Last December, a joint survey by The Economist and the polling organization YouGov claimed to reveal a striking antisemitic streak among America’s youth. One in five young Americans thinks the Holocaust is a myth, according to the poll. And 28 percent think Jews in America have too much power. “Our new poll makes alarming reading,” declared The Economist. The results inflamed discourse over the Israel-Hamas war on social media and made international news. There was one problem: The survey was almost certainly wrong. The Economist/YouGov poll was a so-called opt-in poll, in which pollsters often pay people they've recruited online...