Building your family tree can be confusing, especially when the terms “removed,” “half-cousin,” and “second” or “third” cousin start to crop up. Anyone new to the genealogy world will know that such terms are difficult to get your head around, so allow us to shed some light on one of the most confusing for beginners, and even those who have prior knowledge – “double cousins.”
What is a double cousin?
Double cousins are first cousins, but twice. They share both sets of grandparents. This can happen when both parents of one double first cousin are also the siblings of parents of another double first cousin(s).
It can happen when two siblings meet and have offspring for two other siblings. So two sisters can marry two brothers, and have offspring. Think of it like this, when you met your partner, would you introduce any of your siblings to your partner’s siblings? If they hit it off, then it is perfectly understandable how double cousins can occur. Your children and their children wouldn’t be first cousins, they would be double first cousins.
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How closely related are double first cousins?
Double first cousins are not related in the same way as regular first cousins. In fact, they are considered to be twice as related. They share all four of their grandparents in common and share 100% of their ancestors. This is twice as much as the common 50% that most cousins share.
Double first cousins share their most recent grandparents and have strong connections to both sides of the family. Although this is rare, it can strengthen bonds with more people, as they are often just as related to everyone at the family reunion, which is something not many people can say!
How common are double first cousins?
Double first cousins occur in different circumstances. A common reason is that they were born into a small town or area, or even endogamous populations.
Double cousins may have been more common in days gone by when towns were smaller and access to other towns was not as easy.
Do double first cousins have the same DNA?
Double cousins share twice the amount of DNA as typical first cousins. The average DNA shared by two first cousins is around 12.5%. However, double first cousins share an average of 25% of their DNA.
This is the same approximate figure as half-siblings. Therefore, double first cousins share around the same amount of DNA as half-siblings.
This doesn’t mean that double first cousins are siblings. Rather it means they have more genetics in common than regular first cousins.
Therefore, double first cousins would usually have a close DNA match if they were to take a DNA test. Descendents of their bloodline such as double second, third, fourth, fifth cousins, etc, may also be closer genetically than typical cousins of the same relationship, but not double.
Are double cousins genetically siblings?
No, they are not. Although they share the same amount of DNA as half-siblings, they are not considered genetic siblings. For this to be the case, they would need to share the same parent or parents.
Since double first cousins are the children of two people from different families, they are not considered to be siblings.
Hi, I am a bit curious about the topic too
I wonder what if two siblings married their two double cousins (This is usual in certain parts of the world), are their children going to share 25% average of the DNA as their parents’ scenario?
And what if this circulation has been repeated through many generations (the children of the double cousins marry their double cousins and so-on, all of the family tree share the same grandparents) , will this be harmful to the off-springs?
Many Thanks!
It’s an interesting question for sure, but since it involves some complex genetics, it might be best to reach out to a genetic counselor for more detailed information. Thanks for your curiosity!
If double first cousins have children , are there children double second cousins ?
No they would be called 2nd cousins, but would share more DNA than the average second cousin.
Hi Marc,
My double cousin and I were reunited yesterday after more than 40 years. Her parents’ marriage (my mom’s brother and my dad’s sister) ended in divorce after 5 kids so we didn’t have much interaction with them because of distance and estrangement between my dad and his sister.
Lots of excited chatter and discussion about our shared DNA and life events. She had the “double cousin” name correctly, but I was happy to see your post to better understand our connection.
Nice to discover I have a kind of half-sister 😉
Genetics is fascinating – relationships even more so!
Hi. I am curious…
If double first cousins marry and have children, then these children are siblings, but are they also double second cousins? Then would the children of the siblings be first cousins or third cousins or both?
Thank you. We’re you the Mark McDermott on Jeopardy by chance?
Hey Sue. That’s a tricky scenario. I believe the children of double first cousins would just be siblings, not also double 2c. Then their children would just be first cousins.