Whether cousin marriage is incest depends on the state. Roughly half of U.S. states either criminalize sex or marriage between first cousins as incest or declare any such marriage void from the start. About 20 states and the District of Columbia allow first cousin marriage with no restrictions at all. A smaller group permits it only when the couple meets specific age or fertility conditions. Second cousins are a separate question with a much shorter answer, and the laws almost everywhere leave them alone.
The Three State Categories
Every U.S. state falls into one of three groups on first cousin marriage.
About 20 states plus the District of Columbia treat it like any other marriage. First cousins can apply for a license at the county clerk’s office and marry without any special disclosure or process.
Around 25 states either prohibit the marriage outright or classify sexual relations between first cousins as criminal incest. In these states the marriage is void from its inception, and the couple may also face prosecution. Penalties range from misdemeanors to felonies carrying years in prison.
A smaller group of states takes a middle path, prohibiting first cousin marriage in general but allowing it when the couple satisfies conditions designed to rule out biological children.
Second cousins face almost no legal barriers anywhere. Only two states currently prohibit second cousin marriage, so when people ask whether cousin marriage counts as incest, the real question is almost always about first cousins.
Where It Is Treated as Incest
In the states that classify first cousin relationships as incest, the label carries real weight. Incest is typically charged as a felony, and sentences can run from a few years to a decade or more depending on the state. Some states add substantial fines. The exact penalty often depends on how closely related the parties are, with parent-child and sibling cases drawing the harshest sentences and cousin cases falling lower on the scale in states that distinguish between degrees.
Criminal exposure does not depend on the couple actually attempting to marry. Several states prohibit sexual relations between first cousins regardless of marital status, so a couple who never applies for a license can still be prosecuted if the relationship is discovered. Officiants who knowingly perform a ceremony for a prohibited couple can face separate charges, usually misdemeanors.
Even where the ban is purely civil rather than criminal, the marriage is void from the start. That means it never had legal force, and courts treat the parties as though no marriage ever existed.
States That Allow It With Conditions
A handful of states permit first cousin marriage only if the couple meets specific requirements, built around the idea that the state’s interest in preventing genetic risk disappears when the couple cannot have children.
The most common requirement is an age threshold. Some states set it at 65. Others set it at 55 but add a judicial finding that at least one partner cannot reproduce. In states using the fertility route rather than a pure age cutoff, the couple generally must present medical evidence to a court. A judge reviews documentation from a licensed physician and issues an order before the license can be issued. This is a court proceeding, not a clerk’s-office formality.
Applicants in conditional states still provide standard identification like a birth certificate or passport. They must also accurately disclose the family relationship on the marriage application. Misrepresenting the degree of kinship can lead to perjury charges, which are typically felonies.
Why First Cousins Sit on the Line
Legal systems measure family closeness using degrees of consanguinity, counting the steps between two people through their nearest shared ancestor. First cousins occupy the fourth degree in the civil-law method used by most U.S. jurisdictions: one step from you to your parent, one to your grandparent, one down to your aunt or uncle, and one more to your cousin.
That fourth-degree position is the dividing line in most incest statutes. Third-degree relationships and closer, such as aunts and nephews or half-siblings, are prohibited almost everywhere. Fifth-degree and beyond, including second cousins, draw almost no legal attention. First cousins sit right on the boundary, which is why different states have drawn the line on different sides of them.
What Happens When the Marriage Is Void
A void marriage is treated as though it never existed. That has consequences well beyond the label, because every right that flows from marriage disappears with it.
- No spousal inheritance. If one partner dies without a will, the survivor has no claim under intestacy. Even with a will, the survivor cannot claim the elective share spouses are normally entitled to.
- No joint tax filing at the state level, and often a higher combined bill.
- No spousal benefits. Employer health plans, Social Security survivor benefits, and pension survivor benefits all require a legally recognized marriage.
- No marital property rights. In a separation there is no community property division or equitable distribution; assets belong to whoever holds legal title.
- No automatic hospital visitation or medical decision-making authority in an emergency.
Courts in void-marriage states will not grant a divorce, because there is nothing to dissolve. If the couple has children, parentage and custody follow the rules that apply to unmarried parents, usually through a separate paternity or parentage proceeding.
Crossing State Lines
The legal status of a first cousin marriage can shift when a couple moves. The general rule in American law is that a marriage valid where it was celebrated is recognized everywhere. This place-of-celebration rule has governed interstate marriage recognition for well over a century.1National Constitution Center. Article IV, Section 1: Full Faith and Credit Clause
The rule has an important exception. A state can refuse to recognize an out-of-state marriage that violates its own strong public policy. In practice, courts look at whether the destination state would impose criminal penalties for the relationship. Where the prohibition is purely civil, courts tend to honor the out-of-state marriage. Where first cousin relations are a criminal offense, courts are more likely to invoke the public policy exception and treat the marriage as void.
That creates real legal limbo. A couple can be married for every purpose in one state and legal strangers in another, and the outcome of any dispute over property, custody, or inheritance may depend on which state’s court hears it. Legal advice before relocating is cheaper than litigation after.
Federal Taxes
The IRS follows the place-of-celebration rule. Under Revenue Ruling 2013-17 and the corresponding Treasury regulation, a couple whose marriage was valid in the state where it was performed is treated as married for all federal tax purposes, even if they later move to a state that would not recognize the union.2Internal Revenue Service. Rev. Rul. 2013-17
A first cousin couple who married legally in a permissive state can file jointly on their federal return regardless of where they currently live. The federal regulation confirms that a marriage is recognized for tax purposes “if the marriage is recognized by the state, possession, or territory of the United States in which the marriage is entered into, regardless of domicile.”3Federal Register. Definition of Terms Relating to Marital Status
Immigration and Spousal Petitions
USCIS also starts with the place-of-celebration rule, but adds a public-policy check tied to the couple’s intended state of residence. A first cousin marriage that was valid where it was performed will generally be recognized for immigration benefits, including an I-130 spousal petition, unless the state where the couple plans to live would refuse to recognize it or would criminalize the relationship.4USCIS. Chapter 2 – Marriage and Marital Union for Naturalization
Where the state of residence treats the marriage as void and criminally incestuous, USCIS has denied petitions. A couple cannot sidestep that result by claiming residence in a permissive state when they actually intend to live elsewhere; adjudicators specifically watch for that kind of forum shopping.5USCIS. Chapter 6 – Spouses
The practical question for any couple in this situation is narrow. Does the state where you plan to live allow first cousin marriage, or allow it under conditions you meet? If yes, the petition should proceed normally. If no, the petition faces denial no matter where the ceremony happened.