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."

Individual investors lost significant chunks of their savings during the market's slow-motion crash last autumn. Since then, dozens of companies, looking to cut costs, have ended contributions to employees' 401(k) plans.

The institute said competition has helped keep fees low. The median fee for the 130 plans surveyed in November and December in the study was 0.72% of assets, while the median annual plan fee per participant was $346.

According to the study, secondary drivers of fees included the structure of the retirement plan and sponsor, the allocation of assets to equities and the relationship between the sponsor and the service provider. Factors that did not generally drive fees include the number of payrolls a sponsor has, whether services are provided by a mutual fund sponsor, life insurance company, bank or third party, the plan's tenure with the service provider and the percentage of assets invested in proprietary investments.

Each plan's assets ranged from less than $1 million to more than $500 million, the study found, and the number of participants ranged from 100 to more than 10,000.

According to the Department of Labor, 62% of 401(k) plans have less than $1 million of assets, but only 4% of all 401(k) plan assets and 10% of active participants are in plans that small. Half of 401(k) assets and 30% of active participants are in plans with more than $500 million.

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