In most of the United States, two adult first cousins can legally have sex. Roughly 40 states have no criminal statute that reaches consensual sexual activity between first cousins, while about nine states classify it as incest and prosecute it as a felony. So the honest answer to the question of whether cousins can have sex is: in most states, yes, but in a minority of states it is a serious crime, and the only way to know which rule applies to you is to look at your own state’s criminal code.
Where It Is Legal
Most state criminal codes define incest narrowly. They cover parents and children, siblings, grandparents and grandchildren, aunts and uncles, and nieces and nephews. First cousins sit outside that list in the majority of jurisdictions. Two adult first cousins in a private, consensual relationship in one of these states are not committing a crime, and no agency has grounds to intervene.
This fits a broader constitutional baseline. In Lawrence v. Texas, the U.S. Supreme Court held that the government cannot criminalize private sexual conduct between consenting adults without a legitimate state interest beyond moral disapproval.1Justia U.S. Supreme Court. Lawrence v Texas, 539 US 558 (2003) The decision did not address cousin relationships directly, but it set the backdrop against which any state regulation of adult sexual privacy is read.
Where It Is a Crime
Around nine states treat sexual intercourse between first cousins as a crime, usually by naming first cousins in the incest statute or by reaching them through broader language about relatives within a specified degree of kinship. Penalties are serious. In the harshest jurisdictions, a conviction is a felony carrying up to ten years in prison and a fine of up to $10,000. Some of these states also require registration as a sex offender. In others, the ceiling is five years and a $5,000 fine for a lower-degree felony. A conviction leaves a permanent record that follows a person into employment, housing, and civil rights.
Statutory wording matters. A few states criminalize the marriage but not the sexual act itself, which produces a strange gap where the relationship is legal as long as no one applies for a license. Generalizations don’t hold across fifty states, so the only reliable move is to read the specific statute in your jurisdiction.
Which Cousins the Law Means
The rules turn on how closely two people are related by blood, and the labels aren’t interchangeable.
First cousins share a set of grandparents. Second cousins share a set of great-grandparents. That extra generation of separation makes a real legal difference. Nearly every state treats second cousins the same as unrelated people for both criminal and marriage purposes, with only a couple of rare exceptions.
Half-cousins, who share only one grandparent instead of two, sit in a gray area. Some states treat them exactly like full first cousins; others exempt them. Half-cousins can’t safely assume they fall outside the rules without checking.
Double first cousins come from two siblings in one family marrying two siblings in another. Their children share all four grandparents, making them genetically closer than typical cousins. Most state statutes still classify them the same as regular first cousins, so the same rules apply.
Marriage Is a Separate Question
The biggest source of confusion is the gap between what the criminal code allows in private and what the family code allows at the courthouse. In many states, first cousins can legally have a sexual relationship but cannot obtain a marriage license. The prohibited-relationship lists in the two bodies of law don’t always match. Roughly 32 states either ban first-cousin marriage outright or impose substantial restrictions, even though most of those same states don’t criminalize the sex.
When a first-cousin marriage happens in a state that prohibits it, the marriage is typically void from the start. That is not the same as a divorce. A void marriage is treated as if it never existed, which can wipe out a surviving partner’s claim to property, benefits, and other spousal protections.
Conditional Marriage Exceptions
A handful of states split the difference and allow first-cousin marriage under specific conditions. The most common conditions involve age and fertility. Some states permit the marriage if both parties are above a specified age, often 55 or 65, on the theory that reproduction is unlikely. Others allow it with medical proof of permanent sterility. At least one state requires the female partner to have reached a specified age; another accepts a physician’s affidavit that either party is permanently sterile. A few add genetic counseling as a prerequisite.
These rules apply to the marriage license, not to the sexual relationship itself.
Crossing State Lines to Marry
Couples sometimes try to work around a home-state restriction by marrying in a permissive state and returning. Whether that holds depends on the home state. The traditional rule is that a marriage valid where performed is valid everywhere, but many states have carved out specific exceptions for cousin marriages, and some have evasion statutes that void any marriage a resident contracts out of state to dodge the local prohibition.
The variation is stark. Some states that ban cousin marriage internally still recognize one performed legally elsewhere. Others void all out-of-state cousin marriages. Others take the hardest line and void any out-of-state marriage entered by their own residents who left specifically to evade the rule. A couple that marries across state lines without checking can end up with a marriage that disappears the moment they come home.
Federal Benefits and Immigration
Federal agencies generally don’t make independent calls on whether a cousin marriage is valid. They defer to state law, and that deference bites.
For Social Security survivor benefits, the test is whether the courts of the state where the deceased worker was domiciled at death would consider the marriage valid.2Social Security Administration. Social Security Act Title II 0216 If those courts would not recognize the marriage, the surviving spouse generally does not qualify. The Social Security Administration has specifically concluded that a first-cousin marriage void under state law cannot be “deemed valid” under the statute’s backup provision, because the defect is treated as substantive rather than procedural.3Social Security Administration. SSR 63-20 Section 216(h) – Relationship – Validity of Marriage Between First Cousins
Immigration follows similar logic. For a marriage-based visa petition, federal officers look at whether the marriage would be recognized in the state where the couple lives or intends to live.4U.S. Department of State. 9 FAM 102.8 Family-Based Relationships Establishing residence in a permissive state solely to sidestep the destination state’s rules does not work. If the intended state of residence treats the marriage as void, the petition faces serious problems.
The Policy Reason Behind the Restrictions
The restrictions trace partly to concerns about genetic risk. Children of first cousins carry roughly double the general population’s risk of inheriting a recessive genetic disorder, though the baseline is low enough that the absolute increase is modest. Studies have also found slightly higher rates of developmental delays and increased healthcare use among children of first-cousin parents, even after controlling for income and education.
That is why the conditional marriage exceptions focus on reproductive capacity. If a couple cannot have biological children, the state’s main justification for restricting the marriage falls away. Couples who can reproduce and are considering it may want genetic counseling regardless of whether their state requires it, since a counselor can assess family-specific factors that population statistics can’t capture.