To trace the progress of the wealth tax from a fringe academic idea to the center of the Democratic Presidential primary, it is helpful to begin a bit off-center. On September 15, 2008, the day that Lehman Brothers filed for bankruptcy, a twenty-one-year-old student of Thomas Piketty, Gabriel Zucman, started work as a trainee economic analyst in the offices of a Paris brokerage house called Exane. Zucman felt obviously underequipped for the task before him: to write memos to the brokerage house’s clients and traders helping to explain why the very durable and minutely engineered global financial system appeared to be on the verge of collapse. Poring over some of the data he was given, which concerned the international flows of investments, Zucman noticed some strange patterns. The amount of money that had been moving through a handful of very small economies (Luxembourg, the Cayman Islands, the tiny Channel Islands of Jersey and Guernsey) was staggering. “Hundreds of billions of dollars,” Zucman recalled recently, making the “B” in “billions” especially emphatic. Eventually, he would calculate that half of all foreign direct investment—half of the risk-seeking bets, placed from overseas in India, China, Brazil, and Silicon Valley, and of the safety-seeking investments, placed in the United States and Europe and stock indexes—was moving through offshore hubs like these.

Before the financial crisis, the rise of offshore tax havens hadn’t been ignored—one element of the Enron scandal of 2001, for instance, was the eight hundred and eighty-one overseas subsidiaries the company had created, which had helped it avoid paying federal taxes for three years—but those stories took place within a more confined and more frankly moral framework: it was a cat-and-mouse plot, about the mobility of wealth, and the fruitless efforts to pursue it. Zucman’s intuition was that these arrangements did not describe a moral or a legal drama but a macroeconomic one. That much wealth, poorly documented or regulated, might have helped to destabilize the global economy. It also seemed that, if economists were not attuned to the amount of wealth stored in offshore havens, they might also have missed the extent of global inequality, since it was billionaires who stored money in the Cayman Islands, not retirees. “You know, the way we study inequality is we use survey data, state-tax data,” Zucman told me, “and that’s not going to capture these Swiss bank accounts.” After half a year at Exane, Zucman was back in graduate school, working with Piketty on the study of wealth inequality in the United States and Europe that became Piketty’s landmark book, from 2013, “Capital in the Twenty-First Century,” as well as on his own fixation—on how big the island-shaped loopholes in the global economy would turn out to be.

For the next several years, Zucman followed two tracks. The first led deeper into the mists of offshore banking systems. In obscure monthly reports of the Swiss central bank he discovered that foreigners held $2.5 trillion in wealth there (Zucman would eventually calculate that $7.6 trillion, or eight per cent of global household wealth, was held in tax havens, three-quarters of it undeclared) and that these immense sums were mostly being diverted to mutual funds incorporated in Luxembourg, the Cayman Islands, and Ireland. The second track—the work he did first with Piketty and then with the Piketty collaborator and Berkeley economist Emmanuel Saez—mapped the acceleration of inequality around the world and in the United States. The American story was of a snowball effect, as Zucman described it, in which the very high top incomes of the nineteen-eighties and nineties were saved and invested, “and that creates a spiral which is potentially very powerful and leads to very, very high rates of wealth inequality.” The two stories were in fact one. The concentration of wealth in secretive tax havens was an expression of the broader wealth imbalance—the laissez-faire spirit of the Reagan era working its way through the country and then the world. “One thing that became clear in my mind when I did the study of the U.S. wealth inequality is how hard it is to stop the rise of wealth inequality if you don’t have progressive taxation and, in particular, progressive wealth taxation,” Zucman told me. Without it, the snowball just keeps growing.

This work took place during Obama’s Presidency, a period in which, a bit paradoxically, the global populist reaction to accumulated wealth was consolidating even as liberal institutions, belatedly, began to get a handle on the problem. In 2010, early in Zucman’s doctoral work, Congress had passed the Foreign Account Tax Compliance Act (FATCA), which required tax havens to share banking information with the United States or suffer significant economic sanctions. The program worked, and, by the middle of the decade, European regulators had compelled tax havens to share the same information with them. “That actually had a very big impact on my thinking, because it showed that new forms of international coöperation can emerge very quickly,” Zucman told me. “In particular, sometimes we have this view that, ‘Oh, we can’t do anything about tax havens. Countries are entitled to their own laws, and, if they want to have a zero-per-cent corporate-tax rate of bank secrecy, that’s their own right.’ ” But FATCA had demonstrated that tax havens were not autonomous zones. “At the beginning of my Ph.D., whenever I or N.G.O.s would talk about having some automatic exchange of banking information, policymakers would say, ‘Oh, that’s a pipe dream.’ And so I witnessed the transition from pipe dream to now everybody does it.” He went on, “It can happen very fast.”

As WikiLeaks oriented international relations around a central tension, between transparency and secrecy, similar themes and patterns were emerging in the area of wealth. To parse them required the tools of an investigative journalist, of discovery and cajoling. Zucman is an economist, but he also had some of the qualities—youth and fervency—that investigative reporters often have, and that made him someone people would go to when they thought something was very wrong. A leaked trove of foreign wealth data from the Swiss subsidiary of the banking giant H.S.B.C. made its way to various national tax authorities, and Scandinavian government officials shared it with Danish and Norweigan academics who were collaborating with Zucman. There were limits to what he could see in the H.S.B.C. trove, but it provided a suggestion of how much wealth from Scandinavian countries was being stored away in offshore hubs like Switzerland. In 2015, when the Panama Papers leaked, detailing the evasion efforts of the law firm Mossack Fonseca, it was possible to see the business of tax evasion in action—the lawyers, the pitch decks, the business analysts. Shrouding fortunes was the work of meticulous professionals; when Zucman and colleagues traced this wealth through tax shelters, they found it often was finally invested in ordinary stocks and bonds. “It was very mundane,” Zucman said.

Gradually, Zucman came to see tax evasion differently. “It’s not a psychological thing,” he said. There was a market. The key player wasn’t the billionaire, but the bankers and lawyers who Zucman came to think of as the tax-evasion industry. The professionals in this industry had bosses, and partners or shareholders; they worked within a regulated system. “If you have banks that feel that they are too big to indict then they will continue to commit some form of financial crimes,” Zucman said. “They will budget costs for fines.” In 2009, tax havens seemed like black holes, sucking out so much wealth that it warped the global economy. By 2019, they seemed dependent on the continued dormancy of the great liberal apparatus of international banking regulation, which could be quickly revived. “And the U.S.,” Zucman said, “you know, if there is a U.S. President that is serious about fighting global oligarchy, he or she has a ton of power.”