What are Double First Cousins?

When researching your family tree, you may come across a situation where relatives are not just first cousins, but double first cousins. In this article, we cover what double first cousins are and how they can become doubly related to each other.

In the fascinating world of family history, there are many confusing terms and complicated relationships to figure out. Close family is easy: grandparents, parents, siblings, aunts and uncles. It quickly gets complicated when we get into the world of cousins.

The concept of a first cousin is relatively simple, they are the children of your blood aunt or uncle. As it progresses though we hear about second, third or fourth cousins. Then the ever confusing half cousins and the “removed” term can be daunting.

One term that is often confusing is the “double cousin.” What on earth can that possibly mean? How are they related to us and what makes them double?

What Is a Double First Cousin?

The double cousins sound like a very complicated and maybe distant relation but actually as cousins go they are likely the closest. This interesting family connection comes about when the parents of one cousin are also the siblings of the parents of the other cousin.

In a normal cousin relationship, there is usually only a blood connection between one parent of each cousin. So, for example, if your father's sister has a child then that child is your first cousin. If your father's brother marries your mother's sister and they have a child, then that is your double first cousin.

A unique aspect of this family connection is that whereas first cousins usually share only one set of grandparents, double cousins share both sets.

How Common Are Double Cousins?

In times gone by when we lived in smaller communities and didn’t tend to wander far from home, there were far fewer options in terms of marriage choices. It was certainly not uncommon for cousins as close as first to marry. It was also a lot more likely that multiple children from one family may marry into another family.

The result could be several couples featuring siblings from both families and ultimately quite a few double first cousins being born.

As the years have progressed the likelihood of such marriages has certainly decreased but it definitely has not become a rare occurrence. A couple who gets together tends to meet each other's families and by extension, they all meet one another. It certainly is not unheard of that the siblings of the couple might meet and fall in love as well.

There is certainly nothing taboo or frowned upon with the concept of siblings marrying the siblings of an unrelated family.

Are Double First Cousins More Closely Related?

In terms of the genetic relationship closeness of double cousins, let us first discuss the important measurement unit centimorgans. This unit of measurement is used to measure the genetic match between individuals. Those who have ever taken a test with 23andMe or Ancestry will likely see numbers attached to the matches that have "cm" attached to them.

Centimorgans are not measurement units like length, or weight rather they are a unit of estimate. The higher centimorgan match you have with someone is an indication of how many DNA segments you have in common with them. Obviously, the more DNA matches you have with someone the closer you are genetically related.

So let us first consider how much DNA we usually share with a standard first cousin. This relationship comes through one of your parents and their siblings. A common range that would indicate a first cousin relationship is 553-1225 centimorgans. Usually, however, it is closer to 874 centimorgans, so a DNA match at that level generally indicates a first cousin relationship.

Double cousins though are slightly different. The reason for this is that they both descend from the same two sets of grandparents. This means that the relationship is genetically closer to being a half-sibling than that of a cousin.

Sharing almost double the amount of DNA than an average first cousin connection, the double cousin centimorgan range is 1317-2312 centimorgans. The average DNA match for a double cousin relationship is around 1783 centimorgans.

Does the Double Cousin Match Cause Problems?

In terms of researching family history and using DNA as a tool, the double cousin match would seldom cause issues. It is most likely that a family would be aware of a double cousin situation. But if there were reasons that a person had no idea about their family history due to adoption or abandonment then there may be some confusion.

To an outside observer who does not know any of the immediate family history a double cousin DNA match could look like that of a half-sibling. It would be more likely to be assumed that the connection was that of half-sibling so this could cause some confusion.

Historically though the double cousin relationship has caused issues with contributing to endogamy. This is where the common practice of cousin marriages occurs within a small community. Intermarrying between a small isolated group causes what is known as pedigree collapse.

The gene pool of the group becomes limited and most individuals share a lot of the same DNA. This results in it being very difficult to determine connections between family members based on their DNA match estimates.

The Ultimate Double Cousins

As close as double cousins are genetically, there is a situation from which double cousins could become even more closely related. A standard double cousin would come from siblings of two families who would have received differing levels of DNA from their respective parents.

Imagine then if two sets of identical twins were to marry and both have a child. Identical twins share 100% of their DNA which essentially means that genetically they are almost the same person. As a result, the children created from these two marriages would share the same level of DNA as that of full siblings.

Siblings share 2209-3384 centimorgans with the average match being around 2629 centimorgans. They share the same parents from whom they receive 50% of their DNA from each. The offspring of two sets of identical twins would share the same amount of centimorgans in common as if they were full siblings.