LOOK at the news or social media these days, and you might see a pattern. Stories are about groups in conflict, competing for limited resources, with the gains for some framed as losses for others. If China benefits from trade with America, America must lose. If foreign students enrol at American universities, that must mean fewer spots for Americans. If immigrants find work, they must be taking jobs from citizens. If a diversity initiative helps women or a racial minority, someone else must be left out. More and more, debates are shaped by a mindset that sees the world as a fixed pie—where one person’s or one group’s gain is another’s loss. That mindset is known as zero-sum thinking. And it is crucial to understanding the politics and economics of America today.