Investors far too often are unaware of the high fees that 401(k) plans charge, Bloomberg reports. Consultants who advise plan sponsors on setting up a 401(k) frequently charge ongoing fees that are hidden in expense ratios. In some cases, mutual funds pay consultants or brokers to promote their funds through hidden revenue-sharing agreements that inflate fund fees by as much as a third.

And because U.S. law does not require funds to disclose fees, it's up to investors to inquire about them.

"It's hidden from most people and only a few know where to find revenue-sharing and soft-dollar disclosures at the 'trade' level," said Matthew Hutcheson, an independent pension fiduciary who audits 401(k)s for plan sponsors. "This is possibly the highest area of expense to a plan. Add up all of the fees and you get 1.5% to 5.57%" deducted from your assets annually.

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