For Robert Shiller, the prominent Yale University behavorial economist and Nobel Prize laureate, mathematics and historical example play a key role in predicting market bubbles.
“I think that compared with history,” he says, “US stocks are overvalued. One way to assess this is by looking at the CAPE (cyclically adjusted P/E) ratio that I created with John Campbell, now at Harvard, 25 years ago. The ratio is defined as the real stock price (using the S&P Composite Stock Price Index deflated by the CPI) divided by the ten-year average of real earnings per share. We have found this ratio to be a good predictor of subsequent stock market returns, especially over the long run. The CAPE ratio has recently been around 27, which is quite high by US historical standards. The only other times it has been that high or higher were in 1929, 2000, and 2007—all moments before market crashes. . . This time around, bonds and, increasingly, real estate also look overvalued. This is different from other over-valuation periods such as 1929, when the stock market was very overvalued, but the bond and housing markets for the most part weren’t. It’s an interesting phenomenon.”
Stop. Look. Listen.