SummaryIt has been widely believed that consanguineous marriage was infrequent in northern Europe. As part of ongoing studies into the population structure of northern Sweden, the Demographic DataBase of Umeå University has undertaken digitization of the parish record books of the Swedish Lutheran Church, which date back to the late 17th century. To examine the prevalence and patterns of consanguineous marriage, information from the DataBase was abstracted for the Skellefteå region during the period 1720-1899 and extended family pedigrees constructed. Of the 14,639 marriages recorded, 3,043 (20.8%) were between couples related as sixth cousins or closer. Following changes in the Swedish civil law in 1844 that removed the requirement of royal dispensation for first cousin unions, a significant increase in first cousin marriages occurred during the next two generations, even though the total population of the region grew significantly. There was also strong evidence that consanguineous marriages were favoured within particular families. The findings of the study are consistent with the patterns of single gene disorders reported in specific communities in the region, and they suggest that founder effect, drift and consanguinity all were important influences on population genetic structure in previous generations.

The northern Swedish population has a history of admixture of three ethnic groups and a dramatic population growth from a relatively small founder population. This has resulted in founder effects that together with unique resources for genealogical analyses provide excellent conditions for genetic mapping of monogenic diseases. Several recent examples of successful mapping of genetic factors underlying susceptibility to complex diseases have suggested that the population of northern Sweden may also be an important tool for efficient mapping of more complex phenotypes. A potential factor contributing to these effects may be population sub-isolates within the large river valleys, constituting a central geographic characteristic of this region. We here provide evidence that marriage patterns as well as the distribution of gene frequencies in a set of marker loci are compatible with this notion. The possible implications of this population structure on linkage- and association based strategies for identifying genes contributing risk to complex diseases are discussed.

SummaryMost studies on consanguinity have been conducted on contemporary populations and have focused on the prevalence and types of preferred intra-familial marriage. With its comprehensive birth, marriage and deaths records dating back to the late 17th century, and the legal bar on first cousin marriage removed in the mid-19th century, Sweden offers unique opportunities to examine the factors that determine by whom, where and why consanguineous marriages were contracted. The present study covers the period 1780–1899 and presents a detailed portrait of cousin and sibling exchange marriages in the Skellefteå region of northern coastal Sweden. The combined prevalence of first, second and third cousin marriage increased from 2.3% in 1790–1810 to 8.8% in 1880–1899, and multi-generation consanguinity also increased significantly over the study period. The distribution and prevalence of first cousin marriages was strikingly non-random, with a significantly greater propensity for consanguinity among land-owning families, especially involving first-born sons, within specific pedigrees, and in a number of more remote inland communities. Additional factors associated with a greater likelihood of consanguineous marriage included physical or mental disability among males, and among females the prior birth of an illegitimate child. Besides the inherent interest in the social and demographic structure of this region of northern Sweden during the course of the 19th century, in future studies it will be important to determine the degree to which the observed patterns of consanguineous and sibling exchange marriages in these past generations could have influenced present-day genetic structure.

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Inez Egerbladh holds a PhD. in human geography and is a researcher at the Demographic Data Base, Umeå University, Sweden.ABSTRACT: The long-term development of the households of landed peasants was investigated longitudinally in one village, and cross-sectionally in several parishes in northern coastal Sweden. The results showed a dominance of complex family households in a stem family system during the 1700s and development toward simpler family households in the 1800s. Demographic, economic, ecological, geographic, and, to some degree, cultural factors and their changes over time were taken into consideration in an explanation of this transition.The hypothesis that nuclear families constituted the basic preindustrial household structure in most parts of western and northern Europe, excluding Finland, has generated much research on household structures during the last decades (Laslett