Collective investment trusts are growing in popularity among large companies seeking to provide employees with lower-cost retirement savings options, but some experts worry that these plans don't offer investors enough disclosure.

Companies with thousands of employees have been using these trusts to pool money from participants and invest in stocks, bonds and alternatives. According to Morningstar Inc., approximately 70% of companies with 1,000 employees or more offer collective trusts. Approximately 45% of 401(k) plans currently include collective trusts.

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