On May 21, 2026, the U.S. Supreme Court issued an opinion in M&K Employee Solutions, LLC, et al. v. Trustees of the IAM National Pension Fund, addressing when the actuarial assumptions used to measure unfunded vested benefits (UVBs) may be adopted for purposes of calculating and assessing withdrawal liability to employers withdrawing from a multiemployer defined benefit (DB) plan.

Key takeaways from the opinion include:

• 29 U.S.C. § 1391 requires the UVBs to be calculated as of a specific measurement date. In the Court’s view, the phrase “as of” sets the reference date for factual inputs, such as participant data and asset values, not a deadline for when actuarial assumptions must be selected. The Court does not consider actuarial assumptions to be factual inputs.
• 29 U.S.C. § 1393 requires an actuary to use reasonable actuarial assumptions that offer their best estimate of anticipated experience under the plan for purposes of assessing withdrawal liability. The Court concluded that this section does not provide a deadline for actuaries to select their assumptions but instead supports the notion that an actuary can select assumptions after the measurement date. Importantly, the Court notes that if actuaries were forced to use assumptions selected before the measurement date, it “could prevent them from relying on the most up-to-date data when selecting their assumptions, resulting in assumptions that do not reflect their ‘best estimate.’”²

Calculating withdrawal liability for multiemployer pension plans

In general, an employer that withdraws from a multiemployer DB plan is assessed withdrawal liability equal to its share of the plan’s UVBs, subject to statutory exceptions. The measurement date for determining the UVBs is the last day of the plan year preceding the employer’s withdrawal. For example, an employer withdrawing in 2026 from a calendar year plan would use December 31, 2025, as the measurement date.

It is common for actuaries to review potential changes to actuarial assumptions after a plan’s measurement date. The actuary may need time after the plan’s year-end to gather updated asset information, review capital market expectations, evaluate plan experience, and consider whether assumptions previously used remain appropriate for withdrawal liability. The Court’s decision recognizes that this process may occur after the measurement date without violating ERISA.

Decision on M&K Employee Solutions resolves a circuit court split

The Supreme Court’s decision resolves a federal circuit split over when withdrawal liability assumptions must be adopted. In M&K Employee Solutions, the Court affirmed the D.C. Circuit’s view that ERISA does not require actuarial assumptions to be selected on or before the measurement date, so long as the withdrawal liability is calculated as of that date. That approach conflicted with the Second Circuit’s 2020 decision in National Retirement Fund v. Metz Culinary Management, Inc., where the court rejected a withdrawal liability assessment based on assumptions adopted after the measurement date. In Metz, the plan hired a new actuary who changed the plan’s withdrawal liability discount rate from what it had historically been. The actuary’s decision to select this new assumption came after the end of the plan year to which it would first apply. The Second Circuit rejected this and concluded that the statute required the plan to use the assumptions in effect as of the measurement date. By adopting the D.C. Circuit’s reading, the Supreme Court eliminated that disagreement and established a uniform national rule on the timing of assumption selection for withdrawal liability calculations.

¹M & K Employee Solutions, LLC v. Trustees of IAM Nat. Pension Fund, No. 23-1209, slip op. (U.S. May 21, 2026). Retrieved May 27, 2026, from https://www.supremecourt.gov/opinions/25pdf/23-1209_i3kn.pdf.

² Ibid.