Neo-Nazis, it turns out, dig gene tests—but they often don't like the results. Two sociologists, Aaron Panofsky and Joan Donovan, plowed through years of posts on the white-supremacist website Stormfront in search of accounts of people taking genetic ancestry tests to prove their whiteness. The pair tracked 153 users who'd gotten tested as they discussed their results across 3,000 posts on the site. About two-thirds of them were disappointed with the results, which found that they had something other than white European ancestry in their genome. An excellent piece in Stat on the work talks about how the online community dealt with inconvenient findings. Suffice it to say, things quickly got weird.

Amazingly, though, as the racists scrambled to find ways to discredit the tests, they stumbled into a very real controversy among scientists. The companies that perform the tests, like 23andMe and Ancestry.com, strive to put numbers to the genetic variants they detect. So you might receive, as one prominent white supremacist did, a test claiming that your genes are 86 percent European and 14 percent sub-Saharan African.

As the Stat piece says, testing companies do a lot of work to try to validate these numbers—but they're incredibly hard to pin down. People move around a lot, and our ideas about what constitutes a genetic group by geography are, as a result, pretty fuzzy. That may give racists some wiggle room to question results they don't like. But it also means that any attempts to prove racial "purity" go from simply grotesque to, genetically speaking, completely absurd.