Employers, their retirement service providers and worker advocacy groups are worried confusion in the Department of Labor’s rules for default options could cause 401(k) plan sponsors to make poor investment choices that could hurt retirees’ nest eggs, the General Accountability Office reported Friday.

DOL needs to clarify rules for defined contribution plan sponsors on considering participants’ ages when they select qualified default investment alternatives (QDIAs) and when those default investments protect them from suits by participants for investment losses. Until those rules are clear, sponsors face difficulties in trying to balance participant safeguards with investment choices that can help them establish and maintain financially secure retirement plans for their employees, GAO warned.

Under DOL rules, three kinds of default investments qualify for liability protection: target-date funds (the most popular among sponsors), balanced funds that have a fixed balance or ratio of equity to debt investment, and managed account services that use information about participants to customize asset allocations.

Retirement-plan sponsors surveyed by GAO, the watchdog arm of Congress, cited low costs and ease of implementation as two primary factors that led them to choose target-date funds as their default option.

While DOL has cited liability protection as a key advantage of QDIAs, sponsors said they are concerned they can’t tell if some offer more of a “safe harbor” from litigation than others.

For the study, which researchers acknowledged was not based on a scientific sample, the agency obtained completed questionnaires from 227 plans of various sizes.

In addition, GAO contacted 96 401(k) stakeholders, including service providers, academics and advocacy groups.

While expressing concern over default rules, GAO praised QDIAs for significantly increasing worker participation in defined contribution plans.

The report was requested by Massachusetts Democratic Senator Elizabeth Warren, who has been a leading investor advocate in Congress.