First, from 1970 to the mid-1990s, the variance of permanent income shocks (dotted line) remains relatively flat, while the variance of transitory income shocks (solid line) follows an increasing trend. 35 Second, the variance of permanent income shocks increases from the late 1990s through 2010, while the variance of transitory income shocks decreases from its peak in the early 1990s through 2010. 36 Without extending the moment conditions to include two-year differences, it would have been impossible to detect the increase (decrease) in the variance of permanent (transitory) income shocks beyond the early 1990s.
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