How To Tell if You Have a Half Cousin

By Matthew Jones | | People
How To Tell if You Have a Half Cousin
David Prado Perucha/Shutterstock

Tracing genealogy can quickly give you a headache. While it’s fun tracking down relatives, family trees can get really big and confusing, especially once you get to the outer branches, which includes half cousins, second cousins, cousins twice removed, and so on.

But what is a half cousin, exactly? Curious how related you are to your half cousins? Here are simple explanations for each:

What is a cousin?

To understand what a half cousin is, let’s start from the beginning.

A cousin is your aunt and uncle’s child. In other words, it’s your parents’ siblings’ children. These relatives are also called first cousins.

On average, you share about 12.5% of your DNA with your first cousins, though this can range from 7.31 to 13.8%.

Interestingly, despite this relatively close genetic relationship, 21 states in the U.S. allow cousins to marry one another, and six more permit it with some restrictions (mainly, if the cousins are older and unable to reproduce). It’s also allowed across most of Europe and in other parts of the world. It’s not as genetically risky, as people once thought.

What is a half cousin?

While full cousins, or first cousins, have the same grandparents, half cousins share just one grandparent.

To clarify this, here’s an example:

Your grandmother and grandfather gave birth to your mother. But after your mother was born, your grandfather passed away. A few years later, your grandmother met someone else and had a baby boy. This person is your mother’s half-brother. Any children he has will be your half cousins.

If you share an average of 12.5% of your DNA with your cousin, you share half this, around 6.25%, with your half cousin.

Half cousin chart

Diagram of half cousin relationship

What is a half-second cousin?

A half-second cousin is a cousin with whom you share just one great-grandparent.

How would this work? Imagine your great-grandparents had two kids (one of these being your grandparent). But then your great-grandfather died and your great-grandmother remarried and had two more kids (half siblings to your grandparent). The great-grandchildren your great-grandmother had with her new husband are now your half-second cousins.

In other words, you and your half-second cousins share the same great-grandmother but you have different great-grandfathers.

You share around 3 to 5% of your DNA with your second cousin, so about 1.5 to 2.5% with your half-second cousin.

Half-second cousin chart

Diagram of half-second cousin relationship

What is a half-third cousin?

To understand half-third cousins, we have to go back to your great-great-grandparents. These are your parents’ grandparents’ parents. We all have eight sets of great-great-grandparents, who we will likely never meet.

If you share just one great-great-grandparent with someone else, you are half-third cousins. If you share two, you are full third cousins.

You share just 0.78% of your DNA with your third cousin, and about half that with your half-third cousin. Though there is a decent chance you don’t share any DNA at all with this person.

Half-third cousin chart

Diagram of half-third cousin relationship

What is a half-fourth cousin?

If your head doesn’t hurt yet, let’s dive into half-fourth cousins. As you might expect, we have to go back one step further in the family tree.

In short, you and your half-fourth cousin share one great-great-great grandparent.

Great-great-great grandparents are your great-grandparents’ grandparents. We all have 16 sets of these, so 32 people total. If you share a complete set, you are fourth cousins. But if you share just one (due to one great-great-great grandparent having children with different people), then you are half-fourth cousins.

You share, on average, just 0.2% of your DNA with fourth cousins, so only about 0.1% with your half-fourth cousins, if any at all.

Half-fourth cousin chart

Diagram of half-fourth cousin relationship


Hopefully, you now understand your family tree a bit better. First cousins are your most closely related cousins, but you still share some DNA (though very little) with relatives as distant as your half-fourth cousins.

Chances are, however, if you don’t know them already, you could run into a half-third or half-fourth cousin and not even know you’re related. But thanks to DNA and genealogy services, it’s becoming more common than ever for distant relatives to find one another.

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