I’ve often described what I call the Iron Law of Valuation: the higher the price investors pay for a given set of expected future cash flows, the lower the long-term investment returns they should expect. As a result, it’s precisely when past investment returns look most glorious that future investment returns are likely to be most dismal, and vice-versa.
Market returns and economic growth have underlying drivers. At their core, extended periods of extraordinary growth and disappointing collapse reflect large moves in those drivers from one extreme to another. Extrapolation becomes a very bad idea once those extremes are reached.
For example, from 1982 to 2000, the S&P 500 enjoyed an extraordinary period of total returns averaging just over 20% annually. The primary driver of those gains wasn’t growth in revenue or earnings (though the combination of 4.6% average annual S&P 500 revenue growth and a high starting dividend yield certainly helped). No, the primary driver was expansion in the S&P 500 price/revenue ratio, which rose from a profound low of 0.3 in 1982, to an offensively extreme 2.2 by the 2000 peak.
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