4th Cousin DNA Matches: How Real Are They?

Diahan Southard

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Not all DNA cousin matches are equally valuable! Here’s what to do with those hundreds of fourth cousin matches on your list.

I have always enjoyed math for its absolutes. No matter what, two plus two always equals four. That is comforting somehow when in almost all other disciplines there are multiple interpretations of the same idea or theory.

You see, it is math then that can provide us with so much certainty in genetic genealogy. After all, even if you don’t know who they are, each of us has two biological parents. We have four biological grandparents, and we have eight great grandparents.

The math of DNA matches

However, the farther back we go, the less we can rely on the merits of math. At the fourth or fifth generation back, what happens on paper is simply not reality. For example, on paper you should have 64 3rd great-grandparents. However, many of us do not. For many of us that number is much smaller as the same person occupies more than one slot on our pedigree chart. While this significantly decreases the workload for traditional genealogy, it adversely impacts your ability to use genetic genealogy. Especially when it comes to that long, long list of 4th cousins you have in your match list at any of the major DNA testing companies.

Depending on how intermarried your lines are, you may be seeing individuals on your match list that genetically look like your fourth cousins, but they are genealogically your sixth cousins – EIGHT TIMES OVER!

So how can you tell the difference? Start with these three strategies:

1. Update your pedigree chart.

While your fourth cousins and your eight-time-sixth cousins may look similar genetically, there are often small clues in the genetics that can help you tell the difference. This distinction can sometimes be detected by a testing company who, through research and validation, has been able to fine tune their algorithms to detect these subtle differences. You can participate in this double checking process by using some of the genetic tools that are available to you at Family Tree DNA, or at GEDmatch.com. But since most of you are not aspiring geneticists, let’s focus on the genealogical work you can do to determine whether a match is truly a fourth cousin.

Before we jump in, remember that a fourth cousin designation just means that you and your match are separated by between six and twelve degrees, or people. So that might be five back on your chart to your common ancestor, and five down to your match, which would make you true fourth cousins, or it could be some other permutation of that.

For our example, let’s assume true fourth cousins. That means that the two of you share one of 32 3rd-great grandparent couples. In order to find out which set, you have two genealogical identifiers: surname and location.

Therefore, the first thing you should do is make a list of the 32 surnames and locations of those couples (or, if you want to be an overachiever, make a list of all 64, which would include the maiden names of the ladies). Now, most of us do not know all 32 of those couples, so you are going to have some holes. Feel free to fill in those holes with surnames on subsequent generations that will carry through to this fifth generation. (A great tool to plot your own list of ancestors is to use Google Earth. You can devote an entire Google Earth folder to your 32 couples and easily plot your locations.)

How to Add Family Tree to DNA Test Results 11.png Make sure you’ve got an updated tree connected to your DNA test results. Here’s how to do that at the major testing companies.

2. Identify the cousins

Then, begin to systematically evaluate the fifth generation of your fourth cousin matches for genealogical information that lines up with any of the items on your list. Browse through their trees and run surname searches using your testing site’s tools. (Not sure how? My handy quick reference guides will help you navigate 23andMe, AncestryDNA, MyHeritage DNA, and Family Tree DNA (the Family Finder test*). Consult the tree reconstruction tools at 23andMe, AncestryDNA and MyHeritage DNA (click to read these articles first so you understand the tools).

Even if you don’t immediately see familiar ancestral names as you evaluate the fifth generations of your matches, you may start to see patterns of surnames or locations emerge among your matches. These then become the surnames and locations that might be able to fill the missing spaces in your pedigree chart.

What if your match doesn’t have a tree attached to their test results? See if they have an unlinked tree associated with their profile. If not, you may have to build one. Try these strategies for creating a “quick” family tree for your matches.

Book Cover (1).png Get the ultimate guide to working with your DNA matches: Your DNA Guide—the Book. My book walks you step-by-step through the methods you need to follow to reach your DNA goals. You pick a goal, then follow the plan. It works!

An earlier version of this article appeared on www.genealogygems.com. Revised and updated on YourDNAGuide.com, 2019 and 2020.

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<a href="https://www.yourdnaguide.com/author/guideyourdnaguide-com" target="_self">Diahan Southard</a>

Diahan Southard

As founder and CEO of Your DNA Guide, Diahan Southard has been teaching people how to find family history answers in their DNA for several years, and she's been in the genetic genealogy field since its infancy. Diahan teaches internationally, writes for popular magazines, consults with leading testing companies, is author of Your DNA Guide–The Book, and producer of Your DNA Guide–the Academy, an online learning experience.

2 Comments

  1. Jim

    What is your point of view with this scenario?

    I’d like to prove that my maternal great great grandfather — mother’s father’s father’s father — is the son of the man believed to be my great great great grandfather (b: 1795).

    There is no paper trail connecting the two and the story came from a distant relative who has since passed away.

    There is however a paper trail proving the younger children of this potential g-g-g grandfather. One of the descendants of the younger children has taken an autosomal DNA test.

    I want to know if autosomal DNA will prove if my g-g-g grandfather is this other person’s g-g grandfather and we are in fact 3rd cousins, once removed.

    I read that the major consumer testing companies’ accuracy rate for generations this far back is 45-50%. I do not want to incur the cost of the test if the results will be a flip of the coin.

    What are your thoughts?

    Reply
    • Diahan Southard

      Great question. I would do the Autosomal DNA test as long as the choices are related or not related. So if it isn’t the guy you think it is, then it is a guy unrelated to that guy. To make the best test, I would test more than just the two of you. But just the two of you is a good start.
      If possible, I would also like to see YDNA. Does your mom have a brother or nephew who can test? Can you find a documented direct male descendant of the ggg grandfather? This would be an ideal companion to your Autosomal DNA test. http://www.yourDNAguide.com/ydna.

      Reply

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