Gregory Cochran

Cochran and co-authors Jason Hardy and Henry Harpending suggest that the high average IQ of Ashkenazi Jews may be attributed to natural selection for intelligence during the Middle Ages and a low rate of genetic inflow. Cochran and his colleagues hypothesize that the occupational profile of the Jewish community in medieval Europe had resulted in selection pressure for mutations that increase intelligence, but can also result in hereditary neurological disorders.[4][5][6] Cochran was featured in an episode of the Norwegian television show Hjernevask (in English: "Brainwash") in which he discusses race and intelligence, using Ashkenazi intelligence as compared to the rest of the Israeli Jewish population as an example of differences between groups.[7]

In 2000, Cochran and evolutionary biologist Paul W. Ewald co-authored a paper in which they proposed that most human diseases were the result of pathogenic infections (viruses, bacteria, parasites).[8] They argue that most fitness-reducing diseases would be eliminated through natural selection, but since germs can evolve faster than humans, they are a likely culprit. Cochran and Ewald point to stomach ulcers, which were once thought to be caused by a variety of environmental factors such as smoking, diet and drugs, but were later attributed to bacteria.[9]

Cochran has argued that male homosexuality may be considered a disease because it generally reduces or eliminates reproductive output, and he and Ewald have speculated that homosexuality might be caused by infection with an unknown virus.[10][11][12] However, he does not suggest that the infectious agent that causes homosexuality is spread by homosexuals. Cochran's hypothesis is based on the argument that homosexuality is unlikely to be genetic because it does not follow simple Mendelian inheritance patterns and because natural selection should have largely eliminated genes that cause homosexuality.[10] Cochran says that there is no positive evidence for the gay germ hypothesis.[11] In 1999, journalist Caleb Crain published an article in the gay magazine Out in which he spoke with several sexual orientation researchers about the hypothesis.[13] Geneticist Dean Hamer called it an "interesting idea" which would need to be tested by experimentation, but that he was skeptical as homosexuality doesn't appear in clusters. J. Michael Bailey disagreed with the use of the term "disease", but gave Cochran the "benefit of the doubt". Elaine F. Walker, who researched a pathogenic cause of schizophrenia during pregnancy, did not find it plausible.[13]

The dominant hypothesis in the scientific literature is that male homosexuality may be a result of incomplete masculinisation of the brain during fetal development, under the influence of sex hormones.[14] Maternal immune responses have also been implicated.[15]