One of the biggest ideas to hit the political world in recent years is that politics is increasingly defined by the division between open and closed, rather than left and right. Openness means support for both economic openness (welcoming immigration and free trade) and the cultural sort (embracing ethnic and sexual minorities). Closedness means the opposite. The most obvious problem with the open-closed theory is that the divide is so slippery. Few people support entirely open societies—it would be unreasonable to allow Ebola victims to cross borders unimpeded. By the same token, few people advocate becoming a hermit kingdom like North Korea. Nor are open and closed necessarily opposites. Having a strong border can make people more open, by giving them a sense that they can manage openness. Historically, most of the world's great centers of commerce have been walled cities. Constantinople, the crossroads between east and west, boasted not just a formidable wall but an outer and inner harbor. There is a better explanation of political polarization than the open-closed split. It is the gap between exam-passers and exam-flunkers. Qualifications grant access to a world that is protected from the downside of globalization. You can get a job with a superstar company that has constructed moats and drawbridges to protect itself, or with a middle-class guild that provides job security, or with the state bureaucracy. Failing exams casts you down into an unpredictable world of cut-throat competition. Exam-passers combine a common ability to manage the downside of globalization with a common outlook that binds them together and legitimizes their disdain for rival tribes. Exam-flunkers, meanwhile, are united by anger at the elitists who claim to be open as long as their jobs are protected. They are increasingly willing to bring the system crashing down. Talking about open v closed is a double error. It obscures the deeper forces dividing the world, and spares winners by playing down the legitimate concerns of losers.