American Journal of Trial Advocacy

Photo Credit: Jason Silva, What is the Role of a Digital Forensics Expert Witness?, Cornerstone Discovery (Sep. 27, 2016), https://cornerstonediscovery.com/what-is-the-role-of-a-digital-forensics-expert-witness/

Authored by: James David Greene III

Rule 704(b) of the Federal Rules of Evidence eliminates an expert witness from being able to “state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged[.]”[1] In Diaz v. United States, the court took on the issue of whether an expert witness could testify to a group of people’s mental state other than the defendant in conformity to Rule 704(b).[2] The Court’s holding will impact the future of expert witness’s ability to testify and strategy used by prosecutors in establishing the necessary mental elements of a charged offense.

In Diaz, a United States citizen, Delilah Diaz, was arrested while trying to cross the United States border at a port of entry from Mexico.[3] When Mrs. Diaz got to the port of entry, she was asked to role her window down by a border patrol agent.[4] The agent heard what he described as a “crunch-like” sound coming from inside the door when the window was rolled down.[5] This resulted in the agent performing a series of binary searches on the vehicle, which indicated the presence of narcotics in the door panels of the car.[6] The agents found fifty-six packages of methamphetamine weighing approximately fifty-four pounds with an estimated street value of $368,550.[7] Diaz denied knowing anything about the drugs and told agents she was driving her boyfriend’s car, whom she had only met “two, three times tops.”[8]

Diaz was charged with “importing methamphetamine in violation of 21 U.S.C. §§ 952 and 960” which require her to have “knowingly” transported the drugs.[9] At trial, Diaz claimed she was a “blind mule”[10] and, therefore, could not be convicted under the statute with which she was charged.[11] The government called Special Agent Andrew Flood as an expert witness in the field of “Mexican drug-trafficking organizations.”[12] Agent Flood planned to testify that Mexican drug traffickers “generally do not entrust large quantities of drugs to people who are unaware they are transporting them.”[13] Diaz, of course, objected to this testimony arguing that it violated Rule 704(b).[14] She argued that Agent Flood could not testify that anyone who possessed drugs while crossing the border knew they possessed the drugs because this would include him saying that she had a necessary mental condition for the crime in violation of the rule.[15] The District Court judge ruled that Agent Flood could not testify that all drug mules know they have drugs, but only that most drug mules know they are in possession of drugs.[16] Diaz was subsequently convicted and sentenced to eighty-four months in prison.[17]

The Court held that Agent Flood’s testimony did not violate Rule 704(b) because he testified that “most” drug mules know they have drugs on them, not all.[18] Justice Thomas explained that Agent Flood only testified a majority of drug mules know they have drugs, thereby leaving open the possibility that Diaz was not a part of the majority.[19] The jury was still left with the question: “Is Diaz like the majority of couriers? Or, is Diaz one of the less-numerous-but-still-existent couriers who unwittingly transport drugs?”[20] Therefore, the jury was left with the ultimate question of whether Diaz had the required state of mind and Agent Flood’s testimony did not violate Rule 704(b).[21]

The practical impacts brought by the Court’s ruling in Diaz can be seen from various sides of the issue. First, the Court’s decision will likely embolden the government to call expert witnesses for every criminal case where a charge requires a mental state instead of relying on evidence that requires the jury to infer mental state. Justice Gorsuch argues this as an unfavorable consequence in his dissenting opinion.[22] He points out that the government chose the type of evidence to put forward in establishing the mental state requirement by calling Agent Flood as an expert witness.[23] The government had plenty of other evidence to suggest that Diaz was aware of the drugs, but they chose not to use those available means.[24] Instead, the government relied on expert testimony about what the majority of drug mules know. [25]

