The Influence of Consanguineous Marriage on Reproductive Behavior and Early Mortality in Northern Coastal Sweden, 1780–1899

Abstract

Remarkably few studies have been conducted into the prevalence and possible influence of close kin marriage on fertility and mortality in northern European populations. The Demographic DataBase at Umeå University offers a unique opportunity to correct this situation, with data on births, deaths, and marriages in the Skellefteå region of Sweden for the period 1720–1899 collected by the State Lutheran Church. The data are made more interesting by the fact that until 1680 first cousin unions were prohibited in Sweden; and from 1680 until 1844 a royal dispensation was needed before such unions could proceed. Of the 14,639 marriages initially studied, 20.8 percent were between couples related as sixth cousins or closer, with a significant increase in first cousin marriages post-1844. Using logistic regression, two subsets of marriages contracted from 1780 to 1899 were investigated with respect to fertility and mortality. First cousin marriages were strongly favored by freeholders and peasant landowning families; and in some families they had been preferentially contracted across successive generations. Consanguinity appeared to exert no influence on fertility. However, first cousin couples had higher rates of stillbirths and more deaths in infancy and early childhood among their progeny. This excess mortality was probably associated with the expression of detrimental recessive genes, although nongenetic factors may also have been involved. There was evidence of the clustering of multiple deaths within first cousin families, which likewise would be consistent with a genetic aetiology. Overall, the data confirm the significance of close consanguinity as an important demographic variable in this European population.