A Looming Seismic Shift in the Rules of Evidence: The Far-Reaching Impact of Diaz v. United States

According to the U.S. Department of the Interior, an earthquake is caused by a sudden shift in a fault line. The tectonic plates on fault lines always move slowly, but they remain attached at their edges by friction. When the stress on the edge overcomes the friction, an earthquake occurs, releasing energy in waves that travel through the Earth’s crust, causing the vibrations and structural damage felt on the surface.

The evidentiary weight of our legal system recently shifted when the United States Supreme Court issued its opinion in Diaz v. United States. Justice Clarence Thomas, writing for the court’s majority, held that an expert witness’s conclusion that “most people” in a group have a particular mental state is not an opinion about “the defendant” and thus does not violate long-standing rules prohibiting an expert from offering an opinion on the ultimate question of a defendant’s state of mind. According to Justice Neil Gorsuch’s dissent, this decision “has now given prosecutors a … powerful new tool. … Prosecutors can now put an expert on the stand — someone who apparently has the handy ability to read minds — and have him tell them what “most” people like the defendant are thinking when they commit a legally prohibited act.”