When sophisticated medical attention substantially increases the life expectancy of individuals with a severe homozygous condition without significantly raising their reproductive success, an increase in parental investment should render a concomitant decrease in parental ability to reproductively compensate. This process will lower the expected number of heterozygous carriers produced by heterozygous parents and, in turn, will lower the equilibrium frequency of the gene.
The above document is Part II of a three-part PhD thesis submitted and accepted in 1980 by the Department of Anthropology of Harvard University. Most of the text is from an article published in 1977 (Hartung, J., Ellison P. A Eugenic Effect of Medical Care. Social Biology, 24:192-99, 1977) available here >>