Likewise, some panics follow particularly big declines in house and stock prices, which have damaging effects on their own. The most recent recession would likely have been severe — and the recovery slow — even if the financial system hadn’t been stressed, simply because of the decline in wealth and the climb in household indebtedness.

BUT an even larger determining factor is the policy response. Why was the Great Depression so much worse here than in Spain? According to an influential paper by Ehsan Choudhri and Levis Kochin, Spain benefited from not being on the gold standard. Its central bank was able to lend freely and increase the money supply after the panic. By contrast, in 1931, the Federal Reserve in the United States raised interest rates to defend its gold reserves and stay on the gold standard, setting off further declines in output and exacerbating the banking crisis.

Likewise, the policy response largely explains why output fell after the American banking panics in 1930 and 1931, but rose after the final wave in early 1933. After the first waves, the Fed did little, and President Herbert Hoover signed a big tax increase to replenish revenue. After the final wave, President Franklin D. Roosevelt abandoned the gold standard, increased the money supply and began a program of New Deal spending.

A 2009 study by the International Monetary Fund concluded that fiscal expansion can mitigate the impact of crises. But the Reinhart-Rogoff study points out that policy makers’ ability to take strong fiscal action depends on whether they start with high levels of debt. In the current episode, China and South Korea have recovered faster, partly because they have taken more aggressive fiscal stimulus measures. They could do that because they entered the crisis in good fiscal health.

The response to troubles in the banking system also matters. After its banking panic in 1991, Sweden aggressively restored its banks to health. They were nationalized, recapitalized with public funds, and then returned to private control. After three rough years, Sweden grew rapidly, soon returning to its pre-crisis trend.

Japan, by contrast, put off cleaning up its banks after its 1992 crisis. For years, it allowed financial institutions to avoid realizing losses by simply rolling over loans to unproductive or failing companies. Partly as a result, it has struggled almost two decades with anemic growth and deflation. And, unlike Sweden, it is still miles from its pre-crisis trajectory.