Diaz v. United States

Issues 

Does Federal Rule of Evidence 704(b) allow an expert witness to testify that most drug couriers carry drugs knowingly, and that drug-trafficking organizations do not trust unknowing couriers with large quantities of drugs, when knowledge is an element of the offense?

Oral argument: 
March 19, 2024

This case asks the Supreme Court to decide whether Federal Rule of Evidence 704(b) prohibits expert testimony about the mental states of a group that the defendant is a member of. Diaz was convicted of knowingly importing drugs into the United States after a government expert testified that “in most circumstances, the driver knows they are hired . . . to take the drugs from point A to point B.” Diaz argues that such testimony is impermissible under Rule 704(b) because it is functionally equivalent to stating an opinion that she had a certain mental state. The United States counters that the rule only restricts an expert from explicitly concluding or directly opining on whether a defendant had a certain mental state. The outcome of this case will affect how the government goes about proving its criminal cases, how effectively parties can rebut expert testimony, and how juries make decisions about mental states.

Questions as Framed for the Court by the Parties 

Whether in a prosecution for drug trafficking — where an element of the offense is that the defendant knew she was carrying illegal drugs — Federal Rule of Evidence 704(b) permits a governmental expert witness to testify that most couriers know they are carrying drugs and that drug-trafficking organizations do not entrust large quantities of drugs to unknowing transporters.

Facts 

Delilah Guadalupe Diaz lives in Moreno Valley, California. Brief for Respondent, the United States at 5. One night in August 2020, she drove across the border from Mexico into the United States. Id. She told a border patrol officer who inspected the car that it was her boyfriend’s. Id. During the inspection, the officer tried to roll down a manual window but felt resistance and heard a crunching sound. Id. at 6. A second inspection of the car revealed fifty-six hidden packages containing 24.82 kilograms of pure methamphetamine and two cell phones. Id. Officers arrested Diaz and seized her belongings. Id.

Diaz waived her Miranda rights and claimed during her interview with a Homeland Security agent that she had not known about the methamphetamine in the car. Id. She claimed that she had been visiting her boyfriend in Mexico, that her daughter had driven her there, and that her boyfriend let her use his car to drive back. Id. She did not know her boyfriend’s address or phone number and could not name any of his friends she spent time with. Id. at 7. She refused to say who owned the second phone. Id.

A grand jury charged Diaz with knowingly and intentionally importing at least 500 grams of methamphetamine under 21 U.S.C. § 952 and § 960. Id. at 8. The government planned to call Homeland Security Investigations Special Agent Andrew Flood as an expert witness at trial. Id. Flood was not involved in the particular case and planned to testify that “drug traffickers generally do not entrust large quantities of drugs to people who are unaware they are transporting them.” Id. at 8, 11. Diaz moved to exclude that testimony under Federal Rule of Evidence 704(b), arguing that it was functionally equivalent to saying that Diaz knew she was carrying drugs. Id. at 9.

The United States District Court for the Southern District of California rejected her motion, stating that law-enforcement experts commonly testify, relying on their experience, about how drug traffickers operate. Id. The court permitted Flood’s testimony so long as he did not state in absolute terms that unknowing couriