In a unanimous ruling, the Supreme Court on Thursday (May 21, 2026) affirmed a circuit court decision in favor of a multiemployer pension fund and held that pension plans have the flexibility to update actuarial assumptions after year- end when calculating withdrawal liability rather than fixing those assumptions at the close of the prior year.

The decision was issued in M & K Employee Solutions, LLC, et al. v Trustees of the IAM National Pension Fund. The Word on Benefits® blog previously noted that this was the first withdrawal liability case to reach the Court in three decades.

The timing of actuarial assumptions, including when interest rates are calculated, can significantly impact the magnitude of withdrawal liability, often by hundreds of millions of dollars or more, attorney Deva Kyle explained in her blog published in the fall of 2025. Fixing the assumptions at the close of the prior plan year provides more certainty to employers, she wrote.

Kyle wrote that the question before the Court was whether an actuary of a multiemployer pension plan can adopt assumptions for calculating withdrawal liability after the statutory “measurement date” as long as the assumptions reflect the conditions on that date, or if the actuarial assumptions must be adopted (and never altered) by the measurement date itself.

Following is an excerpt from Kyle’s blog post, providing background on the case.

How Is Withdrawal Liability Determined?

Congress enacted the Multiemployer Pension Plan Amendments Act of 1980 (MPPAA) to prevent employers from abandoning underfunded multiemployer pension plans and leaving remaining employers to shoulder the plan’s unfunded vested benefits (UVBs). When an employer withdraws from an underfunded plan, it must pay its allocable share of the UVBs, which is calculated as the difference between the plan’s vested benefit obligations and its assets. This rule provides a legal basis for a plan to receive (often ongoing) compensation for continuing to provide promised benefits to employees of the withdrawing employer.

Under MPPAA, withdrawal liability is determined “as of the end of the plan year preceding the plan year in which the employer withdraws.” This date (commonly called the measurement date) fixes the point at which the plan’s UVBs are valued. To calculate UVBs, actuaries must discount future benefit payments to present value, which requires selecting assumptions about future investment returns, mortality and other plan experience. This means that the actuary is estimating how those future benefit payments translate into today’s dollars. The statute imposes two substantive requirements on those assumptions: They must (1) be reasonable and (2) “offer the actuary’s best estimate of anticipated experience under the plan.”

However, the statute does not explicitly address when those assumptions must be adopted. This silence—and whether it implies a time-related limit—is at the center of the M&K case.

Relevant Facts

M&K Employee Solutions participated in the IAM National Pension Fund. The company and other employers withdrew during the 2018 plan year. The measurement date for calculating withdrawal liability was set at December 31, 2017. As of that date, the fund’s actuary had used a 7.5% discount rate to value the plan’s liabilities yielding UVBs of roughly $448 million.

On January 24, 2018, the fund’s trustees met with the actuary and adopted new actuarial assumptions, including a lower discount rate of 6.5%. A lower discount rate signifies that the plan expects to generate lower investment returns. Therefore, the change dramatically increased the plan’s UVBs to more than $3 billion, significantly increasing the withdrawal liability assessed to M&K and the other employers.

The employers challenged the assessment in arbitration, arguing that the Employee Retirement Income Security Act (ERISA) required the plan to use assumptions adopted by the 2017 measurement date. The arbitrator ruled for the employers. The district court vacated the award and remanded, holding that post measurement-date assumptions could be permissible if they reflected information available as of the measurement date.

Following an appeal by the employer, the D.C. Circuit affirmed the district court decision.

The D.C. Circuit’s Holding

In its decision to affirm, the D.C. Circuit held that MPPAA permits actuaries to adopt assumptions after the measurement date, if those assumptions are based on information “as of” that later date. The court reasoned that the law requires assumptions to reflect the actuary’s “best estimate” as of the measurement date, and that it would be illogical to force actuaries to make that estimate before they have complete year-end information.

The court rejected the employers’ argument that the law imposes a strict timing requirement, emphasizing that Congress specified substantive requirements for assumptions but not temporal ones. It also noted that employers are safeguarded against manipulation because they may challenge unreasonable assumptions in arbitration. Courts review those findings under a standard that requires a clear preponderance of the evidence to support the arbitrators’ findings.

While other circuit court rulings have touched on questions of timing and actuarial assumptions in withdrawal liability calculations, the D.C. Circuit’s reading in M&K is the first to squarely conflict with the Second Circuit’s decision in a 2020 case Nat’l Retirement Fund v. Metz Culinary Mgmt., Inc. That case held that plans may not change assumptions after the measurement date. In response, the Supreme Court agreed to hear the case to resolve this circuit court split.

