Switzerland will vote this weekend in a referendum on whether to cap its population at 10 million. The Alpine country’s population has increased 23% to 9.1 million since 2002, when it agreed to ease travel restrictions with the EU. If successful, it will be the world’s first explicit population cap. Migration has become a hot-button issue in Europe: The continent’s aging population means ever-fewer workers must support ever-more retirees. Advocates say migration can address the demographic challenge, but opponents think it is changing societies and hurting public finances. The UK saw anti-immigration riots this week, and even Spain — Europe’s migration success story — has seen a rightwing populist party rise in the polls.