What are Double First Cousins

Marc McDermott
February 8, 2022

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. 

It should be noted that we are referring to brothers and sisters who marry other brothers and sisters, and not their own siblings. 

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. 

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