Fault for the crash lies, according to various popular theories, with some combination of over-leveraged banks, a culture of greed on Wall Street, the collapse of mortgage-backed securities, and financial institutions that are too big to fail. But according to Yale University economist Gary Gorton, all of these explanations are wrong, and regulators have done nothing to address the underlying cause of the meltdown.

In “Misunderstanding Financial Crises: Why We Don’t See Them Coming,” published last month by Oxford University Press, Gorton argues that the true culprit was actually a kind of bank run—but not a visible Depression-era bank run, with citizens banging on doors trying to get their money out. Rather it was a run on the “shadow banking system,” a huge, largely unregulated system in which an immense amount of money changes hands daily, all behind the scenes. This “shadow” system is perfectly legal; it funds traditional banking, and the two are deeply intertwined. But its transactions are unregulated and invisible on banks’ balance sheets—so when a crisis of confidence strikes, as it did in 2008, a surprised government must turn to drastic and risky tactics to stanch it.

The shadow banking system still exists, with large financial institutions quietly borrowing and lending money in massive short-term arrangements known as sale-and-repurchase agreements. Although there aren’t definitive measures, the most recent estimate by an international group of regulators put it at $67 trillion worldwide—roughly two-thirds the size of traditional banking.

Gorton is a scholar of financial crashes, and he doesn’t see the most recent meltdown or the one that kicked off the Great Depression as anomalies. Instead, he views the seven crash-free decades between the two as a rare period in which bank regulations were adequate to prevent catastrophic bank runs. And because no new rules have emerged to address the new kind of run that triggered the recent crisis, he says, we could be in for a repeat every few years.

Ideas spoke with Gorton by phone on two occasions from his office in New Haven.

IDEAS: When we think “bank run,” we think of customers mobbing bank buildings. What happened during this crisis?

GORTON: The way it worked was instead of people lining up at banks, people would use the phone or e-mail or Bloomberg Mail, and they would say, “I’m not going to lend my money again. Give it back to me.” And so in the sale-and-repurchase market, about $1.2 trillion was withdrawn in a very short period of time....It just happened that depositors were not individual households, they were institutional investors and other firms.

IDEAS: That’s not the explanation we’ve been hearing.

GORTON: The reason for this I think is in large part because people didn’t see what happened. No one saw a bank run like in “It’s a Wonderful Life.” They only saw the effects of the bank run. They saw firms get into trouble, and they saw house prices go up, but they’ve been unable to connect all this into a coherent picture of what actually happened. And that means that everybody still seems to be quite confused.

IDEAS: A lot of experts say the financial crisis was caused by bankers creating a subprime mortgage bubble. That’s not it?

GORTON: That’s simply not big enough to explain what happened, and in fact the losses on all triple A subprime mortgages are minuscule....That might be thought of as a triggering event, but that by itself does not explain what happened, and that gets wrapped up in that whole notion about bankers—that somehow greedy bankers were at the root of this. People may not like bankers, and people may think they’re greedy, but I can tell you that if greedy bankers explain the crisis, then we would have financial crises every other week.

IDEAS: Why do economists and lawmakers overlook this as the cause?

GORTON: Since the only people who saw it were people on trading floors, I think it’s hard for people to accept an abstract idea about what happened.

IDEAS: Isn’t that what economists are supposed to be good at?