Perhaps you've heard that roughly three-quarters of all mutual funds lose to the market over the long run. The question is why -- why do presumably smart and talented managers serve you worse than simply buying an index fund? Believe me, I've met plenty of fund managers, and the vast majority of these folks are talented and responsible and hard-working -- they're not bad stock pickers.

The best explanation I've found comes from a recent study by professors Cohen, Polk, and Silli. Their study, "Best Ideas," provides pretty convincing evidence that the rules are stacked against smart fund managers. Their few very best ideas do perform well -- beating the market and their other picks by approximately 1% to 4% per quarter (which is significant). However, the very nature of a mutual fund requires managers to pick dozens -- perhaps hundreds -- of stocks.

Who can pick 100 good stocks? No one, really ... and having to load up a fund's portfolio with a bunch of second-tier ideas seriously harms returns.

But what if ..
The knowledge that fund managers' best ideas tend to outperform might be profitable for us individual investors if we knew with certainty which stocks they most loved. One proxy for determining a fund's favorite ideas might be to look at its largest holdings. Take, for example, the famous Fidelity Magellan fund, headed up by Harry Lange:

Fidelity Magellan Top 10 Holdings

% Net Assets

Corning (NYSE:GLW)




Applied Materials (NASDAQ:AMAT)




Newmont Mining (NYSE:NEM)


Goldman Sachs (NYSE:GS)


General Electric (NYSE:GE)


MedcoHealth Solutions




Bank of America (NYSE:BAC)


Data from Morningstar; holdings as of May 31, 2009.

But is Corning really Lange's very best stock idea right now? We can't really know unless we bug his meeting rooms. Maybe it is, or maybe Corning just grew to be Magellan's largest holding. Perhaps Lange has another great idea now, and he's just starting to accumulate shares -- and selling off Corning to do so. There's lag time in reporting fund holdings, and who knows which company is on top now? You undoubtedly see the problem here: We don't want to commit our hard-earned money to a guessing game.

So now you know the big reason for mutual fund mediocrity, and there's just no getting around it.

Unless ...
There's something of a silver lining in this study, however. If you're an experienced and competent investor, you can take comfort in knowing that you're not bound by the mutual fund rules. You don't have to buy dozens and dozens of companies; you can limit your investments to your very best ideas.

Our internal research at Motley Fool Stock Advisor backs up the conclusion of the "Best Ideas" study. Even better, however, you don't have to guess what its best ideas really are. Fool co-founders David and Tom Gardner have been offering up their five "Best Buys Now" every month since June 2006. Our internal tracking shows that these ideas, like the fund managers' best ideas, significantly outperformed the other stocks on our scorecard -- by a margin of 7 percentage points per pick. (The service as a whole is beating the S&P 500 by an average of 42 percentage points each.)

Interested in the five best ideas from Tom and David right now? For the next 30 days you can see them, along with all their recommendations, with a no-obligation free trial. Here's more information.

Rex Moore drives on a parkway and parks on a driveway. He owns no companies mentioned in this article. Apple, MedcoHealth Solutions, and Staples are Motley Fool Stock Advisor recommendations. Nokia is a Motley Fool Inside Value pick. The Fool has a disclosure policy.