Former Federal Reserve Chairman Paul Volcker once remarked that modern finance hasn’t done anyone except finance professionals much good. The automated teller machine is pretty useful, he said, but that’s about it. To which a lot of investors might reply: What about exchange-traded funds?

ETFs have been an astonishing success -- the fastest- growing investment product of the past 20 years. Almost always designed to track an index, they make it as easy to invest in broad segments of a market as it is to buy common stock. Global assets under management doubled in the past four years to $2.4 trillion as of Dec. 31, Bloomberg Markets magazine reports. In 2013, ETFs accounted for 27% of all U.S. equities trading.

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