Expert Testimony Admissible in Drug Case on Mens Rea

It is a well-established, fundamental principle that an expert may not offer testimony in court on an ultimate issue that is reserved for the jury to decide.  Federal Rule of Evidence, Rule 704(b), codifies this, stating an expert witness in criminal cases are prohibited from giving opinion testimony about whether defendant had a particular mental state (mens rea) or condition that is an element of the crime charged or a defense.

In August 2020, Delilah Diaz, a United States citizen, attempted to enter the United States from Mexico.  When Ms. Diaz drove into the port of entry, a border patrol officer asked her to roll down the car’s rear driver-side window.

Diaz responded that the window operation was manual, so the officer left his inspection booth and tried to roll down the window himself.  The officer felt some resistance and then heard a crunch-like sound in the door.

Aware from his experience that car doors are common locations for hiding contraband, the officer investigated further with a “buster,” a handheld tool that measures an object’s density.  After the buster detected an abnormal density in the doors, officers brought a narcotics detection canine and sent the car through a X-ray machine.  

They discovered fifty-six packages of methamphetamine tucked into the car’s door panels and underneath the carpet in the car’s trunk.  The methamphetamine weighed just over fifty-four pounds and had an estimated retail value of $368,550.

Ms. Diaz was arrested and after waiving her Miranda rights (Miranda v. Arizona (1966) 384 U.S. 436), agreed to an interview.  Ms. Diaz claimed she had no idea drugs were hidden in the car, also known as the “blind mule” defense.  The officers, however, found her story difficult to believe.  Ms. Diaz explained that she was driving her boyfriend’s car, but she had only seen her boyfriend “two, three times tops,” did not know his phone number and did not know where he lived.

Ms. Diaz was charged in U.S. District Court with importing methamphetamine in violation of 21 U.S.C. §§ 952 and 960.  Such charges required the Government to prove that Ms. Diaz “knowingly” transported drugs. 

At trial, Homeland Security Special Agent Andrew Flood testified about the common practices of Mexican drug-trafficking organizations.  He testified that, in general, drug traffickers “generally do not entrust large quantities of drugs to people who are unaware they are transporting drugs.”

Ms. Diaz’s attorney objected to Agent Flood’s testimony as violating Federal Rule of Evidence 704(b)), arguing that Agent Flood’s testimony was functionally equivalent to an opinion about whether Ms. Diaz knowingly transported drugs. 

The District Court sustained Ms. Diaz’s objection in part and overruled it in part.  The judge agreed that Agent Flood’s testimony could not include testimony that all couriers knowingly transport drugs, but he could testify that most couriers, when hired, know they are hired to  transport drugs from one point to another point.  Otherwise, the drug-trafficking organization would expose the drug-trafficking organization to substantial risk and they would have to retrieve the drugs without detection from some other location in the United States.  This would put hundreds of thousands of dollars of product at risk of loss.

The jury found Ms. Diaz guilty and the district judge sentenced her to 84 months in federal prison.

Ms. Diaz then appealed her conviction, arguing that the district court judge erred by allowing Agent Flood to testify about the mental state of most drug couriers, in violation of Federal Rule of Evidence 704(b).  

The U.S. Court of Appeals for the Ninth Circuit affirmed the trial court.  

Ms. Diaz then appealed to the U.S. Supreme Court.

Justice Clarence Thomas, writing for the majority, affirmed the district court and the Ninth Circuit.  In his opinion, he explained that while 704(b) bars such testimony, an exception exists that allows an expert to testify about what most couriers’ mental state is, but not defendant’s in particular.

We think this ruling effectively allows the exception to swallow up the rule.  While we respect a juror’s ability to distinguish between what most couriers may know and what a specific courier did or did not know, we think the expert’s testimony does offer an opinion as to what the individual defendant knew, here, Ms. Diaz.  Such testimony should not be permitted.  This ruling is simply terrible.