There’s no question that aggressive risk-taking by financial firms was a key driver of the crisis. But the arguments that Glass-Steagall’s repeal — that is, the commingling of investment banking and commercial banking within the same firm — was a major cause are tenuous.

Of the big firms that got into trouble and helped trigger the crisis, only a few were the mega-banks enabled by the action on Glass-Steagall.

Bear Stearns, Lehman Brothers, Merrill Lynch, Goldman Sachs and Morgan Stanley were all traditional investment banks heading into the crisis. Fannie Mae and Freddie Mac were government-sponsored housing finance companies. A.I.G. was an insurance company. Some of the banks that were most aggressive about making subprime and other risky mortgage loans included Washington Mutual and Countrywide, both of which were organized as a savings and loan in the run-up to the crisis. Wachovia was a big commercial bank that had also gotten into the insurance and securities businesses — but it collapsed not because of those activities but because of its top-of-the-market acquisition of mortgage lender Golden West.

It is true that two of the biggest bailout recipients were mega-banks with both commercial and investment banking arms, Citigroup and Bank of America. And while JPMorgan Chase and Wells Fargo weathered the crisis relatively well, they also accepted bailouts at the insistence of the Treasury and Federal Reserve in 2008.

In other words, the mega-banks that were enabled by Glass-Steagall repeal were certainly among the firms that caused the crisis, and did require bailouts. It is less clear that they were meaningfully more culpable than companies whose structure had nothing to do with the Glass-Steagall repeal, or that the existence of both commercial banking and investment banking under the same corporate entity was a primary reason they got into trouble.

The stronger arguments for Glass-Steagall repeal as a cause of the crisis are also subtler ones. The investment manager Barry Ritholtz, for example, has argued that “the repeal of Glass-Steagall may not have caused the crisis — but its repeal was a factor that made it much worse” by allowing the mid-2000s credit bubble to inflate larger than it otherwise would have and making banks more complex and thus prone to failure.

How does the Volcker Rule fit into this?

The Dodd-Frank financial reform law in 2010 included a rule that aimed to reduce risky activity in mega-banks — in other words, to address some of the same vulnerabilities that Glass-Steagall was designed to prevent without breaking up the banks completely.