Diaz v. United States
602 U.S. 526 (2024)
Premium Feature
Subscribe to Lexplug to listen to the Case Podcast.
Rule of Law:
Federal Rule of Evidence 704(b) does not prohibit expert testimony that "most people" in a group have a particular mental state, because such an opinion is not directly "about the defendant" and thus leaves the ultimate determination of the defendant's individual mental state to the jury.
Facts:
- In August 2020, Delilah Diaz attempted to enter the United States from Mexico while driving a car.
- Border patrol officers searched Diaz's car after an officer felt resistance and heard a 'crunch-like sound' from a window, and subsequently detected abnormal density and found 56 packages of methamphetamine (over 54 pounds) hidden in the car's door panels and trunk.
- Diaz was arrested and claimed she had no idea drugs were hidden in the car, stating she was driving her boyfriend's car but providing inconsistent details about him and two cellphones found in the vehicle.
- Diaz was charged with importing methamphetamine, which required the Government to prove she 'knowingly' transported the drugs.
- The Government sought to call Homeland Security Investigations Special Agent Andrew Flood as an expert witness to testify that drug traffickers generally do not entrust large quantities of drugs to people who are unaware they are transporting them, specifically that "most couriers know that they are transporting drugs."
Procedural Posture:
- Delilah Diaz was charged with importing methamphetamine in federal District Court.
- Diaz filed a pretrial motion under Federal Rule of Evidence 704(b) to object to the Government's expert witness testimony.
- The District Court granted Diaz's motion in part, prohibiting the expert from testifying in absolute terms about whether all couriers knowingly transport drugs, but allowing testimony that most couriers know they are transporting drugs.
- A jury found Diaz guilty, and the District Court sentenced her to 84 months' imprisonment.
- Diaz appealed her conviction to the United States Court of Appeals for the Ninth Circuit, challenging the expert's testimony under Rule 704(b).
- The Ninth Circuit Court of Appeals affirmed the District Court's decision, holding that Rule 704(b) only prohibits an "explicit opinion" on the defendant's state of mind.
Premium Content
Subscribe to Lexplug to view the complete brief
You're viewing a preview with Rule of Law, Facts, and Procedural Posture
Issue:
Does Federal Rule of Evidence 704(b), which prohibits expert witnesses from stating an opinion about whether a defendant had a mental state constituting an element of a crime, apply to expert testimony that "most people" in a group (which includes the defendant) possess the requisite mental state?
Opinions:
Majority - Justice Thomas
No, expert testimony that "most people" in a group have a particular mental state does not violate Rule 704(b). The Court reasoned that Rule 704(b) is a narrow exception to Rule 704(a), which generally permits opinions on ultimate issues. The Rule's text prohibits opinions "about whether the defendant did or did not have a mental state," meaning it applies only to conclusions directly pertaining to the specific defendant's mental state. Agent Flood's testimony that "most drug couriers know that they are transporting drugs" does not necessarily describe Diaz's mental state, as she might be an exception to the general rule. The Court distinguished this from an expert testifying that "all people in the defendant's shoes" have a particular mental state, which would indeed violate Rule 704(b) because it implicitly includes the defendant. The ultimate issue of Diaz's mental state was left for the jury to decide, which is consistent with the Rule's purpose. The Court rejected the argument that "about" should be read broadly to include any testimony that "concerns" the defendant's mental state, emphasizing that the word's context in Rule 704(b) limits it to direct conclusions.
Concurring - Justice Jackson
Justice Jackson concurred, joining the Court's opinion in full and emphasizing the narrow scope of Rule 704(b). She highlighted that the Rule is "party agnostic," meaning neither the Government nor the defense can offer an expert opinion on the defendant's specific mental state, but both can introduce expert testimony on the likelihood of a particular mental state based on the defendant's membership in a group. She pointed out that this type of evidence, such as testimony on mental health conditions (e.g., schizophrenia, battered woman syndrome), can be crucial for defendants to contextualize their actions and challenge a presumed mens rea. Justice Jackson acknowledged the risks of overreliance on expert testimony but noted that other safeguards, such as vigorous cross-examination, Rules of Evidence 401, 402, 403, and 702, and specific jury instructions, remain available to prevent misuse and ensure the jury retains its core duty.
Dissenting - Justice Gorsuch
Yes, Justice Gorsuch dissented, arguing that the expert testimony violated Rule 704(b). He contended that the Rule prohibits any opinion "about whether" the defendant had the mental state, not just definitive ones. He stated that the word "about" broadly includes anything "concerning, regarding, with regard to, in reference to" the defendant's mental state. To allow an expert to say "most" people in the defendant's situation have the requisite mental state is functionally equivalent to saying the defendant "most likely" did, which would surely be barred. He warned that this interpretation creates a "muddled view" that could lead to a slippery slope, where experts offer increasingly specific probabilistic assessments, ultimately "eviscerat[ing] Rule 704(b)." Justice Gorsuch emphasized the fundamental importance of mens rea and the jury's exclusive role in determining it, expressing concern that such expert testimony introduces "junk science" like "mental telepathy" and diminishes the seriousness and reliability of criminal proceedings, turning it into a "competing game of 'I say so.'" He suggested that other rules of evidence, such as 401 (relevance), 403 (unfair prejudice), and 702 (reliability and helpfulness of expert testimony), should operate as bulwarks against such testimony.
Analysis:
This case significantly clarifies the permissible scope of expert testimony regarding a defendant's mental state under Federal Rule of Evidence 704(b), drawing a critical distinction between opinions about a general class of people and direct opinions about the defendant. By permitting testimony on what "most people" in a group know, the Court has potentially opened the door for more frequent use of such 'pattern-of-conduct' expert testimony by both prosecution and defense. This interpretation may influence how defendants, especially those claiming a "blind mule" defense, prepare their cases, requiring them to more actively demonstrate how they are not like "most" people. Future cases will likely test the boundaries of this ruling, particularly how specific such group statistics can be and whether other evidentiary rules (like FRE 403, as suggested by the dissent) might still prevent the admission of highly prejudicial or minimally probative 'most people' testimony.
