This real-world phobia is the fear of going outside. In the investing arena, it typically manifests itself in a debilitating inability to invest outside of one's own employer.

Many investors are intimidated by the countless choices available in their 401(k) plans, for example, and believe -- falsely -- that they are making the safe choice by electing to invest in "what they know." The result: These investors wind up putting all their retirement money in the fortunes of a single company -- the same company upon which they also rely for their salary. It's a high-risk approach.

Treatment: Experts say diversification among various asset classes is the best way to manage risk. For the domestic portion of your portfolio, an investment in a broad index of U.S. stocks is actually a simpler bet to understand than an investment in one's own company, because individual companies sometimes fall on hard times and never recover. But the investor in a broad index fund is betting that the economy will continue to grow and that American business will figure out a way to continue to generate wealth.

Stock markets have ups and downs, to be sure. But when time horizons are still measured in decades, the American economy is still a pretty safe bet. So get your shots, and bring a raincoat. But it's OK to go outside.

These are just a few of the many investmental disorders that sabotage the decisions of millions of investors. It may be tempting to write off behavioral finance as the study of investor stupidity. But this temptation is itself rooted in dysfunction. The reality is that these so-called disorders are simply human nature. And investmental illness, to some degree, afflicts us all.