Inbreeding in Norway

Area of Science:

• Demography
• Human Genetics
• Sociology

Background:

• Marriage records in Norway since 1889 have documented consanguineous unions.
• Data on marriages between relatives were also collected in the 1891 census and the Medical Registration of Births (MRB) from 1967.

Purpose of the Study:

• To assess the reliability of different Norwegian data sources for studying consanguineous marriages.
• To evaluate trends in inbreeding levels in Norway over time.

Main Methods:

• Comparing the ratio of second-cousin to first-cousin marriages across different data sources (marriage certificates, 1891 census, MRB).
• A ratio >= 2 was used as an indicator of data reliability.
• Calculating current inbreeding levels (alpha4) and comparing them to historical estimates.

Main Results:

• The MRB data appears most reliable, followed by the 1891 census.
• Marriage certificates from 1890 onwards are unreliable for second-cousin marriages, and from 1919 for first-cousin marriages.
• Current inbreeding in Norway is low (alpha4 = 0.000242), about 1/8th of the 1891 census estimate.
• The most significant decrease in inbreeding likely occurred after 1920.

Conclusions:

• The Medical Registration of Births and the 1891 census are the most dependable sources for historical consanguinity data in Norway.
• Norwegian marriage certificates are largely unreliable for studying cousin marriages after 1890.
• Inbreeding in Norway has substantially decreased, particularly after 1920.