On the claim that financial transactions can’t be taxed: modern trading is a highly centralized affair. Take, for example, Tobin’s original proposal to tax foreign exchange trades. How can you do this, when currency traders are located all over the world? The answer is, while traders are all over the place, a majority of their transactions are settled  i.e., payment is made  at a single London-based institution. This centralization keeps the cost of transactions low, which is what makes the huge volume of wheeling and dealing possible. It also, however, makes these transactions relatively easy to identify and tax.

What about the claim that a financial transactions tax doesn’t address the real problem? It’s true that a transactions tax wouldn’t have stopped lenders from making bad loans, or gullible investors from buying toxic waste backed by those loans.

But bad investments aren’t the whole story of the crisis. What turned those bad investments into catastrophe was the financial system’s excessive reliance on short-term money.

As Gary Gorton and Andrew Metrick of Yale have shown, by 2007 the United States banking system had become crucially dependent on “repo” transactions, in which financial institutions sell assets to investors while promising to buy them back after a short period  often a single day. Losses in subprime and other assets triggered a banking crisis because they undermined this system  there was a “run on repo.”

And a financial transactions tax, by discouraging reliance on ultra-short-run financing, would have made such a run much less likely. So contrary to what the skeptics say, such a tax would have helped prevent the current crisis  and could help us avoid a future replay.

Would a Tobin tax solve all our problems? Of course not. But it could be part of the process of shrinking our bloated financial sector. On this, as on other issues, the Obama administration needs to free its mind from Wall Street’s thrall.