Financial crises occur with surprising frequency—in every decade in the past century there has been at least one big shock to a major economy’s financial system. Judging from that history, the current upheaval will probably rank among the largest, and we face the prospect of a severe, painful recession. Yet comparing the current financial crisis with those of the 20th century may provide some comfort: the impact of past crises on the real economy was by no means uniform, and it depended, critically, on the way governments acted to recapitalize the banking system and to restore stability and confidence.

The boom that preceded the present crisis uniquely combined several leverage-driven bubbles: a residential-mortgage bubble, an associated one in the real-estate market, and a bubble in corporate earnings. At the time of writing, US financial institutions had taken total credit crisis–related write-offs of almost $1 trillion.¹ McKinsey estimates that the total eventual credit losses in the United States are likely to be between $1.4 trillion to $2.2 trillion in a base case.² The losses will be greater if another major asset area (such as credit default swaps) collapses or if a misguided policy response exacerbates the problems, as it...