Incest laws in the United States prohibit two things: sexual contact between close relatives and marriage between them. Every state criminalizes the core conduct, and most treat it as a felony, but the exact relationships covered, the penalties, and the collateral consequences differ considerably from one state to the next. The hardest questions sit at the edges — step-relatives, adopted family, and first cousins — where the answer depends entirely on which state’s statute applies.
How the Law Defines the Offense
Most statutes treat the sexual act and the marriage as separate offenses. A person can be criminally charged for a sexual relationship with a close relative without ever attempting to marry, and a marriage between prohibited relatives is independently void whether or not anyone is prosecuted.
The Model Penal Code, which many states use as a template, classifies incest as a third-degree felony. Under Section 230.2, a person is guilty if they knowingly marry, cohabit with, or have sexual intercourse with an ancestor, descendant, or sibling of the whole or half blood. The word “knowingly” carries weight. A conviction generally requires proof that the accused understood the family relationship existed, so someone who genuinely did not know they were related has a potential defense to criminal charges. That defense would not rescue an invalid marriage, however.
Which Relatives Are Covered
Blood prohibitions split into two groups. Lineal relatives are people in a direct ancestral line — parents and children, grandparents and grandchildren, and further up or down the line. Every state prohibits sexual contact and marriage between lineal relatives without exception.
Collateral relatives share a common ancestor but are not in a direct line. Siblings fall in this group, and every state bans relationships between them. The law makes no distinction between full siblings and half-siblings who share only one biological parent. Aunts, uncles, nieces, and nephews are prohibited in most states, though a handful of jurisdictions do not extend their statutes that far. These bans apply regardless of the age of the people involved or whether both are consenting adults.
Adopted and Step-Relatives
Many states extend the prohibition beyond biology. Once a court finalizes an adoption, the adopted person is treated identically to a biological child for purposes of incest law. A relationship between an adoptive parent and adopted child, or between adopted siblings, falls within the ban in states that recognize these extensions.
Step-relationships, created by marriage rather than biology, get more uneven treatment. Some states prohibit sexual contact between step-parents and step-children on the theory that the legal family structure creates the same power dynamics and household dependencies the law is meant to address. Other states confine their statutes to blood relatives and adopted family, leaving step-relationships to be prosecuted under other laws when the step-child is a minor. Whether a step-relationship is covered depends entirely on how broadly the local statute defines “relative.”
First Cousins Sit in a Gray Zone
First cousins catch many people off guard. As of early 2025, 25 states prohibit first-cousin marriage outright, 18 states allow it without restriction, and the remaining seven permit it only under specific conditions. Those conditions follow a few patterns:
- Age thresholds, allowing the marriage only when both parties have reached an age at which childbearing is unlikely, such as 50 or 65.
- Proof of infertility, with one or both parties providing medical documentation that they cannot have children.
- Completion of genetic counseling before a marriage license is issued.
The criminal and marital rules do not always line up. In some states, first-cousin marriage is prohibited but the sexual relationship itself is not a criminal offense. In others, both are illegal. Anyone considering formalizing a first-cousin relationship needs to check the statutes of the state where they plan to live, not just the state where the ceremony takes place.
Criminal Penalties
Incest is a felony in the vast majority of states. Prison sentences for a first offense typically run from one year to ten or more, with aggravating factors pushing sentences higher. A relationship involving a minor, or one in which the accused held a position of authority over the other person, routinely draws the harshest punishment.
A conviction frequently triggers sex offender registration on top of any prison time. Some states require registration for a set number of years; others impose it for life. Registration restricts where you can live and work, and it continues long after any sentence and supervised release are complete. For many people convicted of incest, the registry turns out to be the punishment that lasts longest.
How Long Prosecutors Have to File Charges
When a child is the victim, prosecutors have an unusually long window. Forty-four states and the federal government have eliminated the criminal statute of limitations for child sexual abuse entirely, so charges can be filed decades after the conduct. The six states that still impose a deadline are moving in the same direction. For incest between adults, standard felony statutes of limitations apply, which in most states run from three to ten years depending on the offense classification.
What Happens to the Marriage
A marriage between prohibited relatives is not merely invalid going forward. It is void from the start, as if it never existed. This status, called void ab initio, means no court order is needed to end it because there was never a valid marriage to end.
The practical consequences are serious. Because the law treats the marriage as if it never happened, the standard rules for dividing property and awarding spousal support do not automatically apply. Neither party can claim spousal inheritance, survivor benefits, or other protections that flow from a recognized marriage. Assets acquired during the purported marriage may have to be sorted out under general property law rather than family law, which is usually less favorable to the financially weaker party.
Tax Fallout
The IRS treats an annulled marriage the same way the courts do. If your marriage is declared void, you must file amended returns for all affected tax years still within the statute of limitations, with your filing status changed to single or, if you qualify, head of household.1Internal Revenue Service. Filing Taxes After Divorce or Separation Reclassifying years of joint returns as single filings can create substantial additional tax liability, since joint rates are more favorable for most income combinations. Refunds received based on joint status may have to be repaid, with underpayment penalties on the difference.
Children Born During the Marriage
Children born from a void marriage are generally treated as legitimate under the law. Most states have statutes providing that the invalidity of a marriage does not affect the legal status of children born during it. A child still has full inheritance rights, the right to support from both parents, and the same legal standing as any other child. The parents’ legal transgression does not transfer to the child.
Immigration Consequences
Incest creates two separate immigration problems. It can invalidate a marriage for visa purposes, and a criminal conviction can make a non-citizen inadmissible to the United States.
USCIS generally recognizes a marriage as valid for immigration purposes if it was legal where it was performed. The agency also checks whether the marriage is consistent with the public policy of the state where the couple lives or plans to live. For marriages between close relatives, USCIS asks whether the state of residence would refuse to recognize the marriage or would prosecute the relationship. A first-cousin marriage performed legally abroad can be denied for a spousal visa if the couple intends to live in a state that bans it.2U.S. Citizenship and Immigration Services. USCIS Policy Manual Volume 6 Part B Chapter 6 – Spouses
A criminal conviction is a steeper barrier. Under federal immigration law, any non-citizen convicted of a crime involving moral turpitude is generally inadmissible, meaning they cannot obtain a visa or be admitted. A narrow exception exists for a single offense committed before age 18 or one carrying a maximum sentence of no more than one year in prison, provided the actual sentence did not exceed six months.3Office of the Law Revision Counsel. 8 USC 1182 – Inadmissible Aliens Because most states classify incest as a felony well above the one-year threshold, the exception rarely helps. Consular officers evaluate the specific conviction against the elements of the offense.4U.S. Department of State. 9 FAM 302.3 Ineligibility Based on Criminal Activity, Criminal Convictions and Related Activities
Mandatory Reporting When a Minor Is Involved
When incest involves someone under 18, mandatory reporting laws add another layer of obligation. Every state requires certain professionals — teachers, doctors, social workers, therapists, and law enforcement — to report known or suspected child abuse to authorities. Incest involving a minor qualifies as reportable abuse in every jurisdiction, and in many states the duty extends to any adult who becomes aware of it, not just designated professionals.
Failing to report carries its own criminal penalty, usually a misdemeanor with fines and short jail terms for mandated reporters who stay silent. Reporters who make good-faith reports are shielded from civil and criminal liability even if the investigation finds no abuse. The system is built to make reporting the safer choice.