There are experts, and then there's everybody else. In finance, experts have studied the subject and follow the markets closely, so you'd expect that they'd be superior at betting on the stock market as well as on other financial matters, right? Well, perhaps not so much. As the psychologist Philip Tetlock—who did a 20-year study on the subject—famously said: Experts are poorer at predictions than dart-throwing monkeys.

Study after study has shown that low-cost index funds—investments that track major financial market indices—outperform "actively managed" mutual funds. The question of mutual-fund managers' uselfullness is hotly debated, with one study showing that only 24 percent of professional investors beat the market in the long run. If studies don't convince you, perhaps Warren Buffett will: The "Oracle of Omaha" himself recommends low-cost index funds.

A new study spells more bad news for mutual-fund managers. It looks at 84 mutual-fund managers in Sweden and how they do when it comes to their own finances. This is the first study to look directly at fund managers' stock portfolios, real-estate ownership, total wealth, and personality in order to study whether extensive knowledge and day-to-day interaction with the stock market improves one's personal wealth.

The results were striking: The researchers found no evidence that these financial experts make better investments than peers of similar age, income, and education background—casting doubt on how much value is added by so-called expertise.

Financial experts were not better at picking stocks or diversifying investment risks, and they even suffered from known behavioral biases—such as keeping stocks that have dropped in value and trading too much. In short, the researchers say their study shows that financial expertise does not improve investment decisions.

“The point is you have these very educated people who are supposed to know what they are doing, but they are just not that good, on average,” said Andrei Simonov, an associate professor of finance at Michigan State University and co-author of the study with Andriy Bodnaruk.

Simonov adds that the study implies that average investors might be better off managing their own stock portfolios rather than paying a high-fee mutual-fund managers, because beating the market is rare and very difficult.

“I am not disputing that there is a very small fraction of managers who are extremely talented,” Simonov said. “But there are very, very few of these superstars, and the average investor probably can’t afford to invest with them anyway.”