The effect of parental consanguinity and inbreeding in Hirado, Japan

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A census of Hirado, Japan in the summer of 1964 produced data on the reproductive performances of husbands and wives for 10,530 marriages where either the husband, the wife, or both were alive and residing in the city at the time of the census. Approximately one in every 6 of these marriages involves spouses who are biologically related to one another, and in some 10 per cent of marriages the husband, wife, or both are inbred. Analysis of the effects of length of cohabitation, socio-economic status, and consanguinity and inbreeding on total pregnancies, total livebirths, and “net fertility” (total livebirths minus non-accidental deaths in the first 21 years of life) revealed the following insofar as marriages contracted in the years 1920–1939 are concerned: 1. Total pregnancies and total livebirths were significantly increased with consanguinity, but “net fertility” was not when allowance is made for the role of socio-economic factors, and religious affiliation is ignored. The latter finding is thought to reflect the increased risk of death to liveborn children born to consanguineous marriages. Among Buddhists, the only religious group large enough to warrant separate analysis, total pregnancies, total livebirths and “net fertility” are all significantly and positively associated with parental relationship. However, the regression coefficient associated with “net fertility” is less than half the value associated with either total pregnancies or total livebirths. 2. Among non-farm marriages, all three measures of reproductivity were increased significantly with paternal inbreeding when religious affiliation is ignored or restricted to Buddhists. Among farm marriages, these measures were decreased but not always significantly so. Tests of the significance of the differences between farm and non-farm groups were almost invariably significant. No simple explanation other than chance can be advanced for this finding. 3. total pregnancies and total livebirths, but not “net fertility” increase significantly with maternal inbreeding among non-farm marriages; within farm marriages these three metrics also increase, but significantly so only in the case of total livebirths. It is suggested that on Hirado the increased reproductivity of the consanguineous marriage largely offsets the increased mortality among the issue from such unions, and thereby dampens the rate of elimination of deleterious genes and the loss of genetic variability.