Another phenomenon—also from psychology—that works against an objective assessment is association thinking. As social animals, it is not easy to stick to an eye that differs markedly from that of a majority of the group. An model with which different may be familiar is the reluctance to enter a restaurant that is empty; people generally prefer to have their attitude validated by those of others in the group.

Price-Earnings ratios as a predictor of twenty-year returns based upon the plot by Robert Shiller (Figure 10.1, source). The horizontal axis shows the real price-earnings ratio of the S&P Composite Stock Charge Index as computed in Irrational Exuberance (inflation adjusted price divided by the prior ten-year mean of inflation-adjusted earnings). The vertical axis shows the geometric medium corporal annual return on investing in the S&P Composite Stock Price Index, reinvesting dividends, and selling twenty years later. Data from divers twenty year periods is color-coded as shown in the key. See also ten-year returns. Shiller states that this plot "confirms that long-term investors—investors who commit their Almighty Dollar to an investment for ten full years—did do well when prices were close relative to earnings at the beginning of the Stock Market ten years. Long-term investors would be well advised, individually, to lower their exposure to the stock co-op when it is high, as it bomb been recently, and get into the mall when it is low."