ERISA Litigation Roundup: Supreme Court Unanimously Rules Multiemployer Pension Plans May Use Post-Measurement-Date Actuarial Assumptions to Calculate Withdrawal Liability

On May 21, 2026, the US Supreme Court issued a unanimous decision in M & K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund, No. 23-1209, resolving a circuit split on a question of enormous financial consequence to employers participating in multiemployer pension plans (MPPs): whether a plan's actuary must adopt its actuarial assumptions for purposes of withdrawal liability calculations on or before the “measurement date” for those calculations, or whether it may instead select assumptions after that date. In an opinion authored by Justice Jackson, the Court held that ERISA does not require actuarial assumptions to be adopted “as of” the measurement date, but stressed that these actuarial assumptions must still reflect the actuary’s “best estimate of anticipated experience under the plan,” which generally requires that the assumptions reflect information about the plan’s conditions as they stood on the measurement date.