The Iffy Science of Ancestry DNA

Here’s yet another story based on reconciling our individual beliefs about our ethnic or racial identity with the results of DNA testing. It’s a good piece, with several twists. (I won’t give away the surprises.) But it does get at an issue I’ve thought about a lot as an historian and just as a lay person interested in science. Put simply, what if the report about your ancestry just isn’t true?

We can pose this question at a few different levels. All good science is tentative and subject to refinement and disproof. Go back a century and all sorts of things that were believed to be true we now know are not true. Or, more cautiously, our best science now says they’re not true.

This kind of radical doubt is probably too absent from the our view of the world we live in. But that’s not mainly what I’m talking about. I’m quite confident that the basic outlines of DNA science are accurate in our basic understanding. If you take a DNA test and it says you’re not biologically related to your parents, that’s very, very hard science behind the determination, assuming the tests are done correctly. Same with forensic tests that say your DNA is at the crime scene. But using DNA to say where your ancestors are from is a much, much more speculative enterprise. And the testing companies do not make this nearly clear enough.

These ascriptions are based in large part to comparing your DNA with the DNA of other people who seem to have good documentary evidence that they have ancestors that go centuries back in a particular geographic region. So your DNA says 50% of your ancestors come from South Italy because other people who have strong historical or genealogical documentation of that ancestry have DNA that looks a lot like yours.

That’s not a bad methodology. If you get enough samples, outliers should get sifted out. You should end up with fairly good evidence of ancestry in a particular region. But as you can see, there’s a strong non-hard-science (more historical and documentary) bedrock under a lot of the science of how testing companies use DNA to tell you where you’re from. This evidentiary problem amplifies dramatically if your ancestors aren’t from Europe, since most of the data companies are operating from databases made up mostly of people whose ancestry is from Europe. The article I reference above notes one change that one of the big heritage/DNA testing companies recently made in methodology and datasets that led to dramatically different reports about ancestry for the same DNA samples.

There’s an entirely different problem. Much of the DNA companies’ ancestry methodology is based on the idea that until a few centuries ago, people largely stayed more or less in the same place. Actual history says that’s probably not true, though there’s a lot of debate on this point. South Italy was heavily colonized by people from Greece. Later all of Italy had massive and constant imports of slaves from throughout the Mediterranean world and beyond. The story gets less and less clear the more you dig into it. But let’s stick with the clearest problem with these tests, they involve a lot more uncertainty than they let on and that uncertainty gets dramatically amplified if your ancestors aren’t mainly from Europe.

Scientists and historians using DNA to map history and historical migrations work with archeological DNA increasingly. So not someone who we think has ancestors in the Netherlands going back centuries but the DNA of someone who we know from carbon dating died and was buried near Utrecht in 100 AD.

I raise all this because we’re all used to stories about people who had this or that understanding about their ancestries and then learn the ‘truth’ from a DNA test. If it’s a white nationalist who finds out he has some African ancestry well sick-burn! But other people who choose not to believe their results for less dramatic reasons are similarly written off as denialists. But they may be closer to the truth than we’d like to admit. Relatedly, it’s almost a cliche now when someone finds out about some unknown part of their genealogy and then goes on some quest to find out about their Egyptian or Georgian or Polish past. Except maybe that’s just a quirk of the imperfect data models that will be shifted in a couple years.

Our best guide is always science, even though it’s in a constant process of revision. But when science is wrapped up in commerce and marketing, we need to be a lot more skeptical. I don’t say any of the above to mean that these reports are worthless. They’re likely mostly accurate. But we shouldn’t give them the imprimatur of certainty, the certainty that unsettles identities. The science just doesn’t back that up.

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