The U.S. Supreme Court recently handed employers a legal victory by making it harder for workers to prove age discrimination in cases where the employer contends that the adverse action resulted in legitimate corporate restructuring.

In Gross v. FBL Financial Services, Inc., a 5-4 decision, the Court ruled that an employee who files a compliant under the Age Discrimination in Employment Act must clearly prove that age was the sole factor for the adverse employment action.

In 2004, 54-year-old Jack Gross filed an age discrimination claim under the ADEA against his employer, FBL. He argued that the company demoted him, in part, due to his age. The company asserted that the move was a genuine business decision.

Before Gross, the courts allowed employees to assert that age, in part, played a role in their employer’s action, shifting the burden on the employer to prove that other nondiscriminatory factors drove the decision.

Mixed-motive discrimination claims filed under ADEA, however, apply legal standards under Title VII of the Civil Rights Act, because Congress failed to amend ADEA to reflect mixed-motive cases. The Court observed that the mixed-motive standards under Title VII do not apply to ADEA claims.

“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,” Justice Clarence Thomas wrote. “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.”

Justice John Paul Stevens, representing the dissenters, wrote: “The Court’s resurrection of the but-for causation standard is unwarranted. The answer to the question the Court has elected to take up — whether a mixed motives jury instruction is ever proper in an ADEA case — is plainly yes.”

Related coverage:

Spike in class-action suits leaves employers vulnerable

Younger workers hurt boomers can't afford to retire