiest, densest countries in the world, but the data on which countries watch gay porn doesn’t seem to show anything like that. 

I love the gay germ theory, it’s clever and theoretically very enticing. But it has been 20 years since it was proposed - nothing new has come out clearly supporting the theory and what has come out doesn’t obviously seem compatible. By contrast, the genetic theory is performing as would be expected.