The Supreme Court has made it harder to prove discrimination on the basis of age, ruling against an employee in his mid-50s who says he was demoted because of his age.

In a 5-4 decision Thursday written by Justice Clarence Thomas, the court said a worker has to prove that age was the key factor in an employment decision, even if there is some evidence that age played a role. In some other discrimination lawsuits, the burden of proof shifts to the employer once a worker shows there is some reason to believe a decision was made for improper reasons.

Jack Gross had been a vice president of FBL Financial Services of West Des Moines, Iowa. But in 2001, he lost the title of vice president in a reorganization and two years later, some of his responsibilities were given to a colleague.

Gross sued under the federal Age Discrimination in Employment Act and a jury agreed that his age was a motivating factor in his demotion. Gross was awarded $46,945 in lost compensation.