The US Supreme Court on Monday agreed to consider the evidentiary scope of expert witness testimony for demonstrating intent in a “blind mule” drug trafficking case, Diaz v. United States. The petition for reconsideration was brought by Delilah Diaz, a California woman who was convicted of transporting drugs into the US after the government presented expert testimony to disprove Diaz’s claim that she did not know she was carrying drugs. 

In earlier proceedings, the US Court of Appeals for the Ninth Circuit upheld a district court decision to affirm Diaz’s conviction for transporting methamphetamine across the US-Mexico border in violation of 21 U.S.C. §§ 952 and 960. Specifically, the lower courts found that the US government’s expert witness testimony that most “blind mules” know that they are smuggling drugs was admissible under Federal Rule of Evidence 704(b).

Whether Rule 704(b) permits an expert witness to testify that a person transporting drugs knew they were doing so is an issue that has caused a circuit split. The Ninth Circuit, Eighth Circuit, and Eleventh Circuit have all permitted testimony of this kind. However, the Fifth Circuit has repeatedly found that the rule prohibits “the functional equivalent” of a prohibited opinion on an actor’s knowledge, as well as “explicit opinions” on the matter.

Diaz explained this circuit split in support of her request for reconsideration before asserting that the stance of the Ninth Circuit is wrong. According to her petition for certiorari:

The Ninth Circuit’s position … contravenes the text of Rule 704(b), which prohibits all “opinion[s] about” the defendant’s mental state – not just explicit opinions. It also impermissible lightens the Government’s burden to prove knowledge beyond a reasonable doubt, permitting it to substitute a generalization about a particular class of defendants for evidence specific to the actual defendant.

Ultimately, Diaz asserted that allowing this sort of expert witness testimony violates “elementary notions of due process.” The Supreme Court is expected to hear oral arguments on the matter next year.