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What is a 3rd Cousin? Does it matter in genealogy?

image showing title question what is a third cousin

Today, many people get started in genealogy by taking a DNA test from a company like AncestryDNA, MyHeritageDNA, or FamilyTreeDNA.

Suddenly you have this list of cousins (your match list) and a whole lot of questions. In particular, people get confused about what is a 3rd cousin (or a 2nd cousin, a first cousin once removed, etc.). Don't worry! It's not that complicated and it may or may not matter to your situation.

How to determine the type of cousin relationship

Cousin relationships are defined by who the shared ancestor is between the two cousins. Normally when we talk about "our cousins," we mean first cousins (1st cousins). You and your first cousin share a set of grandparents. If you only share one grandparent you are half first cousins. You can probably remember this and that makes it easy to figure out what other types of cousins are.

  • First cousins share a set of grandparents
  • Second cousins share a set of great grandparents
  • Third cousins share a set of great-great grandparents
  • Fourth cousins share a set of third great grandparents
And so on

(So, there's a "math" trick to calculate distant cousins without listing them all out like above---the number of cousinship is one more than how many "greats" for the grandparent---I personally never remember this and simply know my 1st cousins share grandparents (1-1=0 so no great), so 2nd cousins share great-grandparents (2-1=1 so 1 great). Then I remember this little math trick and can jump to say an 8th cousin, 8-1=7 so they share one set of 7th great grandparents).

Anytime cousins share one instead of both grandparents, that makes it the same type of cousin but "half" (half fifth cousins share one 4th great-grandparent).

Why do you need to know what a third cousin is?

So when do you need to know what a 3rd cousin is?

Honestly, it really only matters when working with DNA matches or if you have a legal reason to figure it out. And if you did have a legal reason, someone will hire a professional genealogist to prove any relationships, anyway.

Basically, knowing what a 3rd cousin is makes a big difference when doing genetic genealogy (using DNA for family history research) but isn't a big deal if you are just trying to build a traditional family tree. You can do traditional genealogy without knowing how to define types of cousins.

In normal conversations, even genealogists often refer to cousins as anyone who's most recent common ancestor are grandparents or more distant relatives.

I bet if you head to an average family reunion (not a gathering of genealogists) most people won't know what kind of cousin anyone is and they might even get it wrong, which brings me to...

What about "removed" cousins?

Most people don't understand and even misuse "removed" when talking about cousins. Remember, a cousin is someone where the closest shared relative is a grandparent. 

If the closest shared relationship is a parent you are siblings or it is an avuncular relationship---which is the term for aunt/uncle/niece/nephew.

People think "removed" means a more distant relative but it means the difference in the number of generations to the shared ancestor. Remember, any cousin relationship means the closest shared ancestor is at least a grandparent. 

  • If the closest shared relative for one of the people is a parent, that is a sibling or avuncular relationship (this gets into great-aunts and uncles which is a whole different post). 
  • If the closest shared relative for one of the people is him or herself, that is a parent relationship, including all grandPARENTS (you and your great great grandparents share your great great grandparents, the shared ancestor for one of the people in that relationship is him or herself).

So a first cousin once removed means the closer shared relationship is a grandparent (the "first cousin" part). The other person shares one additional generation (the "once removed" part). That is a great-grandparent.

  • First cousins once removed are one biological generation apart. The shared ancestors are the grandparents of one person and the great-grandparents of the other person.

Removed relationships are the same for both people, it does NOT change depending on which person is talking about the relationship. The child of your third cousin is your third cousin once removed. But when that person looks at it, you are the third cousin of his parent OR the parent of his fourth cousin. No matter what, you are third cousins once removed (often abbreviated in genetic genealogy 3rd cousin 1r or 3C1r or something similar).

This is why it can be difficult to get these relationships correct. You can probably figure out what your relationship to the shared ancestor is but you have to get the other person's relationship to the shared ancestor correct to get this right. That's why we rarely talk about our removed cousins using the specific type of cousinship.

For genealogy, there are cousin charts to make sure you get these relationships correct.

If you are working with DNA it is very important to get the exact cousin relationship right. How much DNA you share with blood relatives is how you use DNA to build your family tree. You can't identify new ancestors if you don't get the specific ancestors shared between you and DNA matches correct.

When you are talking about more distant relationships, 5th cousins, second cousins twice removed, etc., being a generation off can seriously complicate your DNA analysis. Sometimes you are more than one generation off and that is much worse.

Correctly identifying the type of cousins of your DNA matches is how you use DNA testing for genealogy.

Here's a real-life third cousin example.

I found a DNA match to me that is a third cousin. We share 36cM which is a really small amount of DNA for a 3rd cousin. I'm really interested in using DNA to expand the branch we share. I want as much shared DNA between us as possible to increase the chances I can use DNA for this genealogy project.

  • Can I find the third cousin’s parents are tested (specifically the single parent for the shared line)? The father would be my 2C1r. They could shared twice as much DNA but they will at least share the same amount (the child can't have inherited DNA unless the parent had it so the child can't have more DNA from that branch).
  • What else can I do with this third cousin relationship? My father was tested but unfortunately not at the company where this match is found. My father and the match would also be 2nd cousins once removed. If the match's father was tested, he and my father would be 2nd cousins.

I didn't find the match's father but I don't have to stay in my direct line as long as I'm confident with the close family who's tests I'm using. 

My grandfather's sister was tested and she shares that branch with me. The relationship from her to my third cousin is a 1C2r (1st cousin twice removed) and they share 122cM. That's a lot less than you'd expect to be shared for that relationship but it's a lot better than 36cM!

This closer relationship did help me discover something, it looks like we inherited less DNA from this branch which is probably whey I'm struggling using DNA to expand it! I should try and find as many "closer" relatives who have tested to work with as possible. Maybe some of them inherited more DNA from the couple I'm interested in.


So what is a third cousin? 

Third cousins are two people who's shared ancestor is their great-great grandparents.

Why do you care if someone is your third cousin?

Mostly you don't, unless you're doing genetic genealogy, then it's really important.

If you want to make a new discover with DNA, you need to correctly identify cousin relationships. Even with your immediate family, you need to make sure your relationships are accurate. For genetic genealogy you do need to know if the child of your first cousin is your second cousin or your first cousin once removed (the answer is first cousin once removed, assuming you are correct about the person's parent being your first cousin!). When using DNA for family history, you have a huge number of cousins to work with. But they are all different cousin relationships.