WASHINGTON—To minimize potential liability, employers must conduct thorough investigations before taking adverse job actions to be sure no one along the chain of command has a discriminatory motive, experts say.

This is due to the U.S. Supreme Court's ruling last week in Vincent E. Staub vs. Proctor Hospital, in which the court said employers can be held liable for discriminatory conduct even if the person making the decision was not biased but relied at least in part on people who were.

The decision supports the “cat's paw” theory of liability, first cited in a 1990 employment discrimination decision by the 7th U.S. Circuit Court of Appeals. However, the Supreme Court overturned the 7th Circuit in Staub.

“You can't just accept a supervisor's opinion in the face of an employee who claims that their supervisors are discriminating against them,” which is what happened in Staub, said Julie L. Galassi, the attorney for Mr. Staub and a partner with Hasselberg, Rock, Bell & Kuppler L.L.P. in Peoria, Ill.

Melinda J. Caterine, a Portland, Maine-based defense attorney with Fisher & Phillips L.L.P., said although “employees are still going to have to prove that they were terminated for an unlawful reason,” the Supreme Court's ruling “will likely cause employers to do more thorough investigations before they terminate an employee” instead of relying on a personnel file or an immediate supervisor's report.

“They're going to have to look beyond that and do an independent investigation and determine whether there is a legitimate nondiscriminatory reason” for the adverse action, Ms. Caterine said.

Edward A. Brill, a partner with law firm Proskauer Rose L.L.P. in New York, said an employer that does follow a lower-level supervisor's recommendation “will have to be more diligent and conscientious to ensure that there is no stain” connected to the decision and fully investigate any accusation of bias.

In Staub, an investigation that was a “little bit more in-depth would have uncovered some fairly serious anti-military comments,” said Marc E. Alifanz, an associate with law firm Stoel Rives L.L.P. in Portland, Ore.

Michael W. Fox, a shareholder with law firm Ogletree, Deakins, Nash, Smoak & Stewart P.C. in Austin, Texas, said the decision is “just going to put a premium on good (human resources) practices and good documentation.”

“If you have a hint” that bias may have played a role in a termination “you need to do everything you can to make sure that...you have really scrubbed the decision so that it has no relevance or bearing,” Mr. Fox said. “It also means that there's a premium on the conduct of supervisors,” and that more training is called for so they do not make discriminatory statements that may emerge years later, he said.