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 Massachusetts, incest is defined as marriage or sexual intercourse between persons who are closely related by blood or marriage. Specifically, Massachusetts General Laws Chapter 272, Section 17 prohibits marriage or sexual intercourse between a person and his or her ancestor, descendant, brother, sister, uncle, aunt, nephew, or niece. This law applies to both whole and half-blood relations, as well as to relations by marriage. Incest is considered a felony in Massachusetts, and those convicted can face imprisonment. Consent is not a defense to a charge of incest, meaning that the crime can be prosecuted even if both parties consented to the relationship. The severity of the punishment for incest can vary, but it reflects the state's interest in preventing such relationships, which are deemed harmful to family structure and public morality.