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Gregory Cochran's "gay germ" hypothesis posits that male homosexuality is likely caused by a germ.[1]

- The low hereditability of homosexuality (0.22) [2]
- The absence of homosexuality in hunter-gatherer populations
- The relatively high frequency of homosexuality (~4% in the West, and growing)
- The lack of any plausible evolutionary explanation.

A male lack of sexual interest in females would be an evolutionary penalty. If it were genetic, the trait would have been selected out long ago because of a lack of reproductive fitness. Also, the frequency is too high to be caused by a mutation.

There are various examples of infectious agents altering behavior: [3]

- Rabies causes dogs to be more aggressive in the final stage of the disease.
- Influenza causes humans to be more social in the days prior to the noticeable effects.
- Toxoplasma (see below) causes rats to be (sexually) attracted to cat urine;

Consider that there is already a known similar germ, Toxoplasmosis:
A contagion by the microscopic parasite "Toxoplasma gondii" that is thought to be able to infect all warm blooded animals (including humans), but particularly infects rats, rewires their brain to not only stop fearing cats (a natural predator), but also become sexually attracted to cat urine. The parasite does this because it (Toxoplasma gondii) can only reproduce inside of cat guts, so it changes the host-rat's behavior to get the rat eaten by cats so that the parasite can reproduce (to the detriment of the rat). Stated again for emphasis, it's already proven that microorganisms can reprogram warm-blooded animals' sexual attraction. And it is already proven that humans are not immune to this specific microorganism (we do not know all of its effects in humans yet).

Please see the bottom of this page for the many concerning studies regarding said Toxoplasma gondii.

Given that there are many more examples of behavior-altering agents (mostly viruses), this leads to the conclusion that a pathogen changing sexual orientation is far from impossible. It would explain the frequency in Western populations and the absence in hunter-gatherers.

Toxoplasma gondii is a parasite. According to at least one scientific study, homosexual males are fightiningly more likely to have parasites than heterosexuals (~70% vs 16%).[4]

Furthermore, there is the interesting phenomenon of homophobia. Homophobia isn't an actual phobia, but an aversion of gays. Interestingly, homophobia actually is heritable, much more so than homosexuality (~0.5).[5] If it is heritable, what purpose does it serve? In particular, what purpose does it serve if homosexuality is caused by a pathogen?

Research has been done on the nature of homophobia, and this research has found that it exists strongest with regard to homosexuals' contact with children. A subjective 'degree of discomfort' was found much higher when confronted with a scenario of gay people who hold a profession that involved contact with children (such as pediatrician or teacher) than with gay people who were e.g. lawyers or construction workers. A second study showed that for different types of medical doctors, a high degree of contact with children (e.g. child psychiatrist) induced more discomfort than an invasive procedure (e.g. brain surgeon) but only for gay doctors. Two more studies clearly suggested that homophobia is aimed at preventing children, particularly younger children, from being exposed to homosexuals.[6]
Is it not plausible that homophobia is aimed at preventing the spread of the pathogen that causes homosexuality?
Cochran asserts that it’s not likely that the pathogen is spread by homosexual sex. How the putative pathogen is spread is not clear at this time. Even less clear is whether there is a “critical period” for this infection to affect sexual orientation. It’s also not clear how prevalent the pathogen is in the population, or what percentage of infected individuals become homosexual.
Regardless of how the pathogen is spread, it likely that extended periods of fairly close contact with an infected individual is more likely to result in transmission of the infectious agent. If there is a critical period of time, say some time in childhood, for an infection to result in sexual orientation being altered, the evolutionary purpose of homophobia starts to become more apparent.

Indeed, the existence of homophobia may stand as pretty good evidence in support of Greg Cochran’s pathogenic hypothesis for homosexuality. Indeed, for as many have pointed out, homosexuals are otherwise harmless and indeed perhaps beneficial to other males in the group (reduced competition for females, for example). That gays are nonetheless hated seems hard to explain otherwise.

One possible way to test Cochran’s hypothesis is to observe if there is a higher incidence of homosexuality among the adopted children of homosexuals. Using adoptees controls for heredity, and presumably, sharing a household should give plenty of opportunity for the pathogen to be passed on to the adopted child. Ideally, the study should look only at children adopted very young, from strangers (to minimize selection bias among the adopted children). If higher rates of homosexuality were observed among these adopted children, it would point to some environmental factor – the most likely being infection.

The low, but non-zero heritability of male homosexuality may be indicative of some sort of genetic susceptibility to the pathogen, perhaps through weakened defenses or a vulnerable neural architecture.

If homosexuality is indeed caused by an infectious agent, it would have broad implications for society, particularly the prospect of being able to prevent homosexuality (perhaps through a vaccine) if one desires it.


[1] Blog of Gregory Cochran & Henry Harpending: https://westhunt.wordpress.com/2012/02/16/depths-of-madness/
[2] Study: Whitehead, Neil. (2011). Neither genes nor choice: Same-sex attraction is mostly a unique reaction to environmental factors. Journal of Human Sexuality. 3. 81-114.
[3] http://www.theatlantic.com/magazine/archive/2012/03/how-your-cat-is-making-you-crazy/308873/
[4] Study: Keystone, J S et al. “Intestinal parasitic infections in homosexual men: prevalence, symptoms and factors in transmission.” Canadian Medical Association journal vol. 123,6 (1980): 512-4.
[5] Study: Verweij, Karin J H et al. “Genetic and environmental influences on individual differences in attitudes toward homosexuality: an Australian twin study.” Behavior genetics vol. 38,3 (2008): 257-65. doi:10.1007/s10519-008-9200-9
"results suggest that variation in attitudes toward homosexuality is substantially inherited, and that social environmental influences are relatively minor."
[6] http://blogs.scientificamerican.com/bering-in-mind/natural-homophobes-evolutionary-psychology-and-antigay-attitudes/
[97] Cochran's Wikipedia page: https://en.wikipedia.org/wiki/Gregory_Cochran#Pathogenic_hypothesis_of_homosexuality
[98] https://jaymans.wordpress.com/2013/01/12/a-gay-germ-is-homophobia-a-clue/
[99] Well-organized list of notable Cochran posts on this subject, and additional commentary: https://jaymans.wordpress.com/2014/02/26/greg-cochrans-gay-germ-hypothesis-an-exercise-in-the-power-of-germs/

Toxoplasmosis Studies