557 U.S. 167 (U.S. Supreme Court 2009)
Gross v. FBL Financial Services, Inc.
Does the ADEA authorize mixed-motive liability and burden shifting such that, upon a plaintiff's showing that age was a motivating factor, the burden of persuasion shifts to the employer to prove it would have made the same decision; or must an ADEA plaintiff always prove that age was the but-for cause of the adverse employment action?
Under the ADEA, 29 U.S.C. § 623(a)(1), a plaintiff alleging disparate treatment 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 never shifts to the employer to show it would have taken the same action regardless of age. The mixed-motive framework recognized in Price Waterhouse v. Hopkins for Title VII and codified for certain Title VII claims by the Civil Rights Act of 1991, 42 U.S.C. § 2000e-2(m), does not apply to ADEA claims.
A plaintiff bringing a disparate-treatment claim under the ADEA must prove that age was the but-for cause of the adverse employment action; the ADEA does not authorize mixed-motive burden shifting. The Court reversed the Eighth Circuit and remanded for further proceedings consistent with this standard.
Gross is the seminal case establishing the but-for causation standard for private-sector ADEA disparate-treatment claims and rejecting mixed-motive burden shifting in that context. It realigns ADEA litigation around a stricter causation requirement than many Title VII status-based claims, affecting how plaintiffs frame their cases, the evidence they must marshal, and the jury instructions courts may give. For students, Gross is a critical example of textual interpretation, the limits of cross-statutory borrowing of doctrines, and how subsequent congressional amendments to one statute (Title VII) do not necessarily alter another (ADEA). Gross also became a platform for later decisions—most notably University of Texas Southwestern Medical Center v. Nassar (2013), which adopted a but-for causation standard for Title VII retaliation claims—and it informs the modern understanding of causation across employment discrimination law.