As its debt mania progressed, more of the lending was diverted into wasteful speculation. Normally, frenzied borrowing occurs amid excitement about a new innovation like the internet. But this spree spread on conviction that Beijing, obsessed with hitting its growth target, would not let lenders or borrowers fail. More and more unqualified players got in the game. The state banks soon had to compete with “shadow banks,” including crowdfunding websites that offered ordinary people a chance to invest in debt for as little as one renminbi (15 cents), promising fantastic returns.

Try as the Chinese authorities might to steer the money into industry, they could never fully commit to stopping shadow banks from financing an increasingly questionable array of borrowers speculating in real estate. When I visited Shanghai in August 2010, I was stunned to see apartment blocks rising two to three rows deep all along the 110-mile route to Hangzhou. Many of the biggest debtors are front companies set up by local governments to evade national regulators. Small cities are borrowing to build futuristic museums, aquatic centers and apartment blocks that exceed local demand and are often as empty as ghost towns.

My research shows that during the 30 worst debt manias of the past 50 years, private debt — which in China is often held by local governments — rose over five years by at least 40 percentage points as a share of gross domestic product. In all 30 cases, the economy slowed sharply, typically by more than half, in the next five years.

China’s mania is now the largest ever in the postwar emerging world. After holding steady at around 150 percent of G.D.P. for much of the boom, China’s public and private debts surged after Mr. Wen’s about face in 2008, rising to 230 percent of G.D.P. by 2014. That 80-percentage-point increase is also more than three times the increase in the United States before its bubble collapsed in 2008. Since then, United States debt has held steady as a share of its economy. Though many Americans still think the nation is drowning in debt, its burden is much less worrisome than China’s because it is not growing.

Paradoxically, the authoritarian form of government that helped guide China to those years of economic growth may now be undermining its economic stability. My research suggests that compared with democracies, autocracies generate far more unstable growth, and that’s the risk in China now. Looking at the available records going back to 1950 shows that extreme swings between fast and slow growth are much more common under autocratic regimes. On a list of 36 countries that have been whipsawed between rapid growth and recession throughout the postwar era, three out of four were autocracies.