A long-debunked antisemitic conspiracy theory is back—turbocharged by social media—and it’s helping to fuel a sharp rise in anti-Jewish sentiment. A crackpot theory that once lived on the fringes of the Internet now widely circulates on major platforms, repackaged to delegitimize Israel, erase Jewish history, and dress ancient prejudices in the language of “science.”

Last week offered a vivid example. In his interview with U.S. Ambassador to Israel Mike Huckabee, Tucker Carlson demanded that Israeli Jews take genetic tests to prove their descent from the ancient Jews who governed Israel thousands of years ago. Carlson was invoking the Khazar myth—the claim that the “real” Jews died out after the Romans destroyed the Second Temple in Jerusalem in 70 CE, and that modern European (Ashkenazi) Jews instead descend from a Turkic people who converted to Judaism in 805 CE. Once propagated by David Duke, the former head of the KKK, the theory has found new life online. But here’s the thing: The DNA testing Carlson calls for has already been done—and it conclusively refutes the Khazar conspiracy.