M & K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund, No. 23-1209 (May 21, 2026)

Must a Multiemployer Pension Plan (MPP) calculate withdrawal liability for an exiting employer using the discount rate in effect “as of” the plan’s statutory “measurement date,” or may the plan select a different discount rate that more accurately reflects economic conditions at the time withdrawal liability is calculated? In M & K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund, the United States Supreme Court resolved a circuit split on this issue, holding that ERISA permits actuaries to select withdrawal liability calculation assumptions after the statutory measurement date.[1] This ruling has significant financial implications for employers participating in underfunded MPPs: Plan actuaries may adopt new assumptions after the statutory measurement date that dramatically increase withdrawal liability.

Statutory and Factual Background

To protect MPPs from insolvency, ERISA requires withdrawing employers to pay a true up “withdrawal liability” representing the employers’ proportionate share of the plan’s unfunded vested benefits (“UVBs”)—the difference between the present value of what the plan owes in future benefits and the value of the plan’s current assets.[2] ERISA requires that UVBs be calculated “as of” the “measurement date,” which is the last day of the plan year preceding the employer’s withdrawal.[3]

Calculating the present value of UVBs is not a matter of simple arithmetic but instead requires a complex set of calculations. These calculations involve both “hard data,” such as the number of plan beneficiaries and current value of plan assets, and a set of actuarial assumptions used to predict how future events will impact the plan’s assets and liabilities. Actuarial assumptions include estimates of how long beneficiaries will draw benefits, future inflation rates, and the future performance of fund assets.[4] A key actuarial assumption, and the one at issue in M & K Employee Solutions, is the discount rate—the interest rate applied to determine the present value of the plan’s anticipated future liabilities. Applying a lower discount rate to future benefit payments results in a higher present value of assets required to fund those benefit obligations. In an underfunded MPP, applying a lower discount rate results in a higher present value of UVBs and a corresponding increase in an exiting employer’s withdrawal liability.[5]

In M & K Employee Solutions, four employers withdrew from an underfunded MPP in 2018. The measurement date was the last day of the prior plan year, December 31, 2017. As of that day, the plan’s actuary employed a 7.50% discount rate. But in January 2018, the plan’s board of trustees adopted a funding improvement plan that reduced the discount rate to 6.50%. This one percent reduction increased the MPP’s total UVBs from around $500 million to over $3 billion—a six-fold increase. The withdrawing employers similarly faced dramatically higher withdrawal liability under the post-measurement date discount rate than they would have if the discount rate in effect on the measurement date were applied. For example, M & K Employee Solutions’ withdrawal liability increased from $1.8 million to $6.2 million.[6]

Procedural History

The employers initiated arbitration under ERISA § 4221, and the arbitrators ruled in their favor, holding that actuarial assumptions must be those “in effect” on the measurement date. The plan challenged the arbitration awards in the U.S. District Court for the District of Columbia, which reversed and held in favor of the plan. The D.C. Circuit affirmed, holding that nothing in ERISA imposes a temporal constraint on the selection of actuarial assumptions. The Supreme Court granted certiorari to resolve a split with the Second Circuit, which held in National Retirement Fund v. Metz Culinary Management, Inc., 946 F.3d 146 (2d Cir. 2020), that actuarial assumptions must be those in effect on the measurement date.[7]

The Supreme Court’s Holding

In a unanimous opinion authored by Justice Jackson, the Supreme Court reversed, holding that “withdrawal liability can be calculated based on actuarial assumptions adopted after the measurement date.”[8] The holding was based on a textual analysis of two sections of ERISA governing withdrawal liability.

First, 29 U.S.C. § 1391, which addresses the methods plans may use to calculate withdrawal liability, does not mention actuarial assumptions at all, but requires that withdrawal liability be calculated “as of” the measurement date.[9] This “as of” language does not constitute a deadline for the plan to select the discount rate. Dictionaries inform that “as of” means “at the date mentioned” and is understood “to assign an event to one time and the recognition of it at another.”[10] The Court therefore held that § 1391’s use of “as of” meant that (1) hard data inputs for UVB calculations must be fixed on the measurement date, and (2) UVB calculations themselves could be performed at a later date.[11] The key question for the Court to resolve then, was whether actuarial assumptions such as the discount rate were hard facts that were fixed on the measurement date or part of the calculation itself that could be determined after the measurement date.[12]

