Mortality rates in:
Period
Children of first cousins
Children of non-relatives
Young children (US)
 < 1858
22%
16%
Children <20 (US)
18th - 19th cent.
17%
12%
Children <10 (US)
1920-1956
8.1%
2.4%
Children <1 (France)
1919-1950
9.3%
3.9%
Children (France)
1919-1950
14%
10%

Inbreeding Depression in Humans

    First-cousins have a relatedness r = 1/8, and their offspring have an inbreeding coefficient F = 1/16 = 0.0625. This means they are identical by descent at 1/16 of their loci. If any of these loci involve recessive homozygotes that affect pre- or perinatal health (especially, novel mutations to lethal alleles), mortality rates of such children will be higher then those from marriages between distantly related persons. Several studies have compared mortality rates among children of first-cousin marriages versus children of non-relatives, based on extensive parish records of marriages [which include degree of consanguinity], births, & deaths.  In each case, the early mortality of the children born to related parents is higher. Other studies indicate that the mean time to first live birth is lower in marriages of first-cousins. The reduced health of inbred individuals is called inbreeding depression.
Data from Stern (1973); Text material © 2021 by Steven M. Carr