The consequences can be seen clearly from a hypothetical. Suppose a man named George is indicted for purchasing a stolen airplane that had been flown from Oregon to Florida in violation of federal law.[26] To be found guilty, the government would have to prove George knew the plane was stolen when he acquired it.[27] George contends at trial that he had no idea the plane was stolen because he relied on an airplane broker to purchase the plane. The government found text messages, phone calls, and emails showing George knew the plane was stolen and was excited to be able to get such a great deal. However, instead of presenting this evidence to the jury to show George knew the plane was stolen, the government calls their own FBI agent to say in his experience, “Most people buying stolen planes know they are buying stolen planes.” This style of proving the mental requirement may seem to some as lazy and, as Justice Gorsuch argues, is not within the history of the American “commitment” to the mens rea requirement.[28]

The second, and “other side of the coin,” consequence may be seen from the same hypothetical with slightly modified facts. The benefit of this ruling is that the government is provided a remedy if they find themselves with little to no admissible evidence of the mens rea requirement. Suppose the same hypothetical occurs, but the government does not have any of the text messages, phone calls, or emails detailing George’s knowledge that the plane is stolen. Instead, the government has text messages between George and his wife discussing how he is about to “buy this stolen plane for pennies on the dollar.” Naturally, when the government calls George’s wife to testify, she invokes spousal privilege, and the government has nothing to go off. They know that George knew the plane was stolen, but do not have an admissible way to prove it. Under Diaz, the government may call an expert to testify that “most people in George’s shoes” know they are buying a stolen plane.

The Court’s recognition that Rule 704(b) is a narrow exception to Rule 704(a) results in more options for the government when tasked with proving a defendant’s requisite mental state. This development is likely to lead to a more robust approach by government prosecutors to show juries the likelihood a defendant violated the law. Ultimately, Diaz’s holding strengthened the government by allowing them discretion in how to prove their case.

---

[1] Fed. R. Evid. 704(b).

[2] 602 U.S. 526 (2024).

[3] Id. at 528.

[4] Id.

[5] Id.

[6] Id. at 529.

[7] Id.

[8] Diaz, 602 U.S. at 529.  

[9] Id.

[10] See Walter Gonclaves, Busted at the Border: Duress and Blind Mule Defenses in Border-Crossing Cases, The Champion, Feb. 2018, at 46 (defining a blind mule as “a person used by criminals to transport drugs who does not know he is carrying them”).

[11] Diaz, 602 U.S. at 529.

[12] Id. at 530.

[13] Id.

[14] Id.

[15] Id.

[16] Id. (emphasis added).

[17] Diaz, 602 U.S.at 531.

[18] Id. at 536.

[19] See id. (“Here, by contrast, Agent Flood asserted that Diaz was part of a group of persons that may or may not have a particular mental state.”).

[20] Id.

[21] Id.

[22] Id. at 550 (Gorsuch, J., dissenting) (“Yes, proving a defendant’s mental state at trial can require work. Normally, it will require the government to resort to circumstantial evidence and inference. After all, defendants in life do not confess their inner thoughts on the stand nearly as often as they do in courtroom dramas. But there is nothing new about any of that.”).

[23] Diaz, 602 U.S. at 551.

[24] Id. at 550 (discussing that the government had the following evidence to argue Diaz knew she was in possession of the drugs: the 54 pounds of drugs were worth $360,000, she has numerous holes in her story, multiple cell phones were in the car, one of which was locked, and she had no access to it).

[25] Id. at 551.

[26] See 18 U.S.C. § 2313(a) (“Whoever receives, possesses, conceals, stores, barters, sells, or disposes of any motor vehicle, vessel, or aircraft, which has crossed a State or United States boundary after being stolen, knowing the same to have been stolen, shall be fined under this title or imprisoned not more than 10 years, or both.”).

[27] Id.

[28] See Diaz, 602 U.S. at 552 (Gorsuch, J., dissenting) (“None of this serves our criminal justice system well . . . Allowing into our proceedings speculative guesswork about a defendant’s state of mind diminishes the seriousness due them . . . It undermines our historic commitment that mens rea is a necessary component of every serious crime by turning the inquiry into a defendant’s mental state from an exacting one guided by hard facts and reasonable inferences into a competing game of ‘I say so.’”).