Posts Tagged ‘stock market’

Embracing the efficient market hypothesis can be a bit like stepping out on “the Ledge”

July 20, 2009

If Chicago were, say, the world of finance, the efficient market hypothesis would be the Willis Tower. That’s the old Sears Tower, for those who don’t know. As a hulking steel pillar looming over its surroundings, the theory claims that a financial asset’s price reflects every bit of available information related to its value. It is the foundation by which lots of economists tell regular investors they can’t possibly beat the market. After all, how can you figure out a better value for an asset when all the information about it is already wrapped into the current price?

In the world of the EMH, as it is known, bubbles are nothing to seriously worry about because prices cannot deviate from proper valuations for too long. But in the wake of last fall’s stock market collapse, fully embracing the efficient market hypothesis can be a scary proposition – a bit like stepping out on “the Ledge” at the Willis Tower. (An absolute must if you’re in Chicago, by the way.) The events of the past year have sparked a rich debate between behavioral economists and EMH-ers over the the EMH’s validity that is nicely chronicled in this week’s Economist.

“In some ways, we behavioural economists have won by default, because we have been less arrogant,” says Richard Thaler of the University of Chicago, one of the pioneers of behavioural finance. Those who denied that prices could get out of line, or ever have bubbles, “look foolish”. (Myron) Scholes, however, insists that the efficient-market paradigm is not dead: “To say something has failed you have to have something to replace it, and so far we don’t have a new paradigm to replace efficient markets.” The trouble with behavioural economics, he adds, is that “it really hasn’t shown in aggregate how it affects prices.”

Yet EMH-ers and behaviouralists are increasingly asking the same questions and drawing on each other’s ideas. For instance, Mr Thaler concedes that in some ways the events of the past couple of years have strengthened the EMH. The hypothesis has two parts, he says: the “no-free-lunch part and the price-is-right part, and if anything the first part has been strengthened as we have learned that some investment strategies are riskier than they look and it really is difficult to beat the market.” The idea that the market price is the right price, however, has been badly dented.

Read the full piece here.

Revisiting a behavioral economics classic: Why do people hold on to losing stocks?

January 29, 2009

Tyler Cowen poses the following question about stocks, and what he says used to be the conventional behavioral economics answer.

Let’s say you bought two stocks last year. One has tanked and looks likely to fall further. One has gone up and you expect it to keep rising. (Hey, it’s not completely impossible.) Which are you more apt to sell?

Behavioral economists used to think they knew the answer: neither. Studies have shown that people tend to value things more – whether shirts, stereos or stocks – once they own them, no matter what has happened to their actual worth. This phenomenon is called the endowment effect. If it were the only psychological factor at work, you’d be reluctant to sell both losers and winners simply because they’re already tucked into your portfolio.

Cowen’s story is incomplete, and therefore unfair, even to old behavioral economists. In the scenario Cowen describes, two biases, each reinforcing the other, would be in effect: The endowment effect and loss aversion. The endowment effects for both stocks (assuming you bought them at the same price) would cancel each other out, but this would not necessarily mean investor paralysis. For more than twenty years, behavioral economists have been citing something called the disposition effect, which is an implication of prospect theory and the component of loss aversion). The status quo purchase price serves a reference point. Gains and losses are perceived relative to some other aspirational level different from the status quo – say, what you thought the stock would rise to. As the winner is closer to this aspiration, you, as the investor, become more risk-averse and therefore more likely to sell it, while holding on to the loser in the hopes of a roaring comeback, even one with a small probability.

But this isn’t the only explanation for identical behavior. An alternative is a commonly mistaken belief among average investors that stocks will revert to their mean. Stocks that have risen will fall; stocks that have fallen will rise. This story also predicts the selling of winners on the expectation that it will fall. Yes, Cowen’s scenarios says you, the ordinary investor, would expect the winning stock to keep rising. Old behavioral economics says you’d be quite extraordinary for believing this. Both of these potential explanations are laid out in Terrance Odean’s classic paper “Are Investors Reluctant to Realize Their Losses?” His data does allow him to distinguish which of the two stories makes more sense.

Addendum: Cowen’s column is actually an appreciation of a paper by Nicholas C. Barberis and Wei Xiong with yet another explanation for why investors sell winners and hold onto losers: That it’s the pleasure of actual (or what stock traders would called realized) gains – the good feeling you get from making a seemingly smart decision – and the pain of actual losses that leads to selling winners. Read the full paper.