The SPDR, the largest ETF with a market cap of $158 billion, saw $14 billion come out in August after $13.8 billion was added in July. It was the biggest swing in assets since the beginning of 2008, according to data compiled by Bloomberg.

ETFs have been gaining momentum as the stock market surged. In 2013, shares of more than 450 companies have risen, the most since at least 1990, data compiled by Bloomberg show.

Assets in U.S. ETFs have almost tripled to $1.5 trillion in the last five years, according to data from the Investment Company Institute, a Washington-based trade group. About $284 billion was drained from actively managed mutual funds in the same period, data from Morningstar Inc. show. Unlike mutual funds, ETFs, the vast majority of which are pegged to indexes, can be traded throughout the day like stocks.

Greater Influence

“There’s no doubt that ETFs have greater influence than before, and the swings in the ETFs are indicative of general market feelings,” Nick Sargen, who oversees $45 billion as chief investment officer at Fort Washington Investment Advisors in Cincinnati, said in an Oct. 16 phone interview. “They become the market.”

ETF inflows were similarly strong when the S&P 500 was peaking in October 2007. In the four months leading up to the record on Oct. 9 of that year, they attracted more than $54 billion. Money continued to go in even as the equity gauge floundered through the end of the year, with another $65 billion coming in through December, data compiled by Bloomberg show.

Now that the S&P 500’s rally since March 2009 has lasted longer than the average bull market since World War II and corporate earnings growth is slowing, investors shouldn’t expect to make the same kind of easy money, according to Eric Marshall, who oversees $1.4 billion as president and portfolio manager at Hodges Capital Management.

Blindly Investing

“The past strategies of just blindly investing in the ETFs and index funds may be a little bit more challenging,” Marshall said in a phone interview from Dallas. “You really need to go out there and find these pockets of opportunity.”

Earnings growth for the S&P 500 slowed to 2.5 percent last quarter, according to analyst projections, after averaging 4.2 percent since the start of 2012. That’s a fraction of the 28 percent mean for 2010 and 2011, data compiled by Bloomberg show.