What is a double fourth cousin? How much DNA do they share?

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What is a double-fourth cousin, and how are they related? Do double-fourth cousins share more DNA with each other than they would otherwise?

In this post, I’ll explain the meaning of a double-fourth cousin, show you an example of my own double-fourth cousin, and talk about how this “double” relationship affects the amount of DNA shared between cousins of this relationship type.

What is a double-fourth cousin?

Double fourth cousins will generally share two sets of great-great-great grandparents.  In other words, they are fourth cousins – twice.

How can this happen? 

Have you ever heard of two brothers marrying two sisters?  Well, if your great-great grandfather married his brother’s wife’s sister, then the descendants of those two marriages will be “doubly” related to each other. 

Their children will be double first cousins, their grandchildren will be double-second cousins, and their great-grandchildren will be double third cousins, and – you guessed it – their great-great grandchildren will be double fourth cousins.

Sometimes it is easier to understand an explanation when you have a visual, so I made the chart below.  In the image, you can cousins ranging from first to fourth – all double cousins. 

The common ancestors that they share are Sarah and Leigh’s parents AND Mark and Sam’s parents.

In the chart, Jimmy and Amy are double fourth cousins.  They share two sets of great-great-great grandparents, instead of just one set as “single” fourth cousins would.

Technically, it’s also possible for two double fourth cousins to only share one great-great-great grandparent from each line, which would make them half-cousins.  For example, let’s imagine that Sarah and Leigh share the same father, but Sarah’s mother died young, and Sarah’s father remarried. 

After his remarriage, he had Leigh.  This makes Sarah and Leigh half sisters.

Since fourth cousins are generally considered to be distant cousins, people usually don’t get caught up in trying to define the exact relationship (like in the case of Sarah and Leigh being half sisters). 

If Sarah and Leigh were half sisters, and Sam and Mark were half brothers, would that make Jimmy and Amy double-half-fourth cousins?  I think it might, but I’ve never heard anyone use that term before.

Example of a double-fourth cousin

I recently discovered a double-fourth cousin in my family tree, which is actually what got me thinking about writing this article.  I have seen two siblings marry two other siblings several times in different lines of my tree, and I’m just going to go out on a limb here (get it??) and guess that many other people have seen this in their families, too.

My double-fourth cousin (I like saying that – it has a better ring to it than just plain “fourth cousin”) is related on my mom’s side of the family.  I found her on my DNA match list, which is the best place to find distant cousins.

In order to figure out how we are related, I had to build a “quick and dirty” family tree for her.  On her public family tree, she only had her parents and grandparents listed. 

I have found that spending 30 minutes or so seeing if I can work out a simple basic family tree for my DNA matches is a really great way to find out how we are related.

With this match, my strategy worked pretty quickly.  I found out that her great-grandmother, Louise, was the daughter of Julia (last name withheld for privacy). 

When I saw Julia’s full name, I realized that she was the same Julia in my family tree – the sister of my great-great grandmother.  Furthermore, I saw that Julia had married the brother of my great-great grandfather.

How much DNA would double-fourth cousins share?

It’s possible for two fourth cousins to share absolutely no DNA.  In fact, you’ll probably only match about half of your fourth cousins on a DNA test. 

We don’t share DNA with all of our relatives, after all.

So it would be completely normal if you didn’t match a fourth cousin on a test.  Most of our fourth cousin and closer matches will be fourth cousins, however, since we have much more of them than we do first, second, or third cousins. 

For example, you might have about 190 third cousins, but could have about 940 fourth cousins.

My double fourth cousin match had confused me, at first.  We share 44 cMs (centimorgans), which is fine for fourth cousins.  Fourth cousins can share from zero cMs to about 90 cMs. 

The thing that kind of bugged me was that she shared 44 cMs across 4 different DNA segments.  I have noticed that I usually just share one segment with my actual fourth cousins (though this can vary), so I had been wondering if she was more closely related to me than fourth cousin.

See image below:

Now, I understand that the reason that we share four DNA segments is because we share twice as many common ancestors as is typical for fourth cousins, giving much more opportunity for us to both inherit matching segments from some of those ancestors.

It is conceivable that double fourth cousins could share DNA above the range typically seen for single fourth cousins (90 cMs is generally very high for fourth cousins), but it also seems very possible for double fourth cousins to share no DNA.

Conclusion

I hope this post helped you get a better idea of what a double fourth cousin is, and how much DNA you might share.  If you have any questions about something that you read here, or you would like to share your experience, I would love to hear from you below.

Thanks for stopping by!

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