The opinion was written by Justice Thomas. Although we have not had time to digest the opinion thoroughly, the gist of the opinion is that, in a mixed motives age discrimination case, the burden of persuasion never shifts from the plaintiff to the defendant.
Hence, the burden of persuasion necessary to establish employer liability is the same in alleged mixed-motives cases as in any other ADEA disparate-treatment action. A plaintiff must prove by a preponderance of the evidence (which may be direct or circumstantial), that age was the “but-for” cause of the challenged employer decision.
Stripped of the legal hocus-pocus,that means that many age discrimination cases just got harder for the plaintiff to prove.
We'll have more on this decision as time allows us the opportunity to read all 29 pages more thoroughly. (What a great way to spend Father's Day!)