The laws of many U.S. states recognize two separate degrees of incest, the more serious degree covering the closest blood relationships such as father-daughter, mother-son and brother-sister, with the less-serious charge being pressed against more distantly-related individuals who engage in sexual intercourse, usually down to and including first cousins and sometimes half cousins. In New York State, close-blood-relation incest is a felony with a maximum penalty of four years in prison, while the less serious charge is usually only a misdemeanor. In Australia, where incest is punishable by a maximum of 25 years imprisonment for the more serious form of penetrating a child, even if that child is over 18, and 5 years for the less serious charge of sexual penetration of a sibling or half-sibling.

Adult incest
Incestuous relations between adults, such as between an adult brother and sister, are illegal in most parts of the industrialized world. These laws are sometimes questioned on the grounds that such relations do not harm other people (provided the couple have no children) and so should not be criminalized. Proposals have been made from time to time to repeal these laws — for example, the proposal by the Australian Model Criminal Code Officer's Committee discussion paper "Sexual Offenses against the Person" released in November 1996. (This particular proposal was later withdrawn by the committee due to a large public outcry. Defenders of the proposal argue that the outcry was mostly based on the mistaken belief that the committee was intending to legalize sexual relations between parents and their minor children.