An epidemiologic approach to the evaluation of the effect of inbreeding on prereproductive mortality.

Although many studies report deleterious effects of inbreeding on prereproductive mortality (death before age 20 years), such effects are usually measured in terms of genetic load, a concept much debated in the literature. To evaluate the public health impact of inbreeding on prereproductive mortality in terms of relative and attributable risks, the authors reviewed 31 studies with 294 observations comparing various stages of infant and child mortality in offspring of unrelated parents with offspring of first cousin, first cousin once removed, and second cousin marriages, respectively. Compared with offspring of unrelated parents, offspring of first cousin marriages have a higher risk of prereproductive mortality (median relative risk (RR) = 1.41, signed-rank test, p less than 0.01). Offspring of first cousin once removed and second cousin marriages had median relative risks of 1.16 and 1.26, respectively. Countries with relatively higher rates of consanguineous marriages (greater than 5%) had lower median relative risks than did countries with lower consanguinity rates. Also, the higher the mortality rate in the population, the lower the effect of consanguinity (median relative risk). Because of the rarity (less than 5%) and declining trends of consanguineous marriages in many places of the world, and because of their modest effect on prereproductive mortality (RR less than 2.0), it can be shown that the attributable risk of prereproductive mortality due to consanguineous marriages is less than 5%. Except for a few places with very high inbreeding levels, inbreeding has a minor impact on mortality in the prereproductive period.