[W]hen we look at the number of AER equivalent papers instead of pages published at the end of six years... we ﬁnd large and statistically signiﬁcant drop-offs in productivity over time for graduates of both the top and non-top thirty departments. By this measure for graduates of the top thirty programs, the oldest cohort [1986-88 Ph.D. graduates] is 51% more productive than the middle cohorts [1989-94 graduates] and 72% more productive than then youngest [1995-2000 graduates]. The middle cohorts in turn, are 14% more productive than youngest cohorts.
Thus, unless we believe that recent graduates are fundamentally of poorer quality, the same quality of tenure candidate is signiﬁcantly less productive today than 10 or 15 years ago.
--John Conley et al., "Incentives and effects of publication lags on life cycle research productivity in economics," on the real reason economists didn't predict the financial crisis