BOSTON ( TheStreet) -- Pimco's legendary bond fund manager Bill Gross should probably stick with what he's best at -- managing a bond fund -- as his latest claim that stock investing is a dying "cult" may be mere fear mongering.Gross' comments in his firm's August newsletter come at a time when risk-averse investors, still shaken by the 2008-2009 financial crisis, the long-running European debt crisis and the uncertain domestic economy, have been dumping stocks in favor of bonds for well over a year, a good thing for Gross, who runs the world's biggest bond fund. The Pimco co-founder's basic thesis is that investors shouldn't expect anything near stocks' inflation-adjusted 6.6% annual return of the past century. He dramatically labels that performance as "a mutation likely never to be seen again as far as we mortals are concerned." Gross, who's also the firm's investment chief, contends that the widely accepted 6.6% statistic, known as the "Siegel Constant," and based on the research of University of Pennsylvania professor Jeremy Siegel, contains a "commonsensical flaw," because in the 100-year period cited, "wealth or real GDP was only being created at an annual rate of 3.5%" and so to reach 6.6%, "somehow stockholders must be skimming 3% off the top each and every year." Gross is similarly negative on bonds, noting that with long-term Treasuries currently yielding 2.55%, it's unlikely that "long-term bonds -- and the bond market -- will replicate the performance of decades past." He concludes that under that scenario, a 2% return for long-term bonds and 4% return on stocks in a typical diversified portfolio would produce a nominal return of 3%, resulting in an inflation-adjusted return near zero, negating the benefits of investing. But it didn't take long for Siegel to take Gross to task, telling several news outlets that Gross' thesis is false, because there is no direct link between stock market returns and trends in gross domestic product and that investment returns can, indeed, exceed the rate of economic growth. "The thing is that capital gives out dividends, it gives out interest, it gives out return. When you add that all together, it's going to be greater than GDP growth," Siegel told CNBC. "Even in a no-growth economy, you're going to get some return on capital," said Siegel, "so it's not an anomaly, it's not inconsistent to have that phenomenon." And it's worth noting that Gross has previously backed similar doom-and-gloom predictions, including in early 2009, when he said stocks and bonds would average no better than 4% to 5% annual appreciation in a "new normal" world of investing. But since then, the benchmark S&P 500 has about doubled, returning 16.5% per year, which brings its total return over the past decade to an annualized 6.9%, or roughly in line with Siegel's research.