What is a Half-Cousin?

What is a half-cousin? You’ve come to the right place to find all types of half-cousins explained, as well as a half-cousin chart.

If you’ve been doing genealogy for a while, there is a very good chance that you’ve come across the term “half-cousin”.  Genealogy terms can be quite confusing, so in this post, I will use graphics and examples to explain variations of half-cousins. 

In this article, you’ll find:

• meaning of a half-first cousin
• meaning of a half-second cousin
• meaning of a half-third cousin
• meaning of a half-fourth

We all have lots of half cousins, whether they are half-first, half-second, half-third, half-fourth, half-fifth or half-sixth cousins.  There are all sorts of situations that can arise in families that lead to cousins having a “half” relationship.

If you did an autosomal DNA test, understanding half-cousins is very important. Half-cousins typically share about half of the expected amount of DNA than “full” cousins. 

I feel compelled to emphasize that a being “half cousins” to each other doesn’t need to change how two cousins feel about each other or how they navigate their relationship within their own family.  Even so, it’s a useful term for genealogists (and some scientists) who need to be able to specify which common ancestors a pair of cousins share.  

At the end of the day, a cousin is a cousin, whether “half” or “full”.

What is a half-first cousin?

Half-first cousins are two people who share one grandparent,as opposed to full cousins who share two grandparents.  This happens when the parents of the two half-cousins are half-siblings. 

Many people have half cousins. In fact, anyone whose parents have half-siblings who had kids have half-cousins.

An easier way to understand the meaning of half cousin is through this example:

One of the siblings is a child from Grandpa’s first marriage and the other child is from the second marriage, and the offspring of those half-siblings are half-cousins.

The graphic below is a visual explanation of how half cousins occur in families.  You can see that the grandfather had two unions (not at the same time, of course – though it does occasionally happen). 

He had one son with each wife, and those sons are half-siblings.  Each son had a child, and those children are half-first cousins.

What is a half-second cousin?

We all have four sets of great-grandparents, and second cousins share one out of four sets with each other.  Half-second cousins share a single great-grandparent, instead of a pair of great-grandparents. 

This happens when the great-grandparent remarries and has another child with the second spouse.   Alternatively, this can also occur even there was no marriage involved, like when a child is born outside of marriage or in cases of infidelity.

For example, let’s say that Great-grandmother Priscilla married when she was 19 and had three kids with her husband, John.  John got tuberculosis and passed, sadly. 

Great-grandmother Priscilla remarried and had one final child (named Bobby) with her new husband, Michael. 

The child that she had with Michael is half-sibling to her first three children, and her great-grandchildren on Bobby’s line will be half-second cousins to her great-grandchildren on the lines of her other children.

Do you know any of your half-second cousins?

Depending on how close your family is, you might find that you know at least some, if not all, of your second cousins – whether “half” or “full”.  Sometimes, we even personally know the children of our second cousins (our second cousins once-removed).

I’ve always found that the concept of second cousins is usually where people start getting a little mixed up about family relationships.  Having the “half” relationship added in can make matters more complex, and so sometimes a visual explanation can help.

Looking for a half-second cousin chart?

In the image below, you can see how the great-grandfather married once, and then married again. The offspring of the great-grandfather’s two unions are half-siblings. 

Their children are half-first cousins, and their grandchildren are half-second cousins.

What is a half-third cousin?

A half-third cousin is a relative that shares one great-great grandparent with you, instead of two great-great grandparents shared by full third cousins.

Most people will find that they have eight distinct sets of great-great grandparents, and people with whom you share one set of great-great grandparents are your third cousins. 

If, say, you only share one great-great grandparent (instead of a set of great-great grandparents) with a particular cousin, this person is your half-third cousin.

Need another way to think about this?  We have sixteen great-great grandparents.  If you share one out of sixteen great-great grandparents with your cousin, then you are third cousins.

Half-third cousin chart

For some people, it helps to visualize the exact relationship of half-third cousins.  In the image below, notice that the great-great grandfather married twice. 

He had a son with one wife, and a daughter with the other and they are half-siblings to each other. 

Their children are half-first cousins, their grandchildren are half-second cousins, and their great-grandchildren are half-third cousins. 

What is a half-fourth cousin?

I’m sure that if you’ve read this whole article, you are getting the pattern.  We all have 16 sets of great-great-great grandparents, and people who share one set of great-great-grandparents are fourth cousins. 

If two people only share one person as an individual great-great-great grandparent,then they are half-fourth cousins. 

I know I have really picked on Grandpa for this post, but it could just as easily been Grandma that married and got remarried, even several times.  I know that this is the case in my own family most of the time. 

Men were usually the ones out there fighting wars and working dangerous jobs (not to diminish the danger of childbirth, of course).

As I mentioned at the beginning of the post, I generally like to connect my articles back to genealogy and DNA.  There is about a 50% chance that you will share absolutely no DNA with any given fourth cousin.

After all, we do not share DNA with all of our relatives.

Because the range of shared DNA with a fourth cousin includes zero, you can’t always know for sure whether someone is actually your genealogical fourth cousin based only on DNA.

This is especially true for half-fourth cousins – they will share about half as much DNA as full-fourth cousins, and are more likely to share no DNA, or only a very small amount of DNA with you.

Conclusion

I hope that this post has helped give you a good understanding of the meaning of half-cousin. You can use the same pattern in the post to go back as far as you want –even to half-twelfth cousins, if you want to!

If you have any questions about something that you read in this post or would like to talk about finding out who your half-cousins are, Would love to hear from you in the discussion below.

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