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Semi-Rational Exuberance

In 1996, Yale economist Robert Shiller looked around, considered the historical record, and concluded that the American stock market was overvalued. In the past, whenever price-earnings ratios were high, future long-run stock returns were low. But now prices on the broad index of the S&P 500 stood at 29 times the average of the past decade's earnings.

On the basis of econometric regression analyses carried out by Shiller and Harvard’s John Campbell, Shiller predicted in 1996 that the S&P 500 would be a bad investment over the next decade. In the decade up to January 2006, he argued, the real value of the S&P 500 would fall. Even including dividends, his estimate of the likely inflation-adjusted returns to investors holding the S&P 500 was zero – far below the roughly 6% annual real return that we have come to think of as typical for the American stock market.