(FORTUNE Magazine) – The market is back--oh, the joy of it--and so are investors. But should they be? It's not a shock that the average investor can be less than rational. But whatever happened to the notion of "Buy low, sell high"? The chart at right, which compares net inflows into equity mutual funds with the performance of the S&P 500 over the past seven years, demonstrates that investors pile in when the getting is good and flee when stocks are falling. That's precisely the opposite, needless to say, of what they should do. Not only do fund investors react immediately to the gyrations of the S&P, they also often overreact, displaying a volatility normally more associated with, well, the stock market. Buy high, sell low? Poor Benjamin Graham is turning over in his grave. --Matthew Boyle