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Nader makes a good case for it, pointing out that a transaction tax "would certainly raise enough to make the Wall Street crooks and gamblers pay for their own Washington bailout." Also, I support a tax on currency trades across borders. "Called the Tobin Tax after its originator, the late James Tobin, a Nobel laureate economist at Yale University, 10 to 25 cents per hundred dollars of the huge amounts of dollars traded each day across borders would produce from $100 to $300 billion per year."
Economist Robert Pollin explains, “A small tax on all financial-market transactions, comparable to a sales tax, would raise the costs on short-term speculative trading while having negligible effect on people who trade infrequently. It would thus discourage speculation and channel funds toward productive investment.” - Tax The Speculators