Genetic Ancestry Is Basically a Horoscope
In 2014 and 2015, DNA testing companies 23andMe, Ancestry, and Family Tree DNA all reported having more than one million customers. Chief among the companies' offerings are tests to reveal your genetic history, dating back hundreds or even thousands of years. A hundred bucks and a simple cheek swab can show you your true ethnicity and uncover past relations you never knew you had!
Ancestry summarizes the offer's intuitive appeal prominently on their website: “Who knew a kid from Queens was descended from royalty?”
But while testing one's DNA to uncover ancient family links may be popular, that doesn't make it accurate. Many scientists say the tests are about as meaningful as a horoscope.
Think about it. As you travel back in time though your family history, the number of ancestors you have roughly doubles with every generation. Using the most conservative estimate of generation time -- 32 years -- in the year 1152, you had as many as 134,217,728 potential ancestors. And since genes are scrambled with every generation, it's very likely you share little to no genetic relation to most of them. They might as well be strangers!
DNA companies use two DNA tests, a Y-chromosome DNA which provides information about your male line ancestry, and a mitochondrial DNA (mtDNA) test which provides information about your female line ancestry. These tests supposedly yield more accurate information, but they still suffer from major pitfalls. For example, if two males have similar DNA on their Y-chromosome, they likely share a more recent common ancestor than individuals with dissimilar DNA, but any estimate of when or that common ancestor lived and who they are is almost entirely speculative. Mitochondrial DNA tests are similarly limited. The rate of mutation in the whole mtDNA genome is one to three percent per generation, so the time gap between mutations could be as many as 100 generations. This means that a lot of people share the same mtDNA, and their common ancestor could be as close as one generation or as far as fifty or more.
DNA testing companies often take this ambiguity and fill in the blanks with impressive stories that you can show your friends and relatives. Though fascinating, these tales share more in common with astrological horoscopes than historical accounts.
Mark Thomas, a Professor of Evolutionary Genetics at University College London is one of the most vocal advocates of this criticism. On a recent episode of the BBC radio show The Infinite Monkey Cage, he said that that appeal of both horoscopes and genetic ancestry tests arises from the Forer effect.
“If you tell somebody something that seems like it’s highly personalized but in fact is very generic -- you can apply it to anybody -- then people are much, much more likely to believe it.
The effect was revealed almost sixty-eight years ago. In 1948, psychologist Bertram R. Forer gave subjects a psychology test then presented them with a number of personality traits that the test supposedly revealed. Unbeknownst to the subjects, they all received the same set of personality traits, containing vague observations like "You have found it unwise to be too frank in revealing yourself to others" and "While you have some personality weaknesses, you are generally able to compensate for them." Yet when asked to rate the accuracy of the "personalized" evaluation, the subjects on average scored it 4.26 on a five-point scale.
Thomas is not a fan of genetic testing because for a variety of reasons, most of all because it makes science look like a buzzkill.
"It costs unwitting customers of the genetic ancestry industry a substantial amount of hard-earned cash, and it disillusions them about science and scientists when they learn the truth, which is almost always disappointing relative to the story they were told," he wrote in The Guardian.
"Exaggerated claims from the consumer ancestry industry can also undermine the results of serious research about human genetic history, which is cautiously and slowly building up a clearer picture of the human past for all of us."
In 2007, veteran science writer Richard Conniff wrote an excellent piece for Smithsonian explaining why genealogy is bunk. In it, he seemed to relish in puncturing the inflated ego of humanity.
"Almost everyone... has Julius Caesar as a common ancestor. Half of you can probably claim Charlemagne, too. That’s because they lived a long time ago and went about the business of forefathering con gusto. You are probably also descended from every sniveling peasant who ever managed to replicate in ancient times."
Stick that in your cheek and swab it.