Like Kuznets’s analysis, Mr. Piketty’s is based on data. He just has much more: centuries’ worth, from dozens of countries. He distills from them a simple historical regularity. The rate of return to capital — understood broadly to include machinery, land, financial instruments, housing and everything else — is usually higher than economic growth.

This was particularly true before the Industrial Revolution, when economies didn’t really grow, but it prevailed even after economic growth took off in the 19th century.

This means that the income from wealth usually grows faster than wages. As returns from capital are reinvested, inherited wealth will grow faster than the economy, concentrating more and more into the hands of few. This will go on until capital owners decide to consume most of their income and stop reinvesting as much.

Kuznets’s misleading curve is easy to understand in this light. He used data from one exceptional period in history, when a depression, two world wars and high inflation destroyed a large chunk of the world’s capital stock. Combined with fast growth after World War II and high taxes on the rich, this flattened the distribution of income until the 1970s.

But this exceptional period long ago ran its course.

During the Gilded Age — a period of enormous concentration of income and wealth — the stock of the world’s privately held capital amounted to some five years’ worth of global income, by Professor Piketty’s estimate. By 1950, it had fallen to below three, but by 2010, it was back at four. And by the end of this century, Mr. Piketty projects, it will amount to almost seven.

Americans will argue that this description does not fit the United States. Wealth here is largely earned, not inherited, we say. The American rich are “creators,” like Bill Gates of Microsoft or Lloyd Blankfein of Goldman Sachs, rewarded for their economic contributions to society.

Mr. Piketty doubts that the enormous remuneration of top executives and financiers in the United States — enhanced by the decline of top income tax rates since the 1980s — really reflects their contributions. What’s more, he points out, inherited inequality has been lower in the United States mainly because its population has grown so fast — from three million at the time of independence to 300 million today — driving a vast economic expansion.