Starting next month, a US state will ban marriages between first cousins, although it remains legal in 16 other states.

Effective October 1, Connecticut will prohibit marriages between first cousins—individuals who share grandparents, meaning their parents are siblings—due to concerns that are both genetic and social in nature.

The new legislation, supported by both the Democratic and Republican Parties, will not penalize existing first cousin marriages that were legitimate before the law's implementation, nor will it invalidate those marriages retroactively.

This law pertains only to marriages occurring from October 1, 2025, onwards and only applies to those 'knowingly' marrying a first cousin, meaning awareness of the familial relationship is a legal consideration.

Devin Carney, a Connecticut State Representative (Republican), who introduced the bill, told the Connecticut Post: "I received an article about it. Tennessee recently passed legislation to ban it.

A US state will be banning first cousin marriages from the start of next month (1 October) (Christoph Wagner / Getty Images)

"I learned it’s not banned in Connecticut, so I started researching and found that over 30 states do ban it, and Connecticut isn’t among them."

Democrat State Representative Steve Stafstrom added: "We discovered we were somewhat of an outlier state that didn’t prohibit it.

"Science indicates that procreation between first cousins raises the risk of birth defects. We also wanted to avoid penalizing or endangering any marriages that took place when it was legal."

Connecticut will become the 26th US state to fully ban marriages between first cousins.

Once the news broke, people quickly took to social media to express their surprise—especially that such marriages are still legal in many US states.

Connecticut will become the 26th US state to fully ban first-cousin marriages (SimpleImages / Getty Images)

An Instagram user queried: "How is this still possible in 2025 America?"

"Why was it even legal to begin with?" another wondered, while a third commented: "Really? It's still legal in 2025?! Gross."

Another echoed: "This is a thing????"

"It really took this long?" wrote another.

Finally, an Instagram user added: "Why is a law needed? Isn’t that common sense?"

Research from the National Library of Medicine has shown that procreation between family members can result in several birth defects, including limb deformities, heart conditions, premature birth, and neonatal mortality.

There are 16 other US states which still allow first cousins to get married (Getty Stock Images)

States where first-cousin marriage is allowed (no special additional restrictions)

These states permit first cousins to marry under standard marriage laws, according to LegalClarity.

• Alabama
• California
• Colorado
• Florida
• Georgia
• Maryland
• Massachusetts
• New Jersey
• New Mexico
• New York
• Rhode Island
• South Carolina
• Vermont
• Virginia
• North Carolina (with a restriction: “double first cousins” are not allowed)

States where first-cousin marriage is allowed (under special additional restrictions)

Several US states permit first-cousin marriage only under specific restrictions or conditions rather than allowing it freely, with laws varying by age, fertility, counselling, etc.

In Arizona, first cousins may marry only if both are 65 or older, or if one is infertile, while in Illinois, it's permitted only if both parties are 50 or older, or if one of them is permanently and irreversibly sterile.

Similarly, in Indiana, it's only allowed if both are at least 65 years old, while in Maine, first cousins need to obtain a physician’s certificate for genetic counselling beforehand.

In Utah, it's allowed if both parties are 65 or older, or if both are at least 55 and one is unable to reproduce, and lastly, in Wisconsin, it's only allowed if the woman in the marriage is at least 55 or if one of the parties is permanently sterile.

The bill was passed due to genetic and social concerns (Manu Vega / Getty Images)

States prohibiting first-cousin marriage

"A significant number of states have laws that explicitly prohibit the marriage of first cousins. In these jurisdictions, a marriage license will not be issued to first cousins under any circumstances," LegalClarity explains.

The states that have an outright ban on first-cousin marriage include:

• Arkansas
• Delaware
• Idaho
• Iowa
• Kansas
• Kentucky
• Louisiana
• Michigan
• Mississippi
• Missouri
• Montana
• Nebraska
• Nevada
• New Hampshire
• North Dakota
• Ohio
• Oklahoma
• Oregon
• Pennsylvania
• South Dakota
• Tennessee
• Texas
• Washington
• Wyoming

These prohibitions are typically found within state statutes that define prohibited degrees of kinship for marriage. Furthermore, some of these states may classify such a marriage as a criminal offence.