From Hwang, Jakob and Squires (2025) in AER: Insights:
Cousin marriage rates are high in many countries today. While previous studies have documented increased risks of infant and child mortality, we provide the first estimate of the effect of such marriages on life expectancy throughout adulthood. By studying couples married over a century ago, we can observe their offspring across the entire lifespan. US genealogical data allow us to identify children whose parents were first cousins and compare their years of life to the offspring of their parents’ siblings. Marrying a cousin leads to more than a two-year reduction in age-five life expectancy, compounding the documented early-life effects.
About 10% of marriages are cousin marriages so this finding is highly relevant. The good news is that cousin marriage rates have declined. The Economist reports:
In Jordan, 57% of marriages in 1990 were consanguineous, but by 2012 the figure had dropped to 35%. Surveys of Israeli Arabs suggest that 20% of marriages before 2000 were between first cousins, compared with 12% in 2005-09.
You can read the full paper Hwang et al. paper here.
