Incidence of chlamydia, a major cause of infertility. The high polygyny rate among the “female farming” peoples of sub-Saharan Africa may have favored the evolution of STDs. Is this where we should look for the precursor of the hypothetical “gay germ”? (source)
Heritability for male homosexuality is low to moderate (30 to 45%). There is thus some kind of genetic predisposition, but it’s weak and may simply be a low degree of pre-natal androgenization. All things being equal, such individuals would still develop a heterosexual orientation.
But all things aren’t equal. Something out there is tipping these individuals over the threshold that separates heterosexual from homosexual orientation. What is it? I suspect there are several causes, including the rising level of estrogens and estrogen-like substances in the environment over the past century (see previous post).
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.
But so far there’s no smoking gun. No candidate pathogens have been identified, although some STDs seem to have adapted to non-heterosexual modes of transmission, e.g., the bacterium responsible for bacterial vaginosis, particularly Gardnerella vaginalis, and some strains of vaginal yeast (see previous post).
Another objection is that natural selection should reduce host susceptibility. As Ron Unz (2013) has recently argued:
Cochran and others ridicule the gene model as absurd, arguing that strong selective pressure would have rapidly eliminated any such genes from the population, and this is not unreasonable. But similar criticism could applied to their own model, since genetic susceptibility to the germ would obviously be subject to equally powerful selective disadvantage.
A lot of pathogens seem undeterred by this argument. People die all the time from infections of one sort or another. One reason is that pathogens have shorter generation times and thus can evolve faster than their hosts can. An evolutionary equilibrium will eventually fall into place, but it will be heavily weighted in the pathogen’s favor. There are also limits to what a host can do. If the host’s defense system becomes too sensitive, it will attack not only possible pathogens but also host tissues.
Still, the most catastrophic epidemics tend to burn themselves out, largely because they destroy the pool of individuals they can most easily spread amongst. The Plague of Justinian of the 6th and 7th centuries may have wiped out half of Europe’s population. Then it disappeared. The Black Death of the 14th century killed between one and two thirds of all Europeans. It too disappeared, the last possible outbreaks being in the 18th century. This is not the case, however, with STDs, even in places where the consequences are dramatic, such as Africa’s “infertility belt”:
Africa shares the largest burden of infertility in the world. Estimates indicate that an average of 10.1% of couples experience infertility in Africa, with a high percentage of 32% in some countries and ethnic groups within Africa. An infertility belt” spreading through West Africa, through Central Africa to East Africa has been described. In some countries in this belt, up to one-third of women may be childless at the end of their reproductive lives. (Okonofu & Obi, 2009)
A pathogen would not sterilize one third of the population, generation after generation, unless it had something to gain, as Ron Unz notes. In sub-Saharan Africa, infertility can lead to abandonment of the wife, thus making her a better vehicle for pathogen transmission:
The high prevalence of untreated STD, resulting in increased infertility acts paradoxically to increase rather than decrease the fertility in Africa. Infertility is devastating for an African woman, resulting in divorce and diminished social status that often leads to prostitution. The fear of infertility results in refusal of contraception and early childbearing to demonstrate fertility. (O’Reilly, 1986)
Other separated women owe their status to infertility, which is a frequent reason for being driven from marriage and for being unable to marry […] Nadel […] identified such women as a major source of prostitutes: “Adultery and unchastity count less in her than other women. [The] paramount stigma [is] barrenness itself.” (Caldwell et al.,1989)
The existence of Africa’s infertility belt is generally attributed to a high prevalence of STDs, particularly gonorrhea and chlamydia (Collet et al., 1988), which in turn is related to a high polygyny rate (20 to 40% of all sexual unions throughout most of sub-Saharan Africa), which in turn is related to the low cost of maintaining a second or third wife, which in turn is related to year-round hoe farming and the ability of women to support themselves and their children with little male assistance.
We know that the AIDS virus evolved in sub-Saharan Africa, and it may be that syphilis evolved out of yaws, likewise endemic to sub-Saharan Africa. It may be that this region favors the evolution of STDs; if so, we might best look for the precursor of the “gay germ” there as well, assuming of course that it does exist.
Caldwell, J.C., P. Caldwell, and P. Quiggin. (1989). The social context of AIDS in sub-Saharan Africa, Population and Development Review, 15, 185-234.https://www.soc.umn.edu/~meierann/Teaching/Population/Readings/Feb%209%20Caldwell.pdf
Collet, M., J. Reniers, E. Frost, R. Gass, F. Yvert, A. Leclerc, C. Roth-Meyer, B. Ivanoff, and A. Meheus. (1988). Infertility in Central Africa: Infection is the cause, International Journal of Gynecology & Obstetrics, 26, 423–428 http://dx.doi.org/10.1016/0020-7292(88)90340-2
Cochran, G.M., Ewald, P.W., and Cochran, K.D. (2000). Infectious causation of disease: an evolutionary perspective, Perspectives in Biology and Medicine, 43, 406-448.
Okonofu, F.E. and H. Obi. (2009). Specialized Versus Conventional Treatment of infertility in Africa: Time for a Pragmatic Approach, African Journal of Reproductive Health, 13, 9-11.http://www.ajrh.info/vol13_no1/13_1_editorial_english.php
O’Reilly, K.R. (1986). Sexual behaviour, perceptions of infertility and family planning in sub-Saharan Africa, African Journal of Sexually Transmitted Diseases, 2, 47-49.
Unz, R. (2013). “Gay gene” vs. “gay germ”, April 16, The American Conservative, http://www.theamericanconservative.com/gay-gene-vs-gay-germ/