>The proper analogue to a 75 year average is
>one over a business cycle, from peak to
>peak, not "the thirties". If you're not
>an economist, you can be excused from being
>obliged to know this.
Why is that the proper analogue? A 75-year period contains many business cycles, and probably includes incomplete parts of two. A decade is a medium-term periodization that also contains more than one cycle. It's not just the cycles that matter, but the length and relative patterns too. Lots of people think in decades, even economists.
>The trustees base their prediction on the
>slower productivity growth in the U.S.
>since 1972 or so, and lower population
>growth. All good reasons to doubt these
>are welcome (seriously). We'd love to
>hear them. We'll use them for all they're
>worth. Reference to the pre-1972 period
>is not a very good reason. It's not a bad
>reason; it just isn't good enough.
The evidence, at least according to Cutler, Poterba, Sheiner, and Summers in Brookings Papers 1990, is that lower pop growth is associated with higher prod growth. Assuming both fall is redundantly bearish.