Albert Einstein, Charles Darwin, and Queen Victoria did it. The United States is among few countries with laws against it. In some cultures the philosophy - “Why marry a stranger?” - encourages marriage between first cousins.
Chuck that deeply ingrained “yuk” factor, and look at the scientific basis for America's odd legal and social taboos on first-cousin marriages.
Your first cousins are the children of your aunts and uncles - mom and dad's brothers and sisters. They have one set of the same grandparents as you.
Your second cousins have the same great-grandparents as you. Third cousins share the same great-great-grandparents.
Twenty four states, including Ohio, Pennsylvania, and Michigan, forbid first-cousin marriages. Another 7 restrict them. Utah, for instance, permits marriage only between first cousins who are over age 65. The 19 remaining states and the District of Columbia have no restrictions.
For a state-by-state breakdown, check www.cousin-couples.com/info/states.shtml.
The United States is the only western nation with such laws. None exist, for instance, in Canada, Mexico, France, Germany, or any other country in Europe. Worldwide, about 20 percent of couples are first-cousins.
Those laws went on the books - and the social stigma into the national mindset - before the era of modern genetics.
Originally, the basis was not science, but farmers' observations, experts believe. The farmers thought that inbreeding among livestock meant small, sickly offspring with little market value. So farmers mated their livestock with bulls and stallions from other herds.
Science also lent support.
Suppose that it takes two copies of an abnormal gene to cause a hereditary disease, and copies of both are floating around in a family. When family members inbreed, there's a greater chance that kids will wind up with the genes and the disease.
In isolated populations that intermarry for generations - like Ashkenazi Jews and Mennonites - harmful genes accumulate, and there is a greater risk of genetic diseases.
But a single-generation marriage between cousins is different. Although cousins may share many of the same genes, they usually are not members of a population where the number of harmful mutations has hit the ceiling.
In 2000, the National Society of Genetic Counselors convened a panel of experts to study first-cousin marriage risks. It did so partly to help doctors advise growing numbers of immigrants from Middle Eastern and African countries where first-cousin marriages are preferred.
In addition, misconceptions about the risk were causing some first-cousin couples to terminate pregnancies.
First cousins are 1.7 percent to 2.8 percent more likely to have a child with birth defects than children of unrelated couples, the panel reported last year. It said that increased risk is “significant,” but termed it much lower than commonly believed.
The risk is not enough to warrant any special medical screening, care, or counseling for first-cousin couples, the panel said.
And laws restricting first-cousin marriages? Evidence about the actual risks will eventually wipe them off the books, the panel predicted.