The number of participants and average account balances are the primary drivers of fees in 401(k) and defined contribution retirement plans, the Investment Company Institute and Deloitte & Touche found in a study released Tuesday.

"401(k) and other defined contribution plans play an important and growing role in Americans' retirement security," said Paul Schott Stevens, the institute's president and chief executive officer. "Policymakers, employers, plan service providers and workers all need a better understanding of what plans cost and what factors are the key drivers of plan fees."

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