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Everyone has a theory about the financial crisis. These theories range from the absurd to the plausible — from claims that liberal Democrats somehow forced banks to lend to the undeserving poor (even though Republicans controlled Congress) to the belief that exotic financial instruments fostered confusion and fraud. But what do we really know?

Well, in a way the sheer scale of the crisis — the way it affected much, though not all, of the world — is helpful, for research if nothing else. We can look at countries that avoided the worst, like Canada, and ask what they did right — such as limiting leverage, protecting consumers and, above all, avoiding getting caught up in an ideology that denies any need for regulation. We can also look at...