The impact of COVID-19 on the physical health of the world’s citizens has been extraordinary. By April 2021 there were almost 130 million cases spread across more than 190 countries. The pandemic’s effect on mental health could be even more far-reaching. At one point roughly one third of the planet’s population was under orders to stay home. That means 2.6 billion people—more than were alive during World War II—were experiencing the emotional and financial reverberations of this new coronavirus. “[The lockdown] is arguably the largest psychological experiment ever conducted,” wrote health psychologist Elke Van Hoof of Free University of Brussels-VUB in Belgium. The results of this unwitting experiment are only beginning to be calculated.