Also, most laws make no provision for the rare case of marriage between double first cousins. In 24 states of the United States, cousin marriages are prohibited. Most laws regarding prohibited degree of kinship concern relations of r = 25% or higher, while most permit unions of individuals with r = 12.5% or lower. Consanguinity (but not affinity) relationships may be summarized as follows: The degree of relationship is calculated by adding the number of generations back to a closest common ancestor of each individual. Laws regarding incest are sometimes expressed in terms of degrees of relationship. Main article: Coefficient of relationship 2 Laws regarding incest between consenting adults.
