One of the most important statistics in the investment business is that the vast majority of investors -- professional or otherwise -- underperform a benchmark index like the S&P 500 (SNPINDEX:^GSPC). Given the limited potential for outperformance and an unlimited amount to charge in fees and expenses, this is a mathematical certainty.

It's bad enough when professional money-managers underperform an index fund, but if you dig deeper, some of the numbers really make you shake your head. In this video, Fool analysts Morgan Housel and Matt Koppenheffer break down just how bad the pros can be.

Morgan Housel has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.