“Freezes” that limit or stop future pension benefits accruals now affect 21% of workers in single-employer U.S. defined benefit pension plans.

Researchers at the U.S. Government Accountability Office present that figure in a review of defined benefit plan benefits freezes.

The researchers have based their results on an analysis of 471 of the 7,804 defined benefit plan sponsors that have plans with 100 or more total plan participants.

Sponsors of 23% of all single-employer defined benefit plans have imposed a “hard freeze” that will eliminate all future benefits accruals, and 22% have imposed different kinds of freezes, the researchers report.

About 83% of the sponsors of frozen plans now offer 401(k) plans or other alternatives to traditional defined benefit pension plans, but 11% do not, the researchers write.

“Plan sponsors cited many reasons for freezing their largest plans but most often noted two: The impact of annual contributions on their firm’s cash flows and the unpredictability of plan funding,” the researchers write. “Sponsors whose largest plan was hard frozen were significantly more likely to anticipate plan termination as the likely plan outcome.”

Between 1990 and 2006, sponsors voluntarily shut down 61,000 adequately funded defined benefit plans, and the number of single-employer plans that the Pension Benefit Guaranty Corp. oversees has dropped to 28,800, down about 65% from the 1993 total, the researchers write.

“Given the prevalence of plans that are currently frozen and the relationship between plan freezes and plan termination, the shrinking of the single-employer insurance program plan base seems likely to continue,” the researchers conclude.

Many sponsors fund “standard terminations” by buying a group annuity, the researchers write.