Overreaction is probably the most popularly known effect of human behavior on market prices. All things being equal, in a rational market the fundamentals of a company should determine its market price, and there should be a clear relationship between the two. However, research - as well as a casual glance at CNN's stock-ticker on any given day - shows that this relationship doesn't necessarily happen as expected.

Investors regularly overreact, often wildly, so pushing prices up too high or pushing them down too low against their fundamentals. Not only is the market, therefore, not wholly rational in reality, but the effect cannot be attributed to any financial or company-based factor. The most likely reason for the anomaly appears to be the way investors perceive, and react to, earnings surprises or news items, or indeed other investors' actions. This overreaction occurs across the stockmarket and gives rise to several investment strategies.

Contrarian Strategies

The overreaction effect is highly pronounced when comparing 'out of favor' (contrarian stocks) against current 'favorites', or what are also known as value and glamour stocks. 'Out of favor' stocks are not stocks that are bad quality stocks, simply ones that are not attractive to the market, for whatever reason that might be. The interesting thing is, however, that over time the 'out of favor' stocks will, in general, outperform the 'favorites'. Then, when the 'out of favor' stocks become the 'favorites' due to increased buying the effect is reversed and the process is repeated in a cyclical manner, while only minor changes may take place to the stock's fundamentals. 'This occurs,' says David Dremen, who researched the effect with a portfolio of stocks over a ten year period, 'because these stocks will tend to reverse over time as investor expectations change'. Premiums paid for high growth stocks become too expensive while 'out-of-favor' stocks begin to represent greater potential gains. The effect is reminiscent of regression to the mean, a statistical effect where measurements will tend towards their average, and is in fact nothing new. Scientists have known for several hundred years that this kind of effect often occurs when human behavior is involved. What is new is that the effect has been found to occur within a particular domain of stocks.

Whether a stock is an 'out of favor' or 'favored' stock is indicated by their ratios. According to James O'Shaughnessy, whose extensive and well-researched findings were published in What Works on Wall Street, these include: price to book value (P/BV), price to cash flow (P/CF), and price to earnings (P/E). Stocks with the lowest ratios have the most potential to rise, particularly on good news surprises, and are therefore the ones, from this contrarian perspective, that should be sought after, providing they are essentially good stocks.

Momentum Strategies

Contrarian investing would seem to indicate that making money in the stockmarket, over and above the smaller but consistent returns from well-known companies like Microsoft or IBM, requires buying only 'out of favor' or value stocks. However, this is not the case. Indeed, if one were to take this to its logical conclusion no one would buy rising stocks - that were on their way to becoming glamour stocks - and profitable opportunities would be missed. In addition, value stocks take an average of five years to show a worthwhile return. Clearly that is often unacceptable and research bears out, in fact, that momentum pushes many stocks towards new heights regularly, and money can be made on these stocks considerably faster than five years. This doesn't mean that you simply buy any stocks that are rising away from their rational price due to market or behavioral influences. Such an approach would be unsystematic and likely to result in a loss. Although, as Robert Vishny points out, 'You don't necessarily make money on the best stocks in the market but on the stocks everyone thinks are going to be the best'. The rider here, of course, is that you still need to buy stocks that are good or potentially good, even though they may not be the best. Inasmuch as this is true, and you can locate these stocks, there are two momentum strategies that can be implemented.

The first strategy applies to combinations of stocks, and makes use of what is known as the big stock effect. Research on portfolio returns by Andrew Lo and Craig Mackinlay, using a mixture of small and large capitalization companies on the New York Stock Exchange, showed there was a correlation between one weeks return and the next, where around ten-percent of the price change of next weeks return could be predicted from this weeks return. Though the effect only works for portfolios, not for individual stocks, and only in the short-term - that is, daily and weekly returns - there appears to be an observable lead/lag pattern. Which means, big stocks lead little stocks, hence the name. For example, Microsoft goes up dramatically and a few days later there