A century ago, in the midst of a flurry of interest in consanguineous marriages, the Philadelphia Ledger observed, “If one half is true that has been affirmed in regard to the effect of cousins intermarrying, it would seem as if it were the duty of parents and guardians to interfere, and even of all State Legislatures to do what can be done to prevent so injurious a custom”. These sentiments were sufficiently widespread to be echoed in an editorial in the Times of India, dated August 9, 1860, and they might well be justified today if we could establish that, in fact, “one half that is affirmed is true”. Interests in the effect of consanguineous marriage are again high, but they now stem not from a concern for Levitical Law but from the powerful tool consanguinity affords in an appraisal of the relative importance of mutation versus selection in maintaining the genetic burden of a population. Clearly the value of this tool is directly proportional to the quality and quantity of data available.
Recently, numerous studies have been published relating inbreeding to infantile and juvenile mortality. Notable among these are the studies of Sutter and Tabah (summarized in Sutter, 1958) in France, of Böök (1957) in Sweden, of Slatis, Reis, and Hoene (1958) in the United States, of Freire-Maia (1961) in Brazil, of Zerbin-Rüdin (1960) in Germany, and of a number of workers including ourselves in Japan. These studies differ markedly in size, scope, and design. In all, save our own, the mortality data are essentially retrospective in character. Experiences such as those with rubella in pregnancy should make us acutely aware of the possible biased nature of such observations. Be this as it may, it is the purpose of this paper to present the results of a continuing, anterospective investigation of the effects of inbreeding on mortality in Japan.