What is a Fourth Cousin?

What is a fourth cousin? How many fourth cousins does the average person have? In this post, you’ll learn the answer to these questions, as well as:

  • What is a fourth cousin once-removed?
  • Whether fourth cousins share DNA
  • What is a half-fourth cousin?
  • Are fourth cousins blood related?
Image describing the subject of the article, "What is a Fourth Cousin?"

A true fourth cousin can be different than a fourth cousin DNA match. If you want to learn about fourth cousin DNA matches on sites such as Ancestry DNA, click here:

What is a fourth cousin?

A fourth cousin is a person who shares one set of great-great-great grandparents with you. Another way to think about it is that a fourth cousin is descended from a sibling of your great-great grandparent.

The image below shows an example of fourth cousins. The “ancestors” are Abel and Maria. Two of their children are displayed in the example, and you can see how the descendants of their children are related.

A chart depicting the answer to "what is a fourth cousin?"  This is includes an example of a fourth cousin
In this image, you can see what is a fourth cousin. The fourth cousins in this example are great-great-great grandchildren of Abel and Maria, descended from two of their children.

To explain the graphic even further:

  • The children of Abel and Maria are siblings
  • The grandchildren of Abel and Maria are first cousins
  • The great-grandchildren of Abel and Maria are second cousins
  • The great-great grandchildren of Abel and Maria are third cousins
  • The great-great-great grandchildren of Abel and Maria are fourth cousins

As long as the descendants of Abel and Maria are the same number of generations removed from the common ancestor (in the case of fourth cousins, five generations), they are fourth cousins.

If Abel and Maria’s great-great-great grandchild would like to know how to describe their exact relationship to a cousin who is of a different generation (i.e. Abel and Maria’s great-great grandchild), then they will have to read the next section about fourth cousins once-removed.

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If you have a fourth cousin who is descended from a sibling of your great-great grandparent who married a sibling of a great-great grandparent of a different line of your family, then you have a double-fourth cousin. You can read more about double-fourth cousins here:

What is a fourth cousin once-removed?

A fourth cousin once-removed is the child of your fourth cousin or the fourth cousin of your parent (i.e. you are the child of the fourth cousin).

When two cousins share the same common ancestor but don’t share the same number of generations in descent from said ancestor, we use the term “removed”.

The person who is the closest to the common ancestor sets the cousin-relationship type (i.e. first/second/third/fourth cousin) and then the “removed” is calculated from that’s person’s generation level to the other cousin.

For example, in the case of fourth-cousin once-removed, we know that a fourth cousin is a great-great-great grandchild of the common ancestor. If the other cousin is the great-great-great-great grandchild, then there is one generation difference between them and they are fourth cousins once-removed.

If there is more than one difference in generations between the cousins, then you just keep counting. If the cousin in question is a great-great-great-great-great grandchild of the common ancestors, then they are fourth cousins twice-removed.

And so on, and so on.

What is a half-fourth cousin?

As you now know, a fourth cousin is someone that is descended from a full sibling of your great-great grandparent. You share both great-great-great grandparents with your fourth cousin.

A half-fourth cousin is a person who is descended from your great-great grandparent’s half-sibling. In other words, you only share one great-great-great grandparent in common.

For some people researching their family tree, distinguishing between full and half-fourth cousins is very important. This is especially true when it comes to those who use DNA to research their family trees.

Half-fourth cousins will share less DNA, on average, as full fourth cousins.

How many fourth cousins do I have?

The actual number of relatives that you have on any generation depends on lots of variables.

For example, if all of your great-great-great grandparents had lots of kids, and all of those kids survived to adulthood and had lots of kids themselves, then you might have more fourth cousins than the average person.

Alternatively, if your ancestors and their descendants were affected by natural disasters, wars, famines, and illness, you might have fewer fourth cousins than average.

Are fourth cousins blood related?

When people ask if two people are “blood related”, what they might be asking is if fourth cousins share DNA. You will only share DNA with about 50% of your possibly 940 4th cousins.

This is because we don’t actually share DNA with all of our relatives.

If you have 940 4th cousins and all of them took a DNA test, statistics tell us that only about 470 of those cousins would show up as DNA matches to you. In other words you are related genealogically to all of your fourth cousins but you might not share DNA.

Furthermore, if your fourth cousin is a half-fourth cousin, there is a higher chance that you will not share any identical DNA.

What is a Fourth Cousin Pinterest Image


I hope that this post helped you understand more about fourth cousins, how many fourth cousins you have, and whether fourth cousins are always “blood related” (whether they share DNA).

While it’s not very common to grow up knowing any of your fourth cousins personally, it is not impossible! Other times, we get to know our fourth cousins by accident through our genealogy research.

If you have any questions about something that you read in this post, or if you would like to share your own experience finding fourth cousins, I would love to hear from you in the discussion below

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