An irrational and frequently obnoxious overconfidence in one's own investment prowess, this narcissistic condition is so named because it is far more common among men than women. The ailment has been well-known among women for some time but was first quantified by researchers Brad M. Barber and Terrance Odean.

Looking at data from 45,000 different investors between 1991 and 1997, Barber and Odean found that overconfidence in trading ability caused men to trade more often than women -- a lot more. The men in the sample, all told, traded 45 percent more on average than women. As a result, the increased transaction costs and taxes associated with frequent trading shaved 2.45 percentage points off men's total returns every year, compared with just 1.75 percentage points for women.

The thing is, men and women look at money in different ways, says Nancy Langdon Jones, a financial planner who practices in Rancho Cucamonga and Claremont, Calif. "For women, money means security. For men, it means power."

Matt McGrath, a planner with the Miami area wealth management firm Evensky & Katz, agrees. "Most of our couples seem to fit the stereotype," he says. Men tend to be more aggressive, while women are often more conservative and circumspect -- sometimes to the extent that they separate their accounts completely.

Treatment: Treating testosterus can be tricky. Its victims frequently resent the therapy. Consider channeling the same masculine energy into fee reduction instead. Low-cost investment strategies, such as indexing, also tend to be tax-efficient. Women, be sure to praise your brachiating, bellowing spouse for his scintillating foresight when he takes measures to reduce fees. After all, a reduction in fees is free of risk, and the potential upside is powerful.

Consider the erosive effects of trading costs on the portfolios of a typical male afflicted with testosterus versus that of a woman, using Barber and Odean's numbers. Over 30 years, a $10,000 investment earning a 7 percent annualized return, minus costs, would be worth about $38,000 for a man versus roughly $46,400 for a woman. The $8,400 difference might be enough to fund a midlife crisis.