NEW YORK ( TheStreet) -- John Bogle, founder of Vanguard Group, has been a longtime critic of exchange-traded funds.Because ETFs are easy to buy and sell, they can encourage investors to trade rapidly, Bogle wrote in his book Common Sense on Mutual Funds. Constant trading can be a recipe for poor returns. When he began attacking ETFs a decade ago, Bogle was a lone voice. But lately more critics have appeared. After stocks plunged in the Flash Crash of May 2010, some observers argued that high-frequency trading of ETFs had added to the turmoil. Criticisms erupted again this year when analysts said that the increased use of ETFs caused the big market swings in August. Analysts say that ETFs have distorted markets, driving individual investors away from stocks. Concerned about the charges, the Securities and Exchange Commission has begun investigating rapid trading of ETFs. Whether or not the SEC finds a smoking gun, it is clear that the enormous growth of ETFs has helped to reshape markets. There are now more than 1,000 ETFs with $1.1 trillion in assets, according to the Investment Company Institute. That is a big jump from the figures in 2000 when there were 80 funds with $66 billion in assets. While retail investors have embraced ETFs, much of the growth can be traced to institutions and high-frequency traders who constantly buy and sell. ETFs have proven to be ideal vehicles for high-frequency traders. The funds can be shorted or used to profit from small moves in the benchmarks they track. Now that they have become favorite holdings, ETFs account for 40% of all exchange trading. The structure of ETFs encourages volatile market moves, says Harold Bradley, chief investment officer of the Ewing Marion Kauffman Foundation, which manages a $1.7 billion endowment. ETFs represent baskets of stocks or bonds that can be traded on exchanges like stocks. With a single key stroke, an ETF trader can buy the 30 stocks of the Dow Jones industrial average or the 500 companies of the S&P 500.