Marrying a first cousin is legal in many countries and in roughly 19 U.S. states, and the genetic risks, while real, are smaller than most people assume. The baseline risk of a birth defect for any couple is 2 to 3 percent. For first cousins, that rises to about 5 to 6 percent. Whether it’s “okay” depends on where you live, whether you plan to have children, and how you weigh a modest increase in genetic risk against personal and cultural factors.
Why Cousin Marriage Raises Genetic Concerns
Every person carries a handful of faulty gene copies that don’t cause problems because the second copy of that gene works fine. You’re a silent “carrier.” The trouble starts when both parents carry the same faulty gene and a child inherits the broken copy from each side. That child ends up with no working version, and a recessive genetic condition can result. When both parents are carriers, each pregnancy has a 25 percent chance of producing an affected child.
Cousins are more likely to carry the same faulty genes because they inherited DNA from the same grandparents. First cousins share roughly 12.5 percent of their DNA, which means the odds of both carrying the same hidden recessive variant are considerably higher than for two unrelated people. This is the core mechanism behind the increased risk.
How Big Is the Actual Risk?
The numbers are often exaggerated in popular culture. According to the Centre for Genetics Education, unrelated couples face a 2 to 3 percent chance of having a child with a significant physical or intellectual condition. First-cousin couples face a 5 to 6 percent chance. That’s an additional 1.7 to 2.8 percentage points above the baseline, per data cited by the National Society of Genetic Counselors.
To put it plainly: out of 100 babies born to first-cousin parents, roughly 94 to 95 will not have a detectable genetic condition. The risk is real, but it’s not the dramatic picture many people imagine. For comparison, a woman over 40 having a child faces elevated risks of chromosomal conditions that are in a similar range, and society doesn’t treat that as inherently unacceptable.
The picture changes if cousin marriage happens repeatedly across generations. When multiple generations in the same family marry cousins, the shared DNA between partners increases with each generation, and so does the risk of recessive conditions surfacing. This pattern, common in some communities with long traditions of cousin marriage, is where the most serious genetic consequences tend to appear.
Where It’s Legal in the United States
U.S. law on cousin marriage is a patchwork. Nineteen states fully permit first-cousin marriage. Twenty-four states ban it outright. Seven states allow it with conditions: Maine, for instance, requires genetic counseling before issuing a license, and some states only permit it if one partner is unable to reproduce. North Carolina takes an unusual approach, prohibiting marriage only between double first cousins (people who are cousins through both parents’ sides).
Legislation continues to shift. In 2024, Florida’s legislature passed a bill that would have banned first-cousin marriages starting in 2026, but the bill died before becoming law, leaving the practice legal there. If you’re in the U.S. and considering this, checking your specific state’s current statute is essential, since the rules vary widely and change periodically.
Legal Status in Other Countries
First-cousin marriage is legal across much of the world. The United Kingdom, Germany, Australia, and most of Europe permit it. In parts of the Middle East, North Africa, and South Asia, it’s not just legal but culturally common. Pakistan has the highest rate globally, with nearly two-thirds of marriages involving cousins.
That said, the legal trend in some Western nations is moving toward restriction. In late 2024, a UK lawmaker introduced a bill to ban first-cousin marriage, citing health concerns. Sweden’s government proposed a similar ban the same year after allowing the practice since at least 1686. The Netherlands has also debated a ban at the highest levels of government. None of these proposals had passed into law at the time of those discussions, but they signal growing political appetite for restrictions in countries that have historically allowed it.
What Genetic Counseling Can Tell You
If you’re in a relationship with a cousin and considering children, genetic counseling is the most practical step you can take. A counselor can map out your family health history, identify conditions that run in your shared family tree, and order carrier screening tests that check whether you both carry the same recessive gene variants. This kind of screening can narrow your personal risk estimate far beyond the general population statistics.
The National Society of Genetic Counselors emphasizes that counseling for cousin couples should be “evidence-based, nondirective, and culturally responsive.” The goal isn’t to talk you out of anything. It’s to give you specific information about your situation so you can make informed choices. Some states that allow cousin marriage actually require this counseling before granting a marriage license, recognizing it as a reasonable middle ground between outright bans and no oversight at all. Community health settings that offer preconception counseling, especially with language interpreters available, have been effective at reaching couples in communities where cousin marriage is traditional.
Second Cousins and More Distant Relatives
Most of the concern around cousin marriage applies specifically to first cousins. Second cousins (people who share great-grandparents) have much less overlapping DNA, and the additional genetic risk for their children is small enough that it’s difficult to distinguish from the general population’s baseline risk. Second-cousin marriage is legal virtually everywhere, including in U.S. states that ban first-cousin unions, and it’s rarely discussed as a health concern by geneticists.
Social and Cultural Considerations
Beyond genetics and law, social stigma is often the biggest practical barrier. In the United States and much of Western Europe, cousin marriage carries a strong taboo, even in places where it’s perfectly legal. People considering it may face judgment from friends, extended family, or their broader community. This stigma is largely cultural rather than scientific, rooted more in social norms that developed over the past two centuries than in any ancient universal prohibition.
In contrast, cousin marriage is a deeply valued tradition in many cultures, seen as a way to strengthen family bonds, preserve wealth within a family, and ensure compatibility between spouses and in-laws. Roughly 10 percent of marriages worldwide are between cousins or closer relatives. Framing the practice as universally harmful ignores the reality that it has been common throughout most of human history and remains so in large parts of the world today.
The most balanced approach treats this as a personal decision informed by three things: the legal rules where you live, an honest assessment of genetic risk (ideally with professional carrier screening if children are planned), and your own values and circumstances. The genetic risk for a single generation of first-cousin marriage is elevated but modest. It’s not zero, and it’s not catastrophic.