Supreme Court Holds ERISA Permits Multiemployer Pension Plans to Use Actuarial Assumptions Adopted After the Measurement Date in Calculating Withdrawal Liability

In M & K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund, No. 23-1209, ___ S.Ct. ___, 2026 WL 1423319 (U.S. May 21, 2026), a unanimous decision authored by Justice Jackson, the Supreme Court resolved a circuit split over when actuaries must select the assumptions used to calculate an employer’s withdrawal liability from an underfunded multiemployer pension plan under the Multiemployer Pension Plan Amendments Act of 1980. The Court affirmed the D.C. Circuit and held that ERISA does not require actuarial assumptions to be adopted on or before the statutory measurement date, abrogating National Retirement Fund v. Metz Culinary Mgmt., Inc., 946 F.3d 146 (2d Cir. 2020).

Petitioners are four employers that withdrew from the IAM National Pension Fund, an underfunded multiemployer pension plan, between April and December 2018. Pursuant to 29 U.S.C. § 1391, the Fund assessed each employer’s withdrawal liability “as of” December 31, 2017, the last day of the plan year preceding withdrawal. In making that calculation, the Fund’s actuary applied a 6.50% discount rate adopted in January 2018, replacing the previously used 7.50% rate. The reduction dramatically increased Petitioners’ assessed liability; M&K Employee Solutions, for example, was assessed approximately $6.2 million under the 6.50% rate compared to approximately $1.8 million under the prior rate. Petitioners initiated arbitrations under § 1401(a), and the arbitrators ruled that § 1391’s “as of” language required the Fund to use the assumptions in effect on the measurement date. The District Court for the District of Columbia vacated the arbitration awards, and the D.C. Circuit affirmed in a consolidated appeal, 92 F.4th 316 (D.C. Cir. 2024).

The Supreme Court affirmed. Beginning with § 1391, the Court rejected Petitioners’ argument that “as of” establishes a deadline by which actuarial assumptions must be selected. Drawing on dictionary definitions and Wilson Follett’s Modern American Usage, the Court explained that “as of” assigns an event to one time and the recognition of it to another. The provision therefore requires that the hard data feeding the unfunded vested benefits calculation, such as the number of beneficiaries and the value of plan assets, be fixed on the measurement date, but it does not constrain when the calculation is performed. Actuarial assumptions, the Court emphasized, are not observable facts about the plan but predictive judgments and tools used to calculate the unfunded vested benefits. The statute reinforces this by grouping actuarial assumptions with “methods” in § 1393(a)(1) and referring to assumptions as something “used” to determine the unfunded vested benefits. The Actuarial Standards of Practice likewise instruct actuaries to select assumptions for a particular measurement rather than maintaining assumptions “in effect” on any given date.

Turning to § 1393, the Court observed that the provision imposes only substantive requirements, namely that assumptions be “reasonable,” “tak[e] into account the experience of the plan and reasonable expectations,” and “offer the actuary’s best estimate of anticipated experience under the plan.” § 1393(a)(1). The statute contains no deadline, and the Court declined to read one in, citing Romag Fasteners, Inc. v. Fossil Inc., 590 U.S. 212, 215 (2020). The omission is significant because Congress did include a deadline elsewhere; § 1399(c)(1)(A)(ii) requires the amortization period for withdrawal-liability payments to be determined based on “the assumptions used for the most recent actuarial valuation.” Under Russello v. United States, 464 U.S. 16, 23 (1983), the Court presumed the omission in § 1393 was intentional. The “best estimate” instruction affirmatively supports post-measurement-date selection, because relevant data about plan performance and macroeconomic conditions as of the measurement date may not become available until afterward.

The Court rejected Petitioners’ remaining arguments. The retroactivity limits in § 1394(a), which concededly do not apply to actuarial assumptions, undercut rather than support an inferred antiretroactivity principle in § 1393. As for Petitioners’ policy argument that post-measurement-date assumptions invite manipulation, the Court found the concern equally applicable to pre-measurement-date assumptions and reiterated that “policy concerns cannot trump the best interpretation of the statutory text,” quoting Patel v. Garland, 596 U.S. 328, 346 (2022). ERISA already constrains actuarial discretion through § 1393(a)(1)’s reasonableness and best-estimate requirements and through arbitration challenges available under § 1401(a)(3)(B)(i).

The Court left for another day whether assumptions adopted after the measurement date must be based only on information available as of that date. The judgment of the D.C. Circuit was affirmed.