The Long and Large Decline in U.S. Output Volatility
研究发现美国产出波动性自1950年代起持续下降，标准差降至原来的三分之一，这比运气更能解释近两次经济扩张期的异常长度。
The last two U.S. expansions have been unusually long. One view is that this is the result of luck, of an absence of major adverse shocks over the last twenty years. We argue that more is at work, namely a large underlying decline in output volatility. This decline is not a recent development, but rather a steady one, starting in the 1950s, interrupted in the 1970s and eeirly 1980s, with a return to trend in the late 1980s and the 1990s. The standard deviation of quarterly output growth has declined by a factor of 3 over the period. This is more than enough to account for the increased length of expansions.