Under What Circumstances Should You Worry That the Stock Market Is “too High”? | Brad DeLong

Brad DeLong discusses Robert Shiller’s CAPE ratio, a stock-price measure he helped develop:

….on average, at a ten-year horizon, for any CAPE ratio below 35 the S&P has delivered average real asset returns pretty much outclassing all other major asset classes…..

Thus you can see why I am relatively unsatisfied with Shiller’s writing:

“In the last century, the CAPE has fluctuated greatly, yet it has consistently reverted to its historical mean–sometimes taking a while to do so….”

Shiller’s rhetoric leads us to focus on graphs like this one of the Campbell-Shiller CAPE, and to think that what goes up must–someday–come down:


But is there any reason to think that the central tendency of the CAPE today is the same as what it was in the 1880s or the 1950s? There is no unchanging machine buried in the earth for the past 120 years throwing dice to determine the CAPE. It would be much better to say that extreme values of the CAPE are followed by reversion not to but toward the previous historical mean. And dividends and earnings shift too. A much better graph than the CAPE graph is the cumulative reinvested return graph [on a log scale]:

Cumulative Reinvested Returns

It goes way up, and way down, but the dominant feature is not mean reversion but rather exponential growth…..

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