The Court rejected the employers’ view that actuarial assumptions are hard facts frozen in place as of the measurement date. Instead, these assumptions are predictive judgments, or tools, used by actuaries to calculate UVBs. The Court found this distinction clear from ERISA’s text and statutory structure, which group actuarial assumptions with the methods prescribed for how withdrawal liability must be calculated. [13] Rather than being fixed hard facts “in effect” on a given date, actuarial assumptions are tools that an actuary adopts for the purposes of a particular calculation.[14]

Second, 29 U.S.C. § 1393, which governs actuarial assumptions for withdrawal liability calculations, imposes only substantive constraints. Assumptions must be “reasonable in the aggregate,” must “in combination offer the actuary’s best estimate of anticipated experience under the plan,” and must take into account plan experience and reasonable expectations.[15] Section 1393 contains no deadline for when assumptions must be adopted, which is significant because Congress elsewhere included a deadline for selecting actuarial assumptions in determining the amortization period for an employer’s withdrawal liability payments.[16] Because Congress did not impose such a temporal limitation for selecting actuarial assumptions in calculating withdrawal liability, the Court treated that omission as intentional.[17] This outcome is further supported by § 1393’s requirement that the actuary make the “best estimate of anticipated experience under the plan,” which suggests permitting the actuary to rely on current data when selecting the assumptions used to calculate withdrawal liability.[18]

Key Takeaways and Practical Implications

This decision has important practical implications for employers that participate in underfunded multiemployer pension plans:

• Heightened Uncertainty in Withdrawal Planning. Employers considering withdrawal can no longer rely on current actuarial assumptions to estimate their potential liability. Plans may adopt materially different assumptions after the measurement date, resulting in substantially higher (or lower) withdrawal liability than initially anticipated.
• Build Wider Liability Estimates. Employers should model a range of possible liability scenarios—including conservative assumptions about potential discount rate reductions—when evaluating whether to withdraw from an underfunded MPP.
• Challenging Assumptions Remains a Critical Safeguard. The Court emphasized that the “reasonableness” requirement of Section 1393(a)(1) and the right to challenge assumptions remain the primary guardrails against improper assumption changes. Employers should be prepared to challenge post-measurement-date assumption changes where those changes do not reflect the actuary’s genuine “best estimate.”
• Monitor Plan Communications Closely. Employers should pay close attention to plan actuarial reports, funding improvement plans, and trustee communications for early signals that assumption changes may be forthcoming.
• Consider Withdrawal Timing. In light of this decision, timing of withdrawal may be a more significant strategic consideration. Employers may want to evaluate whether withdrawal before an anticipated assumption change is advisable, while recognizing that plans are not required to telegraph such changes in advance.

Conclusion

The Supreme Court’s unanimous decision in M & K Employee Solutions resolves an important question of ERISA statutory construction and provides multiemployer plan actuaries temporal discretion in selecting the assumptions used to calculate withdrawal liability. While this decision increases uncertainty for employers contemplating withdrawal, the statutory reasonableness requirement and right to challenge assumptions provide avenues for protecting employers who believe an actuary selected unreasonable assumptions. Employers participating in underfunded multiemployer plans should consult with their ERISA counsel and actuarial advisors to reassess their withdrawal-liability exposure in light of this decision.

[1] M & K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund, No. 23–1209.

[2] Slip Op. 2 (citing 29 U.S.C. §§ 1381, 1393).

[3] 29 U.S.C. § 1391.

[4] Slip Op. 3.

[5] Id.

[6] Slip Op. 4–5.

[7] Id. at 5–6.

[8] Id. at 6.

[9] Id. at 6 (citing Section 1391(b)(2)(E)(i)).

[10] Id. (quoting Oxford American Dictionary 34 (1980); W. Follett, Modern American Usage 41 (rev. ed. 1998)).

[11] Id. at 6–7.

[12] Id. at 7.

[13] Id.

[14] Id. at 7–8.

[15] Slip Op. 9.

[16] Id.

[17] Id. (citing Russello v. United States, 464 U.S. 16, 23 (1983) (“[W]here Congress includes particular language in one section of a statute but omits it in another section of the same Act, it is generally presumed that Congress acts intentionally and purposely in the disparate inclusion or exclusion” (internal quotation marks omitted)).

[18] Id. at 9–10. The parties disputed below, but the Court did not resolve, whether actuarial assumptions must be limited to information available as of the measurement date. Slip Op. 6 n.2.