A phenomenon closely related to market overreaction is that the stock market appears to display excessive volatility; that is, fluctuations in stock prices may be much greater than are warranted by fluctuations in their fundamental value. In an important paper, Robert Shiller of Yale University found that fluctuations in the S&P 500 stock index could not be justified by the subsequent fluctuations in the dividends of the stocks making up this index. There has been much subsequent technical work criticizing these results, but Shiller’s work, along with research finding that there are smaller fluctuations in stock prices when stock markets are closed, has produced a consensus that stock market prices appear to be driven by factors other than fundamentals