Here’s a Vanguard interview with Richard Thaler on what behavioral economics is all about.

Losses have about twice the emotional impact of an equivalent gain. Fear of losses (and a tendency toward short-term thinking—I’m sneaking in a third one here) can inhibit appropriate risk-taking.

For example, investing in the stock market has historically provided much higher returns than investing in bonds or savings accounts, but stock prices fluctuate more, producing a greater risk of losses. Loss aversion can prevent investors from taking advantage of the long-term opportunities in stocks.

The second bias that causes a lot of trouble is overconfidence. Most people think they are above-average investors, and as a result they trade too much and diversify too little. Overconfidence can also lead people to invest during what appears to be a bubble, thinking they will just get out faster than others. Research shows that the more individuals trade, the lower their returns. Not surprisingly, men suffer from this problem more than women.