Whether federal courts sitting in diversity should exercise their discretion to certify questions to state high courts when a state's courts are silent or split on outcome-determinative questions that implicate subject-matter jurisdiction, core legal requirements like the standard of proof, or both

QUESTION PRESENTED This diversity case involves state-law claims of age discrimination in employment. Petitioner disputed the applicability of the New York City Human Rights Law (“NYCHRV’); it also argued that any punitive damages must be established by clear and convincing evidence rather than by a preponderance. Neither issue, of course, presents a federal question for this Court’s resolution. But how courts proceed in diversity is central to the federal judiciary’s work and to the federal-state balance. In particular, whether federal courts certify doubtful or contentious questions of state law has an outsized effect on E’rie’s aspiration that substantive outcomes—especially substantive legal standards—should not materially differ because a case proceeds in a federal rather than state forum. The question presented is whether federal courts sitting in diversity should exercise their discretion to certify questions to state high courts when a state’s courts are silent or split on questions that implicate subject-matter jurisdiction, core legal requirements like the standard of proof, or (as here) both. i)