Whether you can marry your first cousin depends on the state you live in. Roughly 18 states and the District of Columbia allow it with no special requirements. About 25 states ban it outright. Seven states allow it only under conditions such as a minimum age or proof of infertility. And in nine states, sexual relations between first cousins are a crime, regardless of whether the couple ever tries to marry. Because marriage is regulated state by state, the same relationship can be legal, void, or criminal depending on where you cross the county line.
The Three Categories of State Law
Marriage regulation belongs to the states, so there is no single federal answer. Each state writes its own list of relationships that are too close for marriage, and first cousins land in different places on those lists.
In roughly a third of states, first cousins can apply for a marriage license at any county clerk’s office and marry the same way an unrelated couple would. No doctor’s note, no judicial approval, no extra paperwork. These states simply leave first cousins off the prohibited list.
The majority of states go the other way. First-cousin marriages are void from the start, which means the clerk denies the license once the application shows the relationship. A void marriage has no legal existence at all, so the couple cannot file taxes jointly, inherit as a spouse, or claim any other benefit that flows from a valid marriage. Several of these states add criminal penalties on top of the civil ban.
Seven states sit in the middle. They prohibit first-cousin marriage as the default rule, then carve out narrow exceptions, almost always tied to whether the couple can have biological children.
What Conditional States Require
If you live in a state that permits first-cousin marriage only under specific conditions, expect one or more of the following requirements before the clerk will issue a license.
- A minimum age, typically between 50 and 65. Some states require both partners to meet the threshold; others require only one.
- A physician’s certificate showing that at least one partner is permanently unable to reproduce, filed with the marriage license application.
- Completion of genetic counseling, with a physician’s certificate presented before the license is issued. At least one state requires this regardless of age or fertility.
- Judicial approval, in addition to any age or infertility requirement.
These conditions reflect a compromise. Lawmakers in these states treated genetic risk to future children as the main reason to restrict the marriage, so they lifted the restriction once that risk no longer applies. Plan on gathering medical documentation, and possibly appearing before a judge, before the license moves forward.
Where the Relationship Itself Is a Crime
Most states that ban first-cousin marriage stop at voiding the union. Nine states go further and classify sexual intercourse between first cousins as a criminal offense under their incest statutes. In those states, the physical relationship itself is illegal, whether or not the couple ever applies for a license.
Penalties vary widely. Some states treat it as a misdemeanor. At least one classifies it as a second-degree felony carrying up to ten years in prison, a fine of up to $10,000, and mandatory sex offender registration. Another treats it as a third-degree felony with up to five years and a $5,000 fine. This range matters most for couples who marry legally in one state and then move: their marriage may have been perfectly lawful where performed, but relocating can expose them to prosecution in the new state.
Marrying in Another State to Avoid a Home-State Ban
Couples who live in a state that bans first-cousin marriage sometimes travel to a permissive state to marry, then return home. The general principle of lex loci celebrationis says a marriage valid where performed is valid everywhere. In practice, that principle has a large hole in it.
The hole is the public policy exception. A state that normally recognizes out-of-state marriages can refuse to recognize one that violates a deeply held local policy. If your home state considers first-cousin marriage contrary to its public policy, it can treat the out-of-state marriage as void for every local purpose: property, inheritance, health insurance, and divorce.
Some states make this explicit through marriage evasion statutes, which specifically target residents who leave the state to enter a marriage that would be illegal at home and then return. Under those statutes, the marriage is void the moment the couple comes back, no matter where the ceremony happened. The State Department’s Foreign Affairs Manual describes the same patchwork: some states will recognize a foreign cousin marriage even where local law prohibits it, and others will not.
The practical risk is severe. If the marriage is voided at home, the couple may have no standing to file for divorce or claim spousal support, and can be left in legal limbo. Before traveling to marry, verify both that the destination state allows the marriage and that your home state will recognize it once you return.
Immigration and Federal Benefits
When a first-cousin marriage runs into federal law, the analysis shifts. For visa purposes, the State Department’s Foreign Affairs Manual instructs consular officers that “the law of the place of marriage celebration controls.” If the marriage was legally performed and recognized where it took place, it is generally valid for immigration adjudication. But consular officers are told to request an Advisory Opinion whenever the parties are biological relations such as first cousins, treating the case as “not clearly approvable.”1U.S. Department of State Foreign Affairs Manual. 9 FAM 102.8 Family-Based Relationships
Once USCIS has approved an immigration petition with full knowledge that the couple are first cousins, consular officers are directed to accept that determination. But USCIS policy allows denial if the couple intends to live in a state that voids cousin marriages or criminalizes the relationship. Establishing residence in a permissive state solely to sidestep the public-policy rules of the state where the couple actually plans to live is also grounds for denial.1U.S. Department of State Foreign Affairs Manual. 9 FAM 102.8 Family-Based Relationships
For Social Security spousal and survivor benefits, the agency generally looks to the law of the state where the worker was domiciled when the claim was filed, or when the worker died for survivor claims. If that state does not recognize the marriage, benefits can be denied regardless of where the wedding took place.2Social Security Administration. PR 02706.053 Washington
What Happens to the Children if a Marriage Is Voided
Every state considers a child born during a voided marriage to be legitimate. The Foreign Affairs Manual states plainly that “every state considers a child of a voided marriage to be legitimate or born in wedlock,” and that even after a formal annulment, “a child’s legitimation status usually is not affected.”3U.S. Department of State Foreign Affairs Manual. 8 FAM 304.1 Marriage
Parental rights, custody, and child support survive the voiding of the marriage. Both parents keep their legal relationship to the child, and courts handle custody the same way they would after any divorce. The marriage may not exist in the eyes of the law, but the parent-child relationship does.
The Genetic Risk Behind the Laws
First cousins share roughly 12.5% of their DNA because they have a common set of grandparents. When both parents carry the same recessive gene variant, the child has a higher chance of inheriting two copies and developing the associated condition. This is the concern that drives most cousin-marriage restrictions.
The measured risk is smaller than many people assume. Research puts the increased risk of birth defects for children of first cousins at roughly 2 to 2.5 times the general population rate. In absolute terms, the baseline risk of a significant birth defect for any pregnancy is about 2 to 3 percent; for first-cousin couples, it rises to approximately 4 to 7 percent. A large study found an increased risk of about 4.4 percent for pre-reproductive mortality in children born from first-cousin unions. For comparison, a woman over 40 faces a similarly elevated risk of certain chromosomal conditions in her pregnancy.
Genetic counseling can give first-cousin couples a specific risk profile based on their own family histories, particularly if either side carries a known genetic condition. Even where counseling is not required by law, it turns general statistics into information you can actually use.