Inbreeding and prereproductive mortality in the Old Order Amish. I. Genealogic epidemiology of inbreeding.

Epidemiologic patterns of inbreeding in the Old Order Amish were investigated using a unique genealogic registry of Lancaster County, Pennsylvania, Amish that contains information on 8,163 marriages, dating back to the time of the pioneer migrants in the 1700s and spanning more than 10 generations. The kinship coefficient for each marriage was computed using the path method of tracing common ancestors in the multigenerational pedigrees. Because of extensive genealogic connections, mean kinship coefficients and the proportion of related marriages have increased significantly over time, from 0.004 and 37%, respectively, for marriages before 1850 to 0.012 and 98%, respectively, for marriages after 1950. Demographic factors related to higher kinship levels include young age at marriage, large sibship size for both husband and wife, husband being a farmer, and marriages occurring in the marriage season (November or December). The rise in inbreeding levels in the Amish over time can be uniquely contrasted with the decline in inbreeding in most areas of the world. Furthermore, because some of the demographic factors related to high inbreeding levels may be associated with levels of mortality, such factors have to be taken into account when studying the effects of inbreeding on mortality in the Amish. This study uses an epidemiologic approach to the evaluation of inbreeding patterns in a population over time.