What New Research Reveals About the Health Costs of Cousin Marriages

Marrying one’s first or second cousin is widely considered to be a bad idea in North America, Europe, and in many other parts of the world. This is because of the belief that such consanguineous unions are likely to produce offspring with genetic and/or health defects. This notwithstanding, in many parts of the world such as South Asia, marriages between first and second cousins are common. In Pakistan, the number of such marriages is as high as 50%.

Even though the finding that cousin marriages are likely to negatively impact offspring health is real, the magnitude of this impact appears to be modest. Existing studies have focused primarily on infant and child mortality due to data limitations. In this regard, an influential organisation in the US has said that the stigma associated with cousin marriages in the US and Canada has little biological basis. This conclusion is counterintuitive and hence it is worth examining what thought-provoking new research tells us about this subject.

Health Impact
This research fills a critical gap in the literature by analysing the effect of cousin marriages on mortality across the entire adult lifespan, requiring historical data from a time when such marriages occurred in the US. The research bases its conclusions on a vast dataset of 40 million US genealogical profiles, focusing on individuals born between 1750 and 1920. 

In studies of this sort, the key methodological challenge is to isolate a causal effect when there is potential selection bias, that is, the possibility that individuals who marry their cousins are systematically different, for instance poorer or sicker, from those who do not, and that these differences, not the consanguinity itself, affect their children’s longevity. This challenge is dealt with effectively by the research and it leads to several interesting results. 

First, there is a large reduction in life expectancy. In other words, the research shows that the offspring of first-cousin marriages, or group 1, have a life expectancy at age 5 that is 2.2 years less than the offspring of their parents’ siblings, or group 2, who did not marry cousins.

Second, we see a lifespan-wide effect. This means that the increased mortality risk is not confined to childhood but accumulates throughout adulthood. The survival gap between the two groups mentioned above widens consistently with age, with the offspring of cousin marriages facing a significantly higher probability of death before age 50.

Third, we see robustness and stability in the negative effect. Put differently, this adverse effect is remarkably stable across 150 years of birth cohorts and across the distribution of parental longevity, telling us that the effect is not sensitive to major historical changes in the economy or in public health conditions.

Finally, the total negative effect is understated. In other words, because of significant underreporting of infant deaths in the genealogical records, the authors of this research note that their “2.2 years” estimate is conservative. When they attempt to account for this understatement, the estimated loss in life expectancy at birth increases to at least 3.2 years.

In sum, this research is the first to provide comprehensive evidence that marriage between first cousins has large, negative, and persistent effects on the life expectancy of their offspring, compounding known risks to infant and child health.

Policy Implications
The implications are global. Because the historical US context featured mortality and income levels comparable to many present-day regions where cousin marriages remain prevalent, the findings of this research are likely relevant beyond the US. The historical results align with available modern evidence, which shows consistently higher child mortality among the offspring of cousin marriages across 26 survey waves in countries with high prevalence. Strikingly, the health costs today do not appear lower than in 19^th century America, despite advances in medicine. This research estimates that cousin marriages may account for as much as one-seventh of the life expectancy gap between the US and Pakistan.

In sum, this research demonstrates that cousin marriages entail significant health penalties across the lifespan. It suggests that broader access to genetic counseling and screening may substantially improve public health in regions such as South Asia where the practice is common. More broadly, the findings underscore how deeply rooted cultural practices can impose hidden but enduring biological and economic costs.

*Views are personal