This year we have lived through something more than a financial crisis. We have witnessed the death of a planet. Call it Planet Finance. Two years ago, in 2006, the measured economic output of the entire world was worth around $48.6 trillion. The total market capitalization of the world’s stock markets was $50.6 trillion, 4 percent larger. The total value of domestic and international bonds was $67.9 trillion, 40 percent larger. Planet Finance was beginning to dwarf Planet Earth.

Planet Finance seemed to spin faster, too. Every day $3.1 trillion changed hands on foreign-exchange markets. Every month $5.8 trillion changed hands on global stock markets. And all the time new financial life-forms were evolving. The total annual issuance of mortgage-backed securities, including fancy new “collateralized debt obligations” (C.D.O.’s), rose to more than $1 trillion. The volume of “derivatives”—contracts such as options and swaps—grew even faster, so that by the end of 2006 their notional value was just over $400 trillion. Before the 1980s, such things were virtually unknown. In the space of a few years their populations exploded. On Planet Finance, the securities outnumbered the people; the transactions outnumbered the relationships.

New institutions also proliferated. In 1990 there were just 610 hedge funds, with $38.9 billion under management. At the end of 2006 there were 9,462, with $1.5 trillion under management. Private-equity partnerships also went forth and multiplied. Banks, meanwhile, set up a host of “conduits” and “structured investment vehicles” (sivs—surely the most apt acronym in financial history) to keep potentially risky assets off their balance sheets. It was as if an entire shadow banking system had come into being.

Then, beginning in the summer of 2007, Planet Finance began to self-destruct in what the International Monetary Fund soon acknowledged to be “the largest financial shock since the Great Depression.” Did the crisis of 2007–8 happen because American companies had gotten worse at designing new products? Had the pace of technological innovation or productivity growth suddenly slackened? No. The proximate cause of the economic uncertainty of 2008 was financial: to be precise, a crunch in the credit markets triggered by mounting defaults on a hitherto obscure species of housing loan known euphemistically as “subprime mortgages.”

Central banks in the United States and Europe sought to alleviate the pressure on the banks with interest-rate cuts and offers of funds through special “term auction facilities.” Yet the market rates at which banks could borrow money, whether by issuing commercial paper, selling bonds, or borrowing from one another, failed to follow the lead of the official federal-funds rate. The banks had to turn not only to Western central banks for short-term assistance to rebuild their reserves but also to Asian and Middle Eastern sovereign-wealth funds for equity injections. When these sources proved insufficient, investors—and speculative short-sellers—began to lose faith....

Since these events coincided with the final phase of a U.S. presidential-election campaign, it was not surprising that some rather simplistic lessons were soon being touted by candidates and commentators. The crisis, some said, was the result of excessive deregulation of financial markets. Others sought to lay the blame on unscrupulous speculators: short-sellers, who borrowed the stocks of vulnerable banks and sold them in the expectation of further price declines. Still other suspects in the frame were negligent regulators and corrupt congressmen.

This hunt for scapegoats is futile. To understand the downfall of Planet Finance, you need to take several steps back and locate this crisis in the long run of financial history. Only then will you see that we have all played a part in this latest sorry example of what the Victorian journalist Charles Mackay described in his 1841 book, Extraordinary Popular Delusions and the Madness of Crowds.

As long as there have been banks, bond markets, and stock markets, there have been financial crises. Banks went bust in the days of the Medici. There were bond-market panics in the Venice of Shylock’s day. And the world’s first stock-market crash happened in 1720, when the Mississippi Company—the Enron of its day—blew up. According to economists Carmen Reinhart and Kenneth Rogoff, the financial history of the past 800 years is a litany of debt defaults, banking crises, currency crises, and inflationary spikes. Moreover, financial crises seldom happen without inflicting pain on the wider economy. Another recent paper, co-authored by Rogoff’s Harvard colleague Robert Barro, has identified 148 crises since 1870 in which a country experienced a cumulative decline in gross domestic product (G.D.P.) of at least 10 percent, implying a probability of financial disaster of around 3.6 percent per year....