The 6 biggest investing mistakes

There is one piece of investment advice that, if you follow it, can dependably increase your returns: Minimize your investment costs. We have spent two lifetimes thinking about which mutual fund managers will have the best performance year in and year out. Here's what we now know: It was and is hopeless.

That's because past performance is not a good predictor of future returns. What does predict investment performance are the fees charged by the investment manager. The higher the fees you pay for advice, the lower your return. As our friend Jack Bogle, founder of mutual fund company the Vanguard Group, likes to say, "You get what you don't pay for."

We looked at all equity mutual funds over a 15-year period and measured the rate of return produced for their investors, as well as all the costs charged and the implicit costs of portfolio turnover - the cost of buying and selling portfolio holdings. We then divided the funds into quartiles. The lowest-cost-quartile funds produced the best returns.

If you want to own a mutual fund with top-quartile performance, buy a fund with low costs. If we measure after-tax returns, recognizing that high-turnover funds tend to be tax-inefficient, our conclusion holds with even greater force.

NEXT: Mistake #6: Trusting stockbrokers