As the market moves ever higher, there’s the potential for a sharp decline. The U.S. ETF market has about $2.7 trillion in assets, the majority in products that track indexes. ETFs have attracted more than $160 billion in new flows so far this year, Bloomberg data show.

“This new market structure hasn’t been tested,” Bryan said in a telephone interview, noting that the stock market has never gone through a major downturn when passive investors were as important as they are now. “We could get an onslaught of selling.”

For more than two decades under former manager Robert Rodriguez, Los Angeles-based FPA Capital was among the top-performing stock funds in the U.S. From 1986 to 2010, it returned 14.5 percent a year compared to 8.5 percent for the Russell 2000 Index, according to a data compiled by Bloomberg.

Fund struggles

The fund has struggled in recent years, in part, because the managers, finding too few attractive stocks to buy, have parked 35 percent of their money in cash.

FPA Capital trailed 99 percent of peers over the past five years, according to data compiled by Bloomberg. The fund is a concentrated stock fund. Its biggest equity holding as of March 31 was Western Digital Corp., which makes computer-storage devices.

While several high-profile money managers have raised concerns about ETFs, equity ETFs account for about 7 percent of the U.S. stock market’s value, according to data compiled by Bloomberg.

In a February letter to investors, Seth Klarman, who runs the $30 billion Baupost Group, said that as more investors opt for passive investing over active management “the more inefficient the market is likely to become.”

In the same letter, Klarman cited Nikolaos Panigirtzoglou, a global market strategist at JPMorgan Chase in London, who, according to Klarman, has warned that the inflows into ETFS will “make markets more brittle” and “susceptible to more severe crashes.”

Others say no relationship between indexing, market dynamics

But Jim Rowley, senior investment strategist at Vanguard Group, disagrees with the naysayers. Vanguard, which has roughly $3 trillion in assets in passive products, including almost $700 billion in ETFs, has examined more than 20 years of market history, he said.

The conclusion: markets are as volatile as ever and the dispersion in the performance of individual stocks is as great as it was before indexing became popular.

“We didn’t find any relationship between indexing and market dynamics,” he said.

Ahitov isn’t totally pessimistic. Should stocks sell off indiscriminately, there will be bargains for smart value investors. “Dennis and I will buy the good stocks that are cheap,” he said.

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