The Wall Street herd is at it again.

Even as the cleanup crew is carting away the debris left by the last financial crisis, the investment banks, hedge funds and exchanges are busy working on the next one.

Forget collateralized-debt obligations and credit default swaps -- the new new thing is high-frequency trading. In the last three years, this practice has boosted trading on the country's stock exchanges by more than 150 percent, to the point where it now accounts for two-thirds of the daily trading volume.

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