Have you had problems finding any matches with DNA? Do all of your close matches only share about fifty centimorgans or less with you? If so, you may suffer from the problem of small families.
Family Sizes Throughout History
Family sizes have varied a lot over history, but in the last 200 years, the trend has been to smaller and smaller family sizes. When it comes to DNA and genealogy, how small is too small?
The concept of using DNA in genealogy is that we share DNA with our relatives; however, as I’ve said in previous videos, we don’t share DNA with all of our relatives. Once you get out to third cousins and beyond, you share fifty centimorgans or less with these relatives. Not only that, but you also share DNA with fewer and fewer relatives the further out you go on your tree.
In this video, I further explain why my DNA discoveries have outpaced Devon’s due to the statistics of her smaller family sizes.
Watch this video.
Few People Have Taken DNA Tests
The other difficulty in using DNA as a record is it takes two to tango. Your DNA by itself is worthless. DNA becomes valuable as a genealogical record when others test and then we can match your DNA with their DNA.
As of 2019 only about 5% or less of the US population has had their DNA tested. Many people get discouraged with their DNA tests because they’re not finding matches that they can identify a common ancestor with. This has caused some people to doubt the validity of using DNA as a genealogical tool.
DNA is a record. Just like any other record, it has its advantages and its disadvantages. Mainly DNA is a record of our relationships, but it doesn’t contain information about every relationship.
Consider this. You don’t get upset when a birth certificate can identify your relationship with your Aunt Margie. You wouldn’t expect it to. A birth certificate doesn’t serve that purpose.
The same can't be said of DNA.
Once we understand what relationships we can find with DNA and once we understand how those relationships occurred, then we can better use DNA to find matches that we can identify a common ancestor with.
Understand the limits of Autosomal DNA
Autosomal DNA can reliably match you with people out of about five to seven generations. This doesn’t mean that you’re not going to find 8th, 9th, or 10th cousins. It just means that most of those discoveries are by pure luck.
For instance, I’ve actually matched with a ninth cousin on 23andMe. The reason why we were able to determine that we were ninth cousins is that we both had extensive trees. We were able to see how we matched up. I clued in on the match because I noticed one last name in his tree that was common with some of the surnames that I already knew were in my family.
To what degree do small families affect DNA matches?
Small families mean that there are fewer people that you can match with. I discuss how I built the following table in this video.
Many families had much more than seven children; however, what we’re really concerned with is the number of children that reached adulthood with the possibility of passing on their DNA to somebody else that you may match with.
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The Math of Small Families and Possible DNA Matches
If an average family size is 2 for each generation, then you would have 128 sixth cousins. That may sound like a lot of people, but when we compare that number to an average family size of 3, we see they have more than 2,000 sixth cousins.
The take away is that someone with an average family size of three will find it 17 times easier to identify cousin matches than someone with a family size of two.
Individuals with ancestral family sizes averages of 4-6 will find it hundreds and thousands of times easier to make a cousin connection than somebody who only had a family size of two over the last seven generations.
Are Small Families Impacting Your DNA Discoveries?
If you are uncertain whether a small family size is impacting your DNA discoveries, watch this video to learn how to estimate your potential match pool.
The great news is that Devon has made numerous discoveries as more people have tested. As the percentage of the United States, and the world’s citizens test, the more likely we all will find more matches. That’s a really cool future!
If you have questions about DNA, please leave them in the comments below or join our community discussion that’s popular beneath each of our YouTube videos.