I do love this idea, although I hate the name "The Robin Hood Tax". The idea is that you charge 0.0025% tax on financial transactions.
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Now for most people, that is nothing. If I am buying say $500.00 of stock for my 401K, that would cost me $500.01 (actual $500.0125). So any extra penny on every $400.00 I purchase or pull out of stocks. On $40,000.00 - that's a dollar. Not excessive.
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So how can it raise so damn much, Banks do microtrading all the time. They have superfast computers and huge amounts on fractions of a cent. This generates funds for the banks that they then use to give giant fat bonuses. Not to lend out. And it is risky. Like when all the big banks failed because they bundled up crap mortgages and then made tons of money doing microtrading - but when it all went in the shitter we had to pay up.
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Since we supported them when they needed it, how about a tiny tiny pay-back?
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And if they stop doing this - then stock market volatility goes down and that's a win win.