In Paris in 1876 a 31-year-old banker named Albert took an 18-year-old named Bettina as his wife. Both were Rothschilds, and they were cousins. According to conventional notions about inbreeding, their marriage ought to have been a prescription for infertility and enfeeblement.
In fact, Albert and Bettina went on to produce seven children, and six of them lived to be adults. Moreover, for generations the Rothschildfamily had been inbreeding almost as intensively as European royalty, without apparent ill effect. Despite his own limited gene pool, Albert, for instance, was an outdoorsman and the seventh person ever to climb the Matterhorn. The American du Ponts practiced the same strategy of cousin marriage for a century. Charles Darwin, the grandchild of first cousins, married a first cousin. So did Albert Einstein.
In our lore, cousin marriages are unnatural, the province of hillbillies and swamp rats, not Rothschilds and Darwins. In the United States they are deemed such a threat to mental health that 31 states have outlawed first-cousin marriages. This phobia is distinctly American, a heritage of early evolutionists with misguided notions about the upward march of human societies. Their fear was that cousin marriages would cause us to breed our way back to frontier savagery—or worse. "You can't marry your first cousin," a character declares in the 1982 play Brighton Beach Memoirs. "You get babies with nine heads."

So when a team of scientists led by Robin L. Bennett, a genetic counselor at the University of Washington and the president of the National Society of Genetic Counselors, announced that cousin marriages are not significantly riskier than any other marriage, it made the front page of The New York Times.
. The study, published in the Journal of Genetic Counseling last year, determined that children of first cousins face about a 2 to 3 percent higher risk of birth defects than the population at large. To put it another way, first-cousin marriages entail roughly the same increased risk of abnormality that a woman undertakes when she gives birth at 41 rather than at 30. Banning cousin marriages makes about as much sense, critics argue, as trying to ban childbearing by older women.
But the nature of cousin marriage is far more surprising than recent publicity has suggested. A closer look reveals that moderate inbreeding has always been the rule, not the exception, for humans. Inbreeding is also commonplace in the natural world, and contrary to our expectations, some biologists argue that this can be a very good thing. It depends on the degree of inbreeding.
The idea that inbreeding might sometimes be beneficial is clearly contrarian. So it's important to acknowledge first that inbreeding can sometimes also go horribly wrong—and in ways that, at first glance, make our stereotypes about cousin marriage seem completely correct.
That discussion in my next post.
Based on an article that appeared in DiscoverMagazine.com: https://discovermagazine.com/2003/aug/featkiss