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As is true in most surveys, some factors in the inbreeding study are confounded (eg, the socioeconomic differences in inbred and outbred groups) so that the evaluation and interpretation become very difficult. The authors are fully aware of these and discuss the confounding factors carefully.
The final chapter should be of great interest to all human geneticists, as it discusses one of the most controversial subjects in population genetics. In recent years some geneticists have claimed that the results of cousin marriages can tell us whether a gene is maintained in the population by new mutations or by heterozygote superiority in reproduction ( overdominance in fitness), apparently assuming that there are only two possibilities. Other geneticists do not think so, because all inbreeding does is facilitate the segregation of rare genes in the population. Having reviewed half a dozen theoretical and practical objections and examined their own data, Schull and Neel
Kraft SC. Gastroenterology. JAMA. 1966;195(11):974. doi:10.1001/jama.1966.03100110142059