In other states, however, a judge may grant permission to mentally retarded persons to marry. Every state proscribes marriage between close relatives on the basis of the social taboo of incest. The prohibited degree of relationship is fixed by state law. Every state forbids marriage to a child or grandchild, parent or grandparent, uncle or aunt, and niece or nephew, including illegitimate relatives and relatives of half blood, such as a half brother who has the same father but a different mother. A number of states also prohibit marriage to a first cousin, and some forbid marriage to a more distant relative, in-law, stepparent, or stepchild.
Age is an additional requirement.
Every jurisdiction mandates that a man and a woman must be old enough to wed. In the 1800s the legal age was as low as twelve years old for females. Modern statutes ordinarily provide that females may marry at age sixteen and males at age eighteen. Sometimes a lower age is permitted with the written consent of the parents. A number of states allow for marriage below the minimum age if the female is pregnant and a judge gives permission.