In the United States, incest first became a secular crime in colonial Massachusetts in 1692, when the Bay Colony classified it as a capital offense; most other colonies followed with their own statutes over the next century. England took a very different path and did not criminalize incest in secular law until the Punishment of Incest Act 1908. The question of when incest became illegal has two answers because for most of English history it was a church matter rather than a criminal one, while American colonies pulled it into criminal law almost from the start.
Ancient and Biblical Origins
Prohibitions on sex between close relatives are among the oldest written laws. The Code of Hammurabi, written around 1780 BC in ancient Babylon, prescribed exile for a father who had sexual relations with his daughter and death by burning for a man who slept with his mother. The Hebrew Bible’s Book of Leviticus, chapters 18 and 20, laid out a detailed list of forbidden sexual pairings within families. These “Levitical degrees” became extraordinarily influential in Western law. When colonial American legislatures later wrote their first incest statutes, many copied the Levitical categories almost directly.
Colonial America Criminalized It First
Puritan communities in New England viewed sexual offenses through a theocratic lens and saw no reason to leave enforcement to church courts an ocean away. Massachusetts Bay Colony passed what appears to be the first explicit colonial incest statute in 1692, making it a capital offense. The law declared that anyone who committed incest “in any of the particular instances, made capital by the Law of God” would be put to death. That statute was repealed three years later, and in 1695 the colony replaced it with “An Act to prevent Incestuous Marriages,” which banned both incestuous marriages and sexual intercourse within the prohibited degrees of kinship.
Other colonies followed a similar pattern, drawing their lists of forbidden relationships directly from Leviticus. By the time these territories achieved statehood in the late 1700s and early 1800s, most already had incest statutes on the books. The wave of codification continued through the 1800s as new states built their legal systems. American incest law predates England’s by more than two centuries because colonial legislatures saw no separation between religious morality and criminal law, and they acted accordingly.
England Waited Until 1908
For most of English history, incest was treated as a sin rather than a crime. Ecclesiastical courts run by the Church of England claimed jurisdiction over sexual morality, and incest fell squarely in their territory alongside adultery and fornication. Under the canons of 1604, local churchwardens were required to report anyone who “committed adultery, fornication or incest” so that offenders could face church discipline. Punishments ranged from public penance to excommunication. There was no prison time and no criminal record.
Secular courts stayed out of these cases unless the conduct also involved violence or another recognized crime. By the mid-1800s, church courts had been stripped of nearly all authority over laypeople, but incest still had no home in secular criminal law.
That changed with the Punishment of Incest Act 1908, which made sexual intercourse between close blood relatives a criminal offense for the first time in English law. The statute covered granddaughters, daughters, sisters, and mothers, with penalties of up to seven years of penal servitude or two years’ imprisonment. It applied to both men and women, though women had to be at least sixteen to be prosecuted. The Act specified that “brother” and “sister” included half-siblings, and that the prohibition applied regardless of whether the family relationship was established through marriage.1Irish Statute Book. Punishment of Incest Act 1908
The 1908 Act remained the governing law in the United Kingdom for nearly a century. It was eventually replaced by the Sexual Offences Act 2003, which reorganized all sex crimes into a single comprehensive statute and created separate offenses for familial sexual activity with children and for sexual conduct between adult relatives.2Legislation.gov.uk. Sexual Offences Act 2003
Which Relatives Modern Law Covers
Modern incest statutes define prohibited relationships through two concepts. Consanguinity refers to a biological connection through shared ancestry. This covers the vertical line (parents, children, grandparents, grandchildren) and the horizontal line (siblings). Nearly every jurisdiction includes both full and half-blood relatives.
Affinity refers to relationships created through marriage or legal processes rather than biology. Step-parents, step-children, and sometimes former in-laws fall into this category. A step-parent occupies the same position of trust and authority as a biological parent, and the law treats the potential for exploitation the same way regardless of whether DNA is involved.
Adoption pulls the net wider than many people expect. Many states explicitly include adopted relatives in their incest statutes, meaning that two people with no biological connection can be prosecuted if one was legally adopted into the other’s family. Colorado’s statute covers adopted children and stepchildren. Texas applies harsher sentencing when the offense involves an ancestor or descendant by adoption.
First Cousin Marriage Is a State-by-State Patchwork
First cousins occupy an ambiguous space in American law. Early in the nation’s history, marriage between first cousins was widely legal and socially unremarkable. That acceptance eroded during the mid-to-late 1800s as social reformers and early geneticists raised concerns about health risks to offspring. Legislatures began carving out specific bans, often treating first-cousin relationships differently from the prohibitions covering parents, siblings, and children.
Roughly half the states ban first-cousin marriage outright. About a third allow it without restrictions. A handful permit it only under specific conditions, such as requiring genetic counseling beforehand or allowing the marriage only if one or both parties are past reproductive age or can demonstrate they cannot have children. Couples who marry legally in one state may find their marriage unrecognized if they move to another.
Consent Is Not a Defense
One of the most common misconceptions about incest law is that two consenting adults should be free from prosecution. In the vast majority of jurisdictions, mutual consent between adults is not a defense. The crime is defined by the relationship, not by whether both parties agreed. A prosecutor does not need to prove coercion, force, or a reluctant participant. The fact that the sexual act occurred between people within the prohibited degrees of kinship is enough.
The U.S. Supreme Court’s 2003 decision in Lawrence v. Texas, which struck down laws criminalizing private consensual sexual conduct between adults, did not extend its protection to incest. The majority opinion explicitly noted that the case did not involve “persons who might be injured or coerced or who are situated in relationships where consent might not easily be refused,” a description that maps closely onto familial power dynamics.3Justia. Lawrence v Texas 539 US 558 (2003)
Two narrow exceptions exist. New Jersey does not criminalize incest between people who are 18 or older, and Rhode Island does not criminalize it between people 16 or older. Both states still prohibit incestuous marriages. Outside those two states, age and consent are irrelevant to whether the crime occurred, though the age of the parties may affect who gets prosecuted and how severely.
Civil Consequences for Marriages
Incestuous unions carry serious civil consequences beyond criminal prosecution. A marriage between prohibited relatives is treated as void from the start in most jurisdictions. Unlike a voidable marriage, which is considered valid until a court declares otherwise, a void marriage is legally treated as though it never happened. No court order is needed for this determination, though parties often seek a formal annulment to clean up property records and legal documentation.
A person in a void marriage has no claim to spousal inheritance rights, survivor benefits, or the property protections that divorce law normally provides. If one partner dies, the surviving partner may have no legal standing as a spouse. Courts that discover the void status of a marriage during divorce proceedings will typically treat the parties as unmarried individuals dividing jointly held property rather than as spouses subject to equitable distribution rules.
Criminal Penalties Today
Incest is a criminal offense across the United States, though the severity of punishment varies widely. Most states classify it as a felony. Prison terms range from a year or two at the low end to 20 years in states like Massachusetts. A handful of states apply different sentencing ranges depending on the specific relationship involved, with offenses between parents and children drawing harsher punishment than those between more distant relatives.
Fines also vary. Some states set maximums in the low thousands, while others allow fines up to $500,000 for aggravated offenses. A conviction typically results in a permanent criminal record and, in many jurisdictions, mandatory registration as a sex offender. That registration requirement often lasts for decades and carries its own restrictions on where a person can live and work. The collateral damage to employment, housing, and family relationships extends well beyond whatever a judge imposes at sentencing.