In some ways, it isn’t entirely rational to forbid cousin marriage, if the purpose of the prohibition is that their offspring have an increased risk of some kind of genetic disorder.
For example:
Ashkenazi Jews and, I just learned, French-Canadians and the Cajun community of Louisiana have the same carrier rate for the gene for Tay-Sachs, one in 27. And also, apparently, new research has shown that Irish Americans have a carrier rate for the Tay-Sachs gene of 1 in 50. The carrier rate for Tay-sachs in the general population, excluding these particular groups is 1 in 250. If two carriers reproduce, they have a 1 in 4 chance of producing a child with Tay-Sachs and, whether one or both parents is a carrier, the risk for producing a child who is a carrier is a 2 in 4, producing another genetic time bomb, yet it is not against the law for Ashkenazis, or Jews who may have Ashkenazi ancestry, or French-Canadians, or Cajuns or Irish Americans to marry, and of course, it certainly shouldn’t be, even though their chances of having a child with a genetic defect, either a carrier or at worst a child with Tay-Sachs, is greater than the risk of producing offspring with genetic defects is among first cousins who marry.
Source