What Does Double First Cousin Mean? (Is It a Real Thing?)

Are you mapping your family tree and finding yourself coming across terms you have never heard of? Perhaps you have come across the phrase double first cousin, but don’t know what it means? Or maybe you are curious and want to know more?

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Whatever your reason might be, we have the answer for you!

We know how difficult it can be to navigate the world of family trees and cousins. There are so many different phrases out there that can make it almost impossible to build your family tree and work out what your relation to each relative is. 

And cousins are no different. With so many different types of cousins out there, it can be hard to know what kind you have.

You can find it difficult to complete your family tree and work out who your relatives are. Read on to learn exactly what a double first cousin is.

What Is A Double First Cousin?

Let’s get straight into it, while you normally share one set of grandparents with a regular first cousin, double first cousins share both sets of grandparents. We see this happening when one of your parents and your double first cousin’s parents are siblings. 

Do you have any double first cousins? Find out with an AncestryDNA test!

It sounds a little weird at first, but it’s actually not totally unheard of. Double first cousins happen when two sets of siblings have kids—these kids will be double first cousins.

Personally, I have a brother, so, if my brother marries a woman, and I marry that woman’s brother, our kids would be double first cousins.

Do you need an example to help understand it? We’ve got one for you! Take a woman who meets a man and they start to date, pretty normal, right?

Then, imagine that the man has a great brother and the sister has a fantastic sister. The first couple introduces their siblings to each other and they hit it off! 

Imagine that they hit it off so well that the first couple marries each other and the second couple (their siblings) marry each other and all produce children. The two sisters have married a different brother from the same family and their children would be double-first cousins! 

These cousins would share both their maternal and paternal grandparents, creating a super close family! While it might seem rare to have a double first cousin, it is surprising how many there are out there. 

You could be looking at some on your family tree without even realizing there was a special name for these cousins!

Now that we have covered what a double first cousin is, let’s dive a little deeper and see what else we can learn about them. 

So how are these cousins related? As mentioned earlier, double first cousins are twice as related to each other as normal first cousins because they share two sets of grandparents instead of just one.

Think back to the example we gave you earlier. As the cousin’s mothers and fathers are siblings with one another, the cousins would share a set of maternal grandparents and paternal grandparents. Usually, you would share one or the other depending on which side of the family your cousins are.

As double first cousins have the same grandparents, they share 100% of their ancestors and relatives, compared to the 50% that a typical first cousin shares with their cousin.

Do you have any double first cousins? Find out with an AncestryDNA test!

Are Double First Cousins Technically Siblings?

two cousins smiling

As we said earlier, double first cousins share twice as much DNA with each other than typical first cousins. Usually, first cousins will share 12.5% of their DNA, which works out as ⅛ of their total DNA. Double first cousins, on the other hand, share roughly 25% of their DNA, or ¼. 

Does that sound familiar? Well, half-siblings usually share about 25% of their DNA, so double first cousins actually have the same amount of shared DNA that half-siblings do—about 25%.

So no, they’re not genetically full siblings, but that is still a lot of DNA shared! (For those that don’t know, half-siblings are siblings that share one parent, rather than two.)

They might have the same mother but different fathers, or the same father but a different mother. 

These days half-siblings are far more common than they used to be. We see more people re-marrying or cohabiting with different partners than we did say 50 or 60 years ago. There is also far less stigma these days around divorce and remarriage or cohabitation.

So you are more likely to see half-siblings more recently in your family tree than several generations ago. 

If you see them with your ancestors, it is more likely that half-siblings are the result of a parent being widowed and remarried rather than a divorce. That is not to say that it didn’t happen, but before divorce was more freely available, half-siblings tended to be the result of affairs or widows remarrying. 

Now, back to double first cousins. Just because they share the same amount of DNA that half-siblings do, it does not mean that they are siblings. What it does mean is that from a genetic point of view, double first cousins have more in common than typical first cousins do. 

Double first cousins will have a much larger genetic match than other cousins and you should be able to find double first cousins easily through a DNA test. What’s more, as the generations continue, they will also have a closer genetic match.

So double second, double third, and even double fourth cousins tend to be closer genetic matches than typical cousins! 

Do you have any double first cousins? Find out with an AncestryDNA test!

Final Thoughts 

And there you have it, double first cousins share both sets of grandparents! Their parents are all siblings, giving them a closer genetic relationship to one another than other cousins.

These closer families aren’t as uncommon as you might think and will look fairly similar to normal cousins on family trees. 

However, if it looks as though the parents of children are related, then you could have some double first cousins on your family tree. Why not take a look and see if there are any on your tree?

To find your double first cousins, you can search for them as you usually would through DNA testing or sites like Ancestry. It’s also worth speaking to your parents to find out more about their siblings, as they would be your key to finding double first cousins. 


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