Incest is marriage or sexual relations between close relatives. The criminal offense of incest may also be known as prohibited sexual conduct. Laws vary from state to state, but incest laws generally prohibit marriage or sexual relations between (1) a person’s ancestor or descendant by blood or adoption; (2) a person’s current or former stepchild or stepparent; (3) a person’s parent’s brother or sister of the whole or half blood; (4) a person’s brother or sister of the whole or half blood or by adoption; (5) the children of a person’s brother or sister of the whole or half blood, or by adoption; or (6) the son or daughter of a person’s aunt or uncle of the whole or half blood or by adoption.

Lack of consent to sexual relations between such relatives is not an element of the crime, and persons may be guilty of the crime even if both parties consented.

Laws regarding incest or prohibited sexual contact vary from state to state and are generally located in a state’s statutes—often in the penal or criminal code. The crime is a felony offense in many states, with potential punishment of significant jail or prison time.

In Montana, incest is defined as marriage or sexual relations between close relatives, and it is considered a criminal offense. Under Montana law, specifically Montana Code Annotated 45-5-507, incest occurs when a person knowingly marries, cohabits with, or engages in sexual intercourse or sexual contact with a person known to be an ancestor, a descendant, a brother or sister of the whole or half blood, a first cousin, an aunt, an uncle, a nephew, or a niece. This prohibition applies regardless of whether the individuals are related by blood or adoption. Consent is not a defense to a charge of incest, meaning that the crime can be charged even if both parties consent to the relationship. Incest is classified as a felony in Montana, and those convicted may face severe penalties, including imprisonment.