In a meth-trafficking trial, a government expert testified that most couriers know they are transporting drugs. On Monday, the U.S. Supreme Court agreed to clarify a circuit split over whether that testimony violates a federal rule of evidence.

In Diaz v. U.S., the justices will consider the scope of Rule 704(b) prohibiting experts from testifying about a defendant’s mental state when such knowledge is an element of the crime.