GABRIEL ZUCMAN is a 27-year-old French economist who decided to solve a puzzle: Why do international balance sheets each year show more liabilities than assets, as if the world is in debt to itself?

Over the last couple of decades, the few international economists who have addressed this question have offered a simple explanation: tax evasion. Money that, say, leaves the United States for an offshore tax shelter is recorded as a liability here, but it is listed nowhere as an asset — its mission, after all, is disappearance. But until now the economists lacked hard numbers to confirm their suspicions. By analyzing data released in recent years by central banks in Switzerland and Luxembourg on foreigners’ bank holdings, then extrapolating to other tax havens, Mr. Zucman has put creditable numbers on tax evasion, showing that it’s rampant — and a major driver of wealth inequality.

Mr. Zucman estimates — conservatively, in his view — that $7.6 trillion — 8 percent of the world’s personal financial wealth — is stashed in tax havens. If all of this illegally hidden money were properly recorded and taxed, global tax revenues would grow by more than $200 billion a year, he believes. And these numbers do not include much larger corporate tax avoidance, which usually follows the letter but hardly the spirit of the law. According to Mr. Zucman’s calculations, 20 percent of all corporate profits in the United States are shifted offshore, and tax avoidance deprives the government of a third of corporate tax revenues. Corporate tax avoidance has become so widespread that from the late 1980s until now, the effective corporate tax rate in the United States has dropped from 30 percent to 15 percent, Mr. Zucman found, even though the tax rate hasn’t changed.

Mr. Zucman, an assistant economics professor at the London School of Economics, is part of a wave of data-focused economists led by his mentor, Thomas Piketty, of the Paris School of Economics. Mr. Zucman’s short book on tax evasion, “The Missing Wealth of Nations,” was a best seller in France last year.