Quote from buddiage
The middle east- that's what they do all the time. They marry cousins.
It does occur often in the Middle East. However, there it is often a repeated pattern, occuring over and over again, and there are significant birth defects related to the practice.
It is also seen in Amish groups here in the USA. As Amish are small group in most areas, marrying another Amish person often means that the individuals marrying have distant family relationships (second cousins, twice removed, etc.) and it is very difficult to find two people in a community that are not somehow distantly related. There are defects noted in that community, the most benign being polydactyly (extra fingers or toes). Because of this issue, young Amish men/women may be sent to work/visit with distant family/friends so that they are exposed to distant Amish not of their kin, hoping that they will marry into that group, adding new unrelated blood to a different Amish community, or that they will return and bring new blood into their own community.
Many Ultraorthodox/Hasidic/Eastern European Jews also do the same thing, arranging marriages between young people of separate communities, because their own communities are so insular.
Fundamentalist Latter Day Saints (not to be confused w/the "official" Mormon church) because of their practice of polygamy, have also run into some problems w/birth defects, due to the process of repeatedly intermarrying cousins/close relations. When Mother and daughter and cousin all marry the same man, and their progeny marry uncles or cousins, and their progeny does the same in their community, that is closed to many outsiders, there are problems. The same genetic defect can be compounded/multiplied. And as their births are generally at home, attended by their own community, documentation is poor. And as the marriages, other than the first marriage are generally undocumented, it is difficult to track the issue. The defects are often chalked up to sin or to "the will of G-d".
Again, in these groups, the problem is REPEATED intermarriage, much like the royal families of Europe. A single interfamily marriage between cousins during several generations rarely results in dire consequences, unless there is a known genetic issue like Huntington's, Sickle Cell, or Tay Sachs....diseases that should be checked out by any possible carrier for any marriage, regardless.