If you’ve played Candy Crush, flown on United Airlines, or taken an Uber or Lyft, you’ve been in one of Prof. John List’s experiments without even knowing it. List has revolutionized economics research through his pioneering use of field experiments. A field experiment is conducted in the real world instead of in a lab, testing theories on people in their day-to-day lives. List’s experiments have changed the world by equipping policymakers with real-world data to address issues like climate change, the gender pay gap, and why inner-city schools fail. But now, he’s warning of a crisis that’s threatening the impact of scientific research: Many studies that claim to tell us something about the world fall apart when you test them on a larger scale. It’s something he calls ‘the scale-up problem.’