This article gives simple and clear explanations of the third cousin relationship.
I also answer as many questions around third cousins as I can think of. Let’s start with the basics!
What Is A Third Cousin?
Your third cousins are children of your parents’ second cousins. You each have grandparents who are first cousins to each other.
You share great-great-grandparents with your third cousins. You are both four generations away from your common ancestors.
Here is an illustration of the relationship.
One of your great-grandparents is a sibling of a great-grandparent of your third cousin.
Difference With Other Cousins Relationships
Third cousins are more distantly related than second cousins.
You probably understand first and second cousins. But if you’d like to see clear diagrams, check out these articles:
But let’s focus on other relationships that can be confused with third cousins.
Third cousins are more closely related than fourth cousins. Your child is a fourth cousin to the child of your third cousin.
But what about third cousins once removed? Let’s look at that next.
Third Cousin Once Removed
When your third cousins are “once removed”, this means that they are one generation away from you.
This can be either up a generation (your parents’ generation) or down a generation (your children’s generation).
Children of your third cousins
Probably the easiest to understand are the children of your third cousins. These children are your third cousins once removed.
This illustration shows a family tree with the “once removed” relationship marked.
What about third cousins twice removed?
We haven’t shown it in the diagram above because it would get quite long.
But if your third cousin once removed had children, these kids would be your third cousins twice removed. They are down another generation.
Your parents’ third cousins
We’ve looked at the relationship when the removed is down a generation.
But what if it’s up a generation at your parents’ level?
We need to move out further in the family tree and go back as far as your great-great-great-grandparents.
The third cousins of your parents are your third cousins once removed.
What does 3C1R mean?
You may see articles or charts referring to 3C1R.
3C1R stands for third cousin once removed.
Are third Cousins Blood Related?
Your third cousins are blood relatives because you share common ancestors i.e. two of your great-great-grandparents.
However, if the cousin relationship is through marriage, then you are not blood relatives.
In the diagrams above, we only show your blood relatives.
Do Third Cousins Share DNA?
Third cousins share amounts of DNA ranging from about 0.3% to 2%. The average percentage of shared DNA is about 0.8%.
The number of reported shared centimorgans ranges from zero to 234 cM. The average is about 73 cM.
The reason for the variability is that people inherit different amounts of DNA from common ancestors. DNA gets shuffled, separated, and recombined as it passes down through generations.
You will receive DNA from your great-great-grandparents that your cousins don’t get – and vice versa.
The amount passed down from your great-grandparents also gets smaller with each generation.
Using DNA websites
If you’ve used one of the big commercial DNA testing services, you may be researching a list of DNA relatives who also tested with the same company.
The websites like Ancestry and 23andMe will give you a prediction of relationships based on the amount of shared DNA.
Be aware that the higher end of the range for third cousins will overlap with the lower end of a second cousin. So don’t assume that the labels are correct.
Can Third Cousins Not Share DNA?
DNA from specific ancestors gets reduced as it passes through generations.
Is it possible that you don’t share any DNA with a third cousin? The answer is yes.
There is a possibility of about 2.3% that you will not share DNA with a third cousin.
In other words, between two and three out of a hundred third cousins will not share DNA with you.
This gets worse when we move out a generation to third cousin once removed. The possibility increases significantly to 12.1% of not sharing DNA.
These numbers are based on work by a researcher at Cambridge University. He conducted a study into the probabilities that people don’t share DNA with family members.
You may be wondering if you actually have one hundred third cousins out there in the world. Probably not! But you may have more than you think. We’ll address this in the next section.
How Many Third Cousins Do You Have?
Birth rates in your country and community play a big part in predicting the number of third cousins.
I wrote an article on using birth rates to calculate the numbers for the U.S., the UK, and Ireland. These were the results based on the 2013 statistics:
- United States: 74 third cousins
- United Kingdom: 51 third cousins
- Ireland: 66 third cousins
I suspect that the U.S. number may vary widely between communities and income levels in the country.
Can you find local birth rate stats for your community? Are you from another country?
Just follow this article where I calculate the number of cousins Irish people have. You can work out your own predictions.
Are Third Cousins Really Family?
Are third cousins part of close family? Extended family? Distant family?
This depends on the context. I’ll look at this from a legal context, a genetic context, and normal conversations.
In many jurisdictions, third cousins are not considered immediate family in legal matters.
For example, if your employment has restrictions that mention immediate family then this may not include third cousins.
However, you should always check with the specific department (or state authority).
Because the common ancestors go back so far, many of us have never met third cousins.
For that reason, they are considered distant family in normal conversation.
Let’s say that you hear that a third cousin is only inviting “close family” to their wedding. You’re probably better off not sitting by the letterbox.
If you’ve tested with a commercial DNA site, you may be researching your DNA relatives.
Some of the websites put a label of “distant relatives” on many people in your list of DNA matches.
My third cousins on Ancestry.com are under the “distant relatives” label. In contrast, second cousins are labeled as “close family”.
You can read more in this section on Ancestry’s relationship categories.
But don’t assume that these labels are correct. They are based on a count of shared DNA. Remember, we’ve already said that third cousins fall into a wide range that overlaps with second cousins and also more distant relationships.
Can You Marry Your Third Cousin?
Every state in the U.S. allows marriage between third cousins.
All countries in the western world allow third cousins to marry.
Some states and countries have restrictions on first cousin marriages. However, third cousin relationships are so distant that there are no legal impediments against marriage.
Can Third Cousins Get Married In A Catholic Church?
You may be aware of restrictions and hurdles for first cousins to get a Catholic wedding.
Third cousins can marry in a Catholic church without any restrictions. There is no need to get permission or dispensation from the church hierarchy.
The reason is the way that Canon Law rates the degree of relatedness.
Canon law and third cousins
Canon law is based on counting the number of generations away from the common ancestors for the two potential spouses.
The common ancestors of you and your future third-cousin spouse are your great-great-grandparents.
You are both four generations away from your common ancestors. We add those numbers together to come up with eight degrees.
This is called eight degrees of consanguinity. Don’t get too caught up with the terminology.
The point is that Canon Law used to prohibit marriages up to the sixth degree. This was more recently relaxed to the fourth degree (first cousins).
Third cousins have always fallen outside the restrictions.
Frequently Asked Questions
Do third cousins share the same blood?
Third cousins are blood-related and are considered to share the same blood as their common ancestors, their great-great-grandparents.
If you want to know how much DNA they share, check out an earlier section in this article.
What is my third cousin’s child to me?
Your third cousin’s child is your third cousin once removed.
Check out the earlier section in this article about generations removed.
What are my third cousin’s parents to me?
Your third cousin’s parents are your second cousins once removed.
Check out this section on second cousins once removed to see a diagram illustrating the relationship.