In most states first cousins cannot marry. However, in some states it is allowed. For example, Tennessee law does not forbid first cousins to marry. In Ohio the parties must be no nearer than second cousins. Arizona law provides that first cousins cannot marry unless both parties are over the age of 65 and it is proven that at least one of the parties is incapable of reproducing.

Courts will generally allow parties who are first cousins to remain married even if they reside in a state that does not allow such marriages. For example, in Mason v. Mason (2002), an Indiana appeals court refused to grant a man who had legally married his first cousin in Tennessee a divorce in Indiana. The court wrote that “as a matter of comity (legal reciprocity), Indiana can choose to recognize Tennessee marriages between first cousins, even though such a marriage could not be validly contracted between residents of Indiana.”