What is a Ninth Cousin?

Are you interested to know what a ninth cousin is and whether you are genetically related to them? In this article, we'll help you understand what a ninth cousin is, whether they are considered family, and what DNA results might say.

You have taken a deep dive into your family tree, maybe locating a distant ancestor who was also a distant ancestor to some famous person. I know from personal experience that those famous relative hints on Wikitree can send you down that rabbit hole.

Wikitree then has hooked you in and told you that Ed Sheeran is your ninth cousin. What on earth does that even mean and how close does this make you to that world-famous singer? Well, he may not be coming to Christmas dinner but if you can prove the link using solid genealogy then he’s your cousin. So what then is a ninth cousin?

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What is a Ninth Cousin?

First, if you understand the cousin relationship you know that essentially cousins are people who share a common set of ancestors with you. Your first cousins for example share your grandparents due to the fact that one of your parents is a sibling of one of theirs.

A ninth cousin then shares more distant ancestors, in this case, your great-great-great-great-great-great-great-great grandparents. That’s too many greats to be writing down every time so from now on we’ll use the simpler term 8th, great grandparents.

These 8th great grandparents are the parents of the 7th great grandparents of both your ninth cousin and yourself. You are not part of the same direct line but you do share common ancestors.

How Closely Are Ninth Cousins Related to Us?

There are three classifications of family connections when we consider our family tree: close, extended and distant. Our close family include our parents, siblings, children and any other family who are a regular part of our lives.

When it comes to extended family this usually means ancestors who are two or three generations back and cousins from first to the third level. These are people you would likely have met and are generally known to your close family as family members.

Our distant family starts several generations in the past and at the fourth cousin level. These are individuals that are either not known to any living family member or although living are unknown to the family due to the generational separation between them.

You may know some fourth cousins through your family tree research but technically they are still considered distant cousins. This is why ninth cousins are most definitely considered distant relatives.

It is highly unlikely that you know one of your ninth cousins or have ever knowingly met one. I say knowingly because as we will discuss later in the post we could literally be surrounded by ninth cousins and not even know it.

How Many Ninth Cousins Do We Have?

You know you have two parents and four grandparents but when you start to research your family you begin to discover just how many ancestors you really have. Why do I mention this? Well quite simply to understand how many ninth cousins we have we really need to understand how ancestors we have from whom ninth cousins can come.

Relations Number of Individuals
Parents 2
Grandparents 4
Great Grandparents 8
2nd Great Grandparents 16
3rd Great Grandparents 32
4th Great Grandparents 64
5th Great Grandparents 128
6th Great Grandparents 256
7th Great Grandparents 512
8th Great Grandparents 1024

Take a look at that table above, we potentially have 1024, 8th great grandparents. I say potentially because there is the possibility of pedigree collapse but that’s a topic for another day. So assuming that we do have 1024 different individual ancestors that is 512 pairs from whom our ninth cousins can descend.

It’s time for a little theoretical math here. Imagine that your 8th great grandparents had 5 children who each had an average of 5 children themselves. This would mean your 8th great grandparents would have 25 grandchildren.

Taking that math to the next generation those 25 grandchildren could produce an average of 5 children each creating a total of 125 great grandchildren. If this trend continued all the way to your generation can you imagine how many descendants they may have? Well assuming perfect numbers which obviously isn’t the case it could be close to 10 million descendants.

True, 10 million is a lot but wait a second, that’s just one set of 8th great grandparents, you actually have 512 sets. Is the magnitude starting to sink in yet? The truth of the matter is that there are things like pedigree collapse and varying family sizes to consider but regardless the number of ninth cousins you have is astronomical.

There are some statistical estimates of how many sixth cousins a person may have but they do not go as far as the ninth cousin level. Still, let's take a look at the numbers and see if they can help our perspective.

Relations Number of Individuals
Parents 2
Grandparents 4
Great Grandparents 8
2nd Great Grandparents 16
3rd Great Grandparents 32
4th Great Grandparents 64
5th Great Grandparents 128
6th Great Grandparents 256
7th Great Grandparents 512
8th Great Grandparents 1024

As you can see from the above table on average we have an estimated 174,000 sixth cousins. You might also note that the growth is not merely exponential but that the multiplication factor also increases. This could mean millions of seventh cousins, tens of millions of eighth cousins and potentially hundreds of millions of ninth cousins.

That’s a lot of cousins especially if you consider the entire population of the United States is a little over 326 million. Depending on your family if there has been a long history of big family sizes you could have enough ninth cousins to match the entire U.S. population.

This all assumes that every child in every generation lives to adulthood and has children of their own and that no pedigree collapse occurs. The truth is that realistically it wouldn’t be in the hundreds of millions. We simply can’t say how many ninth cousins we have.

The point here is that there are a lot of ninth cousins and we could not possibly trace them all even if we spent the entire rest of our lives researching.

Are Ninth Cousins Related by DNA?

Interestingly, the answer to this question is that most of our ninth cousins are likely not related to us through DNA. This doesn’t mean they are not relatives, only that we do not share DNA from our common ancestor. Regardless of a DNA match as long as we are directly descended by blood from those common ancestors they are still our ninth cousins.

Conclusion

We have a lot of ninth cousins, too many for us to possibly trace. They are the descendants of our 8th great grandparents through the siblings of our 7th great grandparent. If we assume there is no pedigree collapse involved we have 512 sets of 8th great grandparents so all of our 7th great aunts and uncles from those couples could start the line that produces our ninth cousins.

They are family through a common ancestor even if we do not know them or for the most part share any matching DNA.