The Employers’ Argument: “As of” Means “By”

The employers argued that by directing plans to calculate UVBs “as of the end of the plan year,” the Multiemployer Pension Plan Amendments Act (MPPAA) freezes both the data and the assumptions as of that date. Because actuarial assumptions are embedded in the definition of UVBs, the employers claim that those assumptions must be the ones in effect on the measurement date.

They stressed that Congress knew how to authorize the use of later-adopted assumptions but did not do so in the law. They provide tax law as a similar example, where valuation with an “as of” a date requires using contemporaneous assumptions.

They also emphasize the importance of predictability and fairness. Employers that are contemplating withdrawal rely on disclosed plan data, including the assumptions in place before the measurement date, when making their decision. The employers argue that allowing plans to adopt new assumptions after a withdrawal enables retroactive manipulation that can inflate liability and game the system. This is the reason they argue that the fund increased liability from $448 million to $3 billion in UVBs after January 24, 2018.

Finally, the employers warned that the D.C. Circuit’s standard is unworkable, requiring litigation over subjective questions like what an actuary “knew” as of a date in the past. They claim that Congress could not have intended to hinge withdrawal liability on these uncertainties.

The Fund’s Argument: “As of” Means “With Reference To”

The fund responded that ERISA imposes no timing requirement for adoption of assumptions. Instead, the fund argued that MPPAA requires assumptions to represent the actuary’s “best estimate” as of the measurement date. Forcing actuaries to set assumptions before they have completed year-end data, the fund contends, would undermine the “best estimate” mandate.

The fund also notes that Congress included timing language elsewhere in ERISA but omitted it here, suggesting that no time-related limit was intended. And it argues that because the statute allows the parties to seek arbitration and can contest arbitrators’ findings, employers are protected from unreasonable assumptions without imposing a rigid deadline.

In response to the employers’ assertion that withdrawal liability assumptions should be predictable so employers can calculate their liabilities when considering withdrawal, the fund states that employers can never know their exact withdrawal liability in advance, because liability depends on plan data and actuarial decisions that are unavailable until after year end. Even under the rule that the employers support, employers would not learn the selected assumptions until after withdrawal. Thus, they argue that the employers have overstated their claims of unpredictability.

Finally, the fund characterized the circuit split as narrow and tolerable because only two circuits have addressed the question directly, and forum-selection clauses in plan documents often dictate where cases must be heard. These clauses would prevent parties from choosing a circuit that is more favorable to them because the plan documents would dictate where the case would be heard.

The Meaning of “As of the Measurement Date”

The heart of the dispute is the meaning of the phrase as of the measurement date. The statutory phrase can have two interpretations:

1. Fixed date (M&K employers’ reading): Everything—data and assumptions—must be set on the measurement date itself. The phrase acts as a cutoff and no changes can be made after that date.
2. Reference point (the fund’s reading): The phrase defines the conditions to be measured, not the date of adoption. The actuary may adopt or amend assumptions later, so long as they describe reality on the measurement date.

Both points of view have support from the law, other court rulings and past policies, Kyle explained.

Update: May 21, 2026 Supreme Court Decision

In a unanimous decision delivered by Justice Ketanji Brown Jackson, the Court held that ERISA Sections 1391 and 1393, which govern the calculation of withdrawal liability, do not require the actuarial assumptions to be selected on or before the measurement date.

Instead, the Court said that the “as of” language in Section 1391 means that while the hard data that feeds the UVB calculation must be fixed on the measurement date, the calculation itself can be performed after the date. “Because actuarial assumptions are tools used to calculate UVBs rather than hard data about the plan, they cannot be ‘frozen’ on the measurement date,” the opinion stated. The “as of” requirement in Section 1391 sets the reference point for factual inputs into the UVB calculation, but it has no bearing on when actuaries must select the tools, including assumptions, that they use to calculate a plan’s UVBs, the decision stated.

In addition, the Court said that Section 1393 states that assumptions must be “reasonable,” “take into account the experience of the plan and reasonable expectations” and “offer the actuary’s best estimate of anticipated experience under the plan.”  The section provides no deadlines by which actuaries must select their assumptions, the Court said, adding that it presumes that the omission was intentional because there is a similar deadline in another section of the law.

Also, the “instruction that actuarial assumptions reflect the actuary’s ‘best estimate of anticipated experience under the plan’ . . .  supports the conclusion that actuaries can select their assumptions after the measurement date,” the opinion said. Requiring actuaries to use assumptions selected before the measurement date could prevent them from relying on the most up-to-date data when selecting their assumptions and result in assumptions that do not reflect their best estimate, the Court continued.

In addition, although the employers contended that allowing plans to adopt actuarial assumptions after the measurement date would “open the door to manipulation,” the Court said that employers’ proposed rule would not address those concerns. Employers also can challenge actuarial assumptions in arbitration, the Court pointed out.

Stay tuned for more information about the impact of this decision.