While isolated instances of first cousin marriage aren't especially harmful, contrary to what some comments here suggest, if done across successive generations,

While isolated instances of first cousin marriage aren't especially harmful, contrary to what some comments here suggest, if done across successive generations, these unions can produce a catastrophic unmasking of harmful recessive alleles, and the nations with the highest rates of consanguineous marriages also often have the highest rates of birth defects (see p. 22): https://www.marchofdimes.org/materials/global-report-on-birth-defects-the-hidden-toll-of-dying-and-disabled-children-full-report.pdf https://www.marchofdimes.org/materials/global-report-on-birt... However, while the dangers of inbreeding depression are common knowledge, it's worth pointing out that outbreeding depression exists as well. Where lies the ideal balance between these two extremes? Surprisingly, it appears to be marriages between third or fourth cousins, if you use fecundity as a proxy for general offspring health: https://www.sciencedaily.com/releases/2008/02/080207140855.htm https://www.sciencedaily.com/releases/2008/02/080207140855.h... A third or fourth cousin is a person with whom you share first cousin grandparents or great grandparents--essentially a distant relative, or simply put, someone of your own "race," which is quite at odds with the "hybrid vigor" claims often made to tout interracial marriage. Apparently just marrying any old white person (if you're white) gives you all the hybrid vigor your offspring need.