Here are the facts from the opinion:
Jack Gross began working for respondent FBL Financial Group, Inc. (FBL), in 1971. As of 2001, Gross held the position of claims administration director. But in 2003, when he was 54 years old, Gross was reassigned to the position of claims project coordinator. At that same time, FBL transferred many of Gross’ job responsibilities to a newly created position—claims administration manager. That position was given to Lisa Kneeskern, who had previously been supervised by Gross and who was then in her early forties. Although Gross (in his new position) and Kneeskern received the same compensation, Gross considered the reassignment a demotion because of FBL’s reallocation of his former job responsibilities to Kneeskern.
Gross filed suit . . . alleging that his reassignment to the position of claims project coordinator violated the ADEA, which makes it unlawful for an employer to take adverse action against an employee "because of such individual’s age." 29 U. S. C. §623(a). The case proceeded to trial, where Gross introduced evidence suggesting that his reassignment was based at least in part on his age. FBL defended its decision on the grounds that Gross’ reassignment was part of a corporate restructuring and that Gross’ new position was better suited to his skills. . . .
The courts below wrestled with the proper standard of proof, assuming that Title VII's frameworks and analyses equally applied to the ADEA. The Supreme Court, which accepted review of the case to determine the proper time to give a "mixed motive" instruction in an ADEA case, answered: Never.
The Court's 5-4 majority reasoned that the ADEA statute is worded differently from Title VII, and that Congress passed a law amending Title VII to allow "mixed motive" cases, but did not simultaneously amend the ADEA. So, to sum up:
We hold that a plaintiff bringing a disparate-treatment claim pursuant to the ADEA must prove, by a preponderance of the evidence, that age was the "but-for" cause of the challenged adverse employment action. The burden of persuasion does not shift to the employer to show that it would have taken the action regardless of age, even when a plaintiff has produced some evidence that age was one motivating factor in that decision.
The dissent argued strenuously that the Court should not have reached the question that it decided because it was not presented for review. Then the dissenters, in two opinions, would have held that the language in the ADEA did not require "but-for" causation, and that courts had used Title VII precedent to interpret the ADEA's causation standards.
Congress can overturn this decision by simply incorporating Title VII's causation standards into the ADEA, or by simply adding "age" to Title VII and ending the separate statutory schemes. The majority pointed out Congress has taken up Title VII and ADEA amendments before without harmonizing the causation standards. I guess we'll find out soon enough if Congress omitted that amendment intentionally.
The case is Gross v. FBL Fin. Servs. and the opinion is here.