Saul, euphoria in the market is expressed - a l’outrance - by Shiller’s CAPE for the S&P standing at 27.1 with a mean of 16.6, implying a 38.7% reduction in market value
This issue has been discussed on a number of boards, including I think this one at some time in the past. There is an inherent difficulty in assuming a reversion to the mean in this measure because the basis of the measure has changed substantially, most notably that it is based on GAAP earnings and standards have changed requiring companies to include a number of things in GAAP expenses which they didn’t use to. This means a systematic reduction in GAAP earnings without the company doing one smidge worse. Thus, if anything, I would expect a reversion to what the mean would have been had current GAAP policies been in effect. To my knowledge, no one has attempted to compute what that would be.
Moreover, I think one has to be systematically about assuming that what once tended to be true will always be true. There is much about today’s market which is fundamentally different from the market in the past. The types of companies today are different. The economy is different. We have tons of ETF and MF that we didn’t used to have. HFT is common. Etc., etc. Including, note, the change in GAAP standards. While some observed ratio like CAPE might tend to remain constant across some of this change, that is, at best, an assumption, not a natural law. I think there is just as good a reason to assume that such observations would exhibit systematic secular shifts in response to that change as to expect it to remain constant.