The article is referring to the business cycle, a well-established economic phenomenon of unpredictable cycles of expansion and contraction. http://en.wikipedi

The article is referring to the business cycle, a well-established economic phenomenon of unpredictable cycles of expansion and contraction. http://en.wikipedia.org/wiki/Business_cycle http://en.wikipedia.org/wiki/Business_cycle > This is extrapolating from a pretty small data set I wouldn't call the last century of the US stock market a small data set. Perhaps what you mean is that the author's choice of decade-long windows is too arbitrary? But regardless, the business cycle is a broadly accepted and easily observable phenomenon. Exactly what causes these cyclic fluctuations is hotly debated (at least it was ten years in my macro-econ classes.) > it's quite possible average returns in the 21st century will not be as good as average returns in the 20th century The article wasn't arguing that one should expect those exact returns, but rather that positive returns are cyclical-- good times generally follow bad times.