Gabriel Zucman started his first real job the Monday after the collapse of Lehman Brothers. Fresh from the Paris School of Economics, where he'd studied with a professor named Thomas Piketty, Zucman had lined up an internship at Exane, the French brokerage firm. He joined a team writing commentary for clients and was given a task that felt absurd: Explain the shattering of the global economy. "Nobody knew what was going on," he recalls.

At that moment, Zucman was also pondering whether to pursue a doctorate. He was already skeptical of mainstream economics. Now the dismal science looked more than ever like a batch of elaborate theories that had no relevance outside academia. But one day, as the crisis rolled on, he encountered data showing billions of dollars moving into and out of big economies and smaller ones such as Bermuda, the Cayman Islands, Hong Kong, and Singapore.