Investors are increasingly exposed to American stocks through funds that track indexes, such as the S&P 500 Index. With the occasional hiccup, this strategy has served them well since the market’s recovery in early 2009. At current valuation levels, however, the most popular benchmark index is priced to deliver negative returns in future years. To believe otherwise is to suggest that “this time is different”, which, as every investor knows, are four of the most dangerous…