M & K Employee Solutions, Inc. v. Trustees of IAM Nat. Pension

Summary

Justice Jackson delivered the unanimous opinion of the Court, holding that ERISA does not require actuarial assumptions used to calculate an employer’s withdrawal liability to be selected on or before the statutory measurement date. Section 1391 fixes the date for the factual inputs into the calculation, but actuarial assumptions are predictive tools rather than historical facts. Section 1393 imposes reasonableness and best-estimate requirements without imposing a selection deadline. The Court rejected arguments based on statutory antiretroactivity provisions and concerns about manipulation, and affirmed the D. C. Circuit. It expressly reserved whether assumptions must be based solely on information available as of the measurement date.