Age discrimination — the fastest-growing category of job bias complaints filed last year with the Equal Employment Opportunity Commission — is the backdrop for a U.S. Supreme Court challenge asking a key question about what employees and employers must prove when litigating cases under the Age Discrimination in Employment Act.

The underlying age bias claim in Jack Gross’ case before the Supreme Court represents the somewhat typical controversy when an employer takes an adverse job action against an employee, triggering a discrimination complaint, said employment law scholar Paul Secunda of Marquette University Law School.