The best way to get investors the lowest 401(k) fees possible is to automatically enroll as many of them as possible into big plans with high contribution rates, according to a study released last week on the economics of 401(k) and defined contribution retirement plans by Deloitte and the Investment Company Institute.

"Commitment by employers and employees is really key," said Sarah Holden, senior director of retirement and investor research at the ICI. "The study found that the plans tend to be lower cost when the participants' and employers' contribution rates are higher."

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