Inbreeding effects on fertility and sterility in southern India.

The effects of consanguineous marriages on couples' fertility and sterility were explored through an interview survey of 20 626 women, chosen randomly from the rural and urban areas of the North Arcot District of Tamil Nadu State. Qualified women investigators obtained relevant information about reproductive performances of all married women resident in well defined rural and urban samples chosen randomly from North Arcot District. For each marriage, a family pedigree was drawn, extending upwards to two earlier generations on both sides of each spouse, in order to determine the existence and type of consanguinity involved. Of marriages in rural areas, 46·9% were consanguineous, and in urban areas, 29·1%. In more than 80% of the consanguineous marriages, the spouses were first cousins or more closely related. The extent of primary and secondary sterility and the level of fertility were examined in relation to each type of consanguineous marriage with the duration of the marriage and the age of the woman. The frequency of primary sterility appeared to be lower in the consanguineous marriages compared to that in the non-consanguineous marriages. However, the differences were only marginal and only occasionally attained statistical significance. No trends were seen in the degree of consanguineous relationship, and there did not appear to be any association with the duration of marriage or the age of the woman. The frequencies of secondary sterility did not differ significantly in consanguineous marriages in either the rural or the urban areas. No consistent associations were observed with degree of relationship. There were no specific associations in terms of the duration of marriage or the age of the woman observed in the frequencies of secondary sterility. The mean levels of fertility were slightly raised among the consanguineous marriages and attained significance merely because of the large sample sizes involved. These findings are discussed and compared with relevant published work. Comparisons are made difficult because of paucity of data based on community studies, and also because great differences exist in the methodology adopted by various investigators. The findings from the present study seem to show that long-term inbreeding results in only marginal or non-significant effects on fertility of inbred populations.