Can Third Cousins Legally Marry in the United States? Everything You Need to Know

Hey there! As a tech geek and data analyst who loves genealogy, I‘ve had a growing number of friends and readers ask me about the genetics, risks, and laws around marrying third cousins. It‘s an interesting topic with a complex history, so I wanted to really break it all down in this post. Let‘s dig in!

Defining What Third Cousins Are

First things first – what exactly are third cousins? Here‘s a quick refresher:

• First cousins share grandparents (2 generations back)
• Second cousins share great-grandparents (3 generations back)
• Third cousins share great-great-grandparents (4 generations back)

Here‘s how it looks in a family tree:

[Simple family tree diagram showing 3rd cousin connections]

So third cousins have just one set of great-great-grandparents in common. Seems pretty far removed!

On average, third cousins will share about 0.78% of their DNA. But due to randomness in inheritance patterns, some third cousin pairs may share no detectable DNA at all. More on the genetics next.

Assessing the Genetic Risks of Third Cousin Marriage

Now let‘s get into the science! Just how risky is third cousin marriage genetically?

Here‘s an overview of the main risks and what research has found:

• Increased recessive mutation pairing – 1.7-2.8% higher risk
• Increased risk of rare autosomal recessive disorders – about 2.3% higher
• Slightly higher risks (~1.3-1.6x) of conditions like cleft palate, spinal bifida, heart or limb defects
• No increased risk seen for common chronic illnesses like cancer or heart disease

Now those sound scary at first. But looking closer, the baseline risk for the general population is very low for the rarest disorders we‘re talking about. Let‘s break it down:

Disorder	Baseline Risk	Risk with 3rd Cousin Marriage
Spinal Muscular Atrophy	1 in 6,000 to 1 in 10,000	Approx. 1 in 3,750
Cystic Fibrosis	1 in 2,500	Approx. 1 in 1,600

As you can see, the absolute increased risk is only about 0.1-0.5% higher. Researchers deemed this statistically insignificant.

For context, the risks from much more distant second cousin marriage was comparable at 1.7-2.8%. The risks don‘t lower substantially until beyond 6th-8th cousins.

So genetically, third cousins are arguably distant enough. But what‘s the optimal level of shared DNA in a mate from an evolutionary standpoint? Fascinatingly, second or third cousins might be the "goldilocks" sweet spot…

The Evolutionary Biology Perspective on Cousin Marriage

Evolutionarily, mating with a complete stranger can increase risks of genetic incompatibility. But mating with a close relative increases risks of expressing damaging recessive mutations.

Turns out second or third cousins might be the optimal balance point. Their genomes are just similar enough to avoid rejection risks, while not being so similar as to raise recessive disorder risks.

In fact, one study found third cousin marriages produced the optimal number of children and grandchildren compared to other cousin pairings!

This aligns with mathematical models predicting second or third cousins as the optimal mate choice for reproductive success. Quite amazing how the math works out that way!

Of course, we marry for love not mathematical evolutionary optimization these days. But the science shows cousin marriage can make sense biologically.

Just How Common Are Third Cousin Marriages?

Third cousin marriages end up being far more common than you might think! Here are some stats:

• Roughly 1 in every 1,000 marriages in European populations is between third cousins or closer.
• In Iceland, marrying third cousins is so common that software exists to avoid accidental cousin marriages!
• Among Ashkenazi Jewish populations, the rate of third cousin marriages reaches as high as 10%!
• Experts estimate all individuals on Earth are at least 50th cousins. So distant cousin relations are inevitable if you go back far enough.

With global travel and population mixing, chances are many third cousin pairs have connected randomly. They just don‘t realize it without a DNA test!

The Changing Social Stigma Around Cousin Marriage

The legality and social acceptance of cousin marriage has varied greatly across history. Here‘s a brief overview:

• 1800s: 20 of 24 US states permitted first cousin marriage.
• 1860s: cousin marriage bans began passing state by state, justified by avoid "defective offspring."
• Early 1900s: cousin marriage opposed on pseudoscientific eugenics theories.
• Today: 25 states permit first cousin marriage, but social stigma remains. Beyond second cousins widely accepted.

So why did attitudes change? In the 19th century, fears spread of "inbreeding" and passing on family defects through cousin pairings. But modern genetics has shown moderate kin marriage does not necessarily have drastic risks.

The social stigma against cousin marriage lessens significantly for third cousins and beyond. Unlike closer pairings, it‘s unlikely to be seen as controversial or taboo.

Famous Examples of Third Cousin Marriages

Want a few real-world examples of successful third cousin marriages? Check these out:

• Edgar Allen Poe married his third cousin Virginia in 1836.
• Jesse James married his third cousin Zerelda Mimms in 1874.
• Franklin D. Roosevelt was a third cousin of his wife Eleanor Roosevelt! Their last common ancestors were in the early 1700s.

With so many generations back to find the connection, third cousins are really very distantly related. Their family relation ends up fairly trivial.

DNA Testing Makes Finding Relatives Easier Than Ever

Nowadays services like 23andMe and AncestryDNA make it possible for anyone to uncover distant relatives at the click of a button! But what do third cousins look like genetically?

On average, third cousins share around 73 centimorgans (cM) of DNA. However, the actual amount shared can vary widely due to randomness in inheritance. Some third cousins share hardly any detectable DNA at all.

Here‘s a chart with average shared DNA for different cousin levels:

[Add data viz showing Shared cM by cousin level]

So a third cousin match could show up with anywhere from 0-200+ cM shared. The amount isn‘t necessarily telling! You have to use genealogical records to confirm the actual relationship.

Legalities: Third Cousin Marriage Is Permitted Nationwide

Okay, let‘s talk laws! Today, third cousin marriage is legal nationwide in the United States. No federal or state laws prohibit cousin marriage at this level.

The most restrictive states permit first cousin marriage with conditions but ban uncle-niece and aunt-nephew marriages. No states restrict third cousins or beyond.

So if you discover a romantic interest happens to be a distant third cousin, legally speaking you are in the clear! At this range, genetic risks are minimal as well.

My Take: The Science Supports Third Cousin Marriage

Personally, after reviewing the research on genetics, risks, and evolutionary biology, I don‘t see an issue with third cousin marriages. The science shows genetic risks are low and social stigma fades quickly for third cousins and beyond.

I think viewing third cousins as essentially unrelated is reasonable. With only 0.78% average DNA shared and four generations back to find the common ancestor, the familial connection ends up negligible.

Of course personal and religious views differ on cousin marriage in any form. But if the marriage otherwise seems right, a distant third cousin relation shouldn‘t stand in the way. That‘s my two cents!

The Bottom Line

I hope this overview was helpful and clearly explained the realities around third cousin marriage – genetically, legally, and socially. The short answer is yes, third cousins can marry legally nationwide. And the genetic risks are minimal. But personal choice always factors in.

Let me know if you have any other questions! I‘m always happy to chat more about the latest genetic genealogy research and tech.