What is a Half-Cousin?

Marc McDermott
First Published: | Updated: April 2, 2024

When looking at your family tree, you may notice that you have numerous half relations. From siblings to cousins, they make up part of your genealogy in some form, but what is a half-cousin? Half cousins share one grandparent and not the usual two grandparents. 

This isn’t just half-cousins we’re talking about though. You may also have half-first, half-second, and half third cousins, and so on. Read on to discover everything there is to know about half-cousins to get some clarity over these relations, and what they mean to your family tree. 

What is a half-cousin?

A half-cousin is someone who shares one grandparent with a cousin, rather than both like regular cousins. 

For example, your grandmother might not be related to you, but your grandfather is. So, your first cousins are your half cousins as a result. Half cousins share around half the normal amount of DNA as full cousins.

It should be noted that being half cousins should not lessen the bond between two people. Most half cousins treat each other no different from their full cousins. It is like having a half-brother or sister in the family. The “half” doesn’t make you care about them any less. 

However, half is a useful term when researching your family tree, so genealogists use it.

How common are half cousins? 

Very. Although it is more common to have full cousins in your family tree, half cousins are becoming more and more common. 

If your parents have half-siblings then their kids are your half cousins. One sibling can be from one marriage, then the other from a second marriage. This makes the offspring of these two half-siblings half cousins. 

So, in a world where it is divorce and remarriage are more and more common, half-siblings and half cousins are increasing in number. 

How much DNA do you share with half cousins?

Half cousins share around 50% of the usual DNA they would have with first cousins. It is thought that 1st cousins share around 12.5% of DNA, meaning half of the first cousins share approximately 6.25%.

However, DNA doesn’t follow strict averages, so half of the first cousins can indeed share the same amount of DNA (even more in some cases) than full first cousins. 

What is a half first cousin?

If a grandparent marries once, has a child, then marries again and has another child, these siblings are half brothers and/or sisters. In turn, the grandparents’ grandchildren are known as half-first cousins. 

What is a half-second cousin?

Everyone has four sets of grandparents. Second cousins share one of these four sets. Half second cousins share one of these great-grandparents, rather than both.

As you may expect, great-grandparents must have another child with someone else. This may or may not happen while married which is why we haven’t highlighted the fact that great-grandma or great-grandpa must be in wedlock before they have children. 

So, if a great-grandfather Bob marries, has two kids but then great-grandmother Joy passes away, Bob may or may not remarry. Either way, if he has another child with someone else, let’s call his new flame Martha, this child is the half-sibling of his first two children. 

Bob’s great-grandchildren on Martha’s line will be known as half-second cousins to his great-grandchildren on the line he shared with Joy.

Know your second cousins?

It’s always down to how well you know your extended family, and with the remote working world we live in, families are often spread around the world. You may or may not know your half or full cousins well, but it is nice when we do get to spend time with the children of our second cousins – these children are known as our second cousins once removed. This “once” relates to the number of generations between us

Second cousins are often a tricky part of genealogy and an area where many people get confused. This becomes even more complicated when introducing the “half” into your genealogy. 

Allow things to get just a tiny bit more complex…

What are half-third cousins?

Third cousins share one great-great-grandparent, but not two that are shared by full third cousins. That wasn’t so painful, right? 

In most cases, we all have eight sets of great-great-grandparents. The people who share a set of great-great-grandparents are third cousins. 

If two cousins share one great-great-grandparent and not both, these two people are half-third cousins.

What are half-fourth cousins?

Now we’re getting into half cousins. With 16 sets of great-great-great-grandparents, fourth cousins are people who share one set of great-great-great-grandparents. 

Two cousins who share one, but not both great-great-great-grandparents are half-fourth cousins. Seeing the pattern? Understanding it? 

Don’t worry reading this through a couple of times can help you better understand. 

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  1. Thank you so much for this explanation. I am working on a family that has so many remarriages/relationships that I couldn’t figure out what the relationships were. I think I can rework it now and figure it out.

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