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.

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