Fresh off a weekend visit to the "capitalist Woodstock," the annual shareholders meeting for Warren Buffett’s Berkshire Hathaway BRK.A, +0.28>#/span### BRK.B, +0.24>#/span### , Vanguard Founder John Bogle addressed an interesting question that must vex any retirement investor who relies on index funds. Bogle was asked the following Monday to explain if the move by investors toward index-style exchange-traded funds (ETFs) was creating a new kind of risk. Namely, what will those investors do when the market inevitably turns sour? Over the weekend, Bogle had said that a stock market that is 75% indexed would be a tipping point, but in the interview he backtracked. "What indexing does is neutralize a large part of the stock market. There's no trading in those stocks, or almost none," Bogle explained. If that neutralization effect were to double to half the market, yes, trading levels would decline, Bogle said. "Turnover in the market is now around 250%. Theoretically, it would turn down to 125%," Bogle told CNBC.via