THE EFFECT OF INBREEDING ON LONGEVITY, FERTILITY AND DISABILITY

Background

In the Skellefteå region in northern Sweden, cousin marriages were common. In 1750 the national prevalence of marriages between first cousins was estimated at 0.2% while in Skellefteå it was 0.34%. In early 19th century the national prevalence was 1% and in Skellefteå around 1.3%, and from the mid 19th century it had reached 1.5% for Sweden while Skellefteå had 2.65%. This makes Skellefteå an interesting case to study if there are any effects of inbreeding. This study aims at assessing these effects on mortality, fertility and disability. Laws and rules against marriage between relatives have existed in Sweden since at least the Middle ages. During the 1170’s all marriages between individuals who were related at sixth cousins or closer were forbidden. This changed in 1215 when up to fourth cousins could marry. In 1680 the law was changed so that up to second cousins were allowed to marry. The king could however give an exemption and then cousins could also marry. In 1845 marriages between cousins were legalized.

Data and sample

Digitized parish registers stored at the Demographic Data Base (DDB) at Umeå University, Sweden, based on original records of birth and baptism, marriage, migration, death and burial, and annual catechetical examination records from the 18th to 20th century. Everybody born in the Skellefteå region 1890-1910 is followed until death or censoring. For the fertility and disability study they have to be observable until the age of 45. Life length (birth date to death date), fertility (number of children) and disability (impairment notes in the registers) are the outcome variables. Inbreeding coefficient is calculated using parish registers back to the 18th century.