Arguments by Profs. Emily Murphy, Matt Coles Cited in SCOTUS Case on Expert Testimony

Professors Emily Murphy and Matt Coles’ arguments were cited in a concurring Supreme Court opinion that could shape how expert testimony is used in future criminal trials.

Faculty Who Lead: Emily Murphy and Matt Coles

  • Professors Emily Murphy and Matt Coles influenced a Supreme Court case on the admissibility of expert testimony about a criminal defendant’s likely mental state.
  • In Diaz v. United States, the Supreme Court interpreted a Federal Rule of Evidence to permit expert testimony providing probabilistic insights without directly stating a defendant’s mental state.
  • Such expert testimony is crucial for jurors to assess evidence while maintaining compliance with evidentiary rules, Murphy, Coles, Chancellor & Dean David Faigman, and over 20 evidence law professors argue in their amicus brief, as highlighted in a concurring opinion by Justice Ketanji Brown Jackson

UC Law San Francisco professors played an important role in a recently decided Supreme Court case testing the limits of expert testimony in criminal trials.

In a case challenging whether experts can opine on a defendant’s likely mental state, Supreme Court Justice Ketanji Brown Jackson penned a concurring opinion citing arguments co-written by Professors Emily Murphy and Matt Coles along with University of Utah Law Professor Teneille Brown.

The case, Delilah Guadalupe Diaz v. United States (No. 23-14), involves a woman convicted of smuggling illegal drugs across the U.S.-Mexico border. The defendant said she didn’t know drugs were hidden in her car door panels. A Homeland Security agent testified that couriers like her often know they were hired to take drugs across the border.

Murphy and Coles filed an amicus brief in the case on behalf of more than 20 evidence law professors, including UC Law SF Chancellor & Dean David Faigman. The professors argued that such experts offer important “framework” evidence on the likelihood of a fact being true. They said such experts do not violate the rules of evidence because they do not directly state what was in a defendant’s mind, allowing jurors to make the ultimate determination based on a variety of factors.

“What we get from this case is that in a criminal trial, an expert can still testify about mental state evidence as long as their testimony is probabilistic and doesn’t explicitly include the defendant themselves,” Murphy said.

Murphy added that it remains to be seen if courts will pick up on Justice Jackson’s suggestion that trial court judges should use jury instructions and judicial management techniques to ensure such testimony is relevant, reliable, and “not unduly persuasive to a jury such that they skip over making the final, critical inferences about this particular defendant’s mental state.”

Read the full amicus brief.