"The study shows that investors who pay brokers for mutual fund investment-related advice, that invest in traditional broker-sold fund shares, experience materially worse performance than investors who get no such advice," says Jeff Buckner, founder and president of Plancorp, a fee-based investment advisory firm based in St. Louis.

The ABCs of mutual fund investing
Investors who seek the advice of a broker or commission-based adviser generally must pay a sales charge or load.

To accommodate this fee structure, many funds come with different share classes. Class A shares refer to front-end loads, which typically range from 3 percent to 5 percent. Your money is taken off the top before one dollar is invested. Example: You invest $10,000 in a fund with a 4 percent front-end load; your actual initial investment is $9,600 (less the $400 load fee).

Class B shares are those with deferred sales loads (also called back-end loads) of more than 1 percent. Investors in this class are penalized when they exit a fund, if they do so within a certain number of years. Class B shares are terrible fund shares to own if you move in and out of funds regularly because you'll get clobbered with fees.

Class C shares have either a deferred sales load of 1 percent or a 12b-1 fee above 0.25 percent. No-load funds that charge a 12b-1 fee of 0.25 percent were also studied, as were "pure" no-load funds that have no sales charges and no 12b-1 fees.

The study reveals that investors in Class B shares suffered the worst performance gaps. Let me reiterate: This underperformance has nothing to do with load fees. Those who followed a market-timing strategy with their brokers sacrificed 2.28 percentage points of performance annually, on average, versus fund investors who employed a buy-and-hold strategy.

Let's illustrate the impact of a 2.28 percent return shortfall over time. In our example, Joe buys and holds a particular fund, earning an annualized return of 8 percent over 10 years. Market-timer Dan only earns 5.72 percent annually, due to bad broker advice. Joe's $10,000 investment grows to $21,589, while Dan's grows only to $17,441. That's a $4,000 premium that Dan is paying for broker advice. And again, it doesn't take commissions into consideration.

Below is a summary of returns among market timers within the various types of funds. Legal no-load refers to funds with 12b-1 fees, while pure no-loads have no such distribution fees.

Annual timing performance gap by share class