First-cousins have a relatedness r = 1/8, and their offspring would
have an inbreeding
coefficientF = 1/16
= 0.0625. This
means they would beidentical 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.