Double First Cousins: Everything You Need To Know

By Elizabeth Tirk | | People
Double First Cousins: Everything You Need To Know
Monkey Business Images/Shutterstock

What are double first cousins? The term isn’t all that common among the already broad scope of unique familial relationships, but how exactly can people be double first cousins to begin with? The simplest answer is that when two siblings of one family have kids with two siblings from a different family, the resulting children are first cousins on both sides—otherwise known as double first cousins.

If we take that branch of familial relation even further, what does it mean to be double half first cousins or irregular double first cousins? Don’t worry, it’s not as complicated as you might think.

What are double first cousins?

As the name suggests, double first cousins are cousins that share the same four grandparents with each other. They are first cousins once on their mother’s side, and once on their father’s side, making them a first cousin twice. This happens because two of the children from one set of grandparents reproduced with two of the children from the second set of grandparents.

Surprisingly those who have a double first cousin are only 25% related to them, according to Barry Starr, a biochemist and retired tech geneticist with The Tech Interactive, a Stanford program that brings geneticists in to answer questions. That means double first cousins are twice as related as first cousins.

“This is the same amount of DNA that you would share with a grandparent, a half-sibling or an aunt or uncle,” Starr said.

Double first cousins chart

Double cousins diagram

Are double first cousins genetically siblings?

No. First cousins are only related by about 12.5%. Since double first cousins are double that, they are genetically more like half-siblings. However, Marc McDermott with the National Genealogy Society saids they wouldn’t be recognized as true half-siblings since none of them share any parents.

What if both parents are identical twins?

The answer is probably still no. If all four parents came from two sets of identical twins, their children will genetically resemble siblings and share around 50% of their genetics with their cousins like siblings would. That doesn’t actually make them siblings though. Much like the double first cousins, they won’t be recognized as siblings because none of them share any parents together regardless of their close relations.

What are double half first cousins?

Similar to double first cousins, double half first cousins happen when two half-siblings of one family have children with two half-siblings of another family. These children are double half first cousins because they share a single grandparent on each side of their family. Comparatively, first cousins share their two grandparents on one side and double first cousins share all four grandparents.

Double half first cousins are only about 12.5% related to each other, similar to first cousins. However, it should be noted that double half first cousins and first cousins are not interchangeable due to their differing variations in grandparentage.

What are irregular double first cousins?

This is a very specific arrangement: An irregular double first cousin happens when two siblings of one family have children with two half-siblings of another family.

While the irregular double first cousins are more closely related than the double half first siblings, the irregulars are not as closely related as the double first cousins. The irregulars share about 18.75% of their DNA between each other, meaning they sit roughly halfway between a first cousin and a double first cousin.

Conclusion

So, what are double first cousins? Well, we know they share all four grandparents with a 25% genetic overlap. While they are incredibly similar to half-siblings, their distinct relation falls on the grandparental line instead of parental.

Then how can you be double first cousins? If your parents married into the same family as your aunt and uncle, and they then had kids of their own, congratulations, you’re double first cousins!

Frequently Asked Questions

How common are double first cousins?

Double first cousins were more common in the past when travel was limited. They are less common now.

Can double first cousins marry?

In the U.S., second cousins are legally allowed to marry but first cousins and closer relations are limited depending on the individual state’s laws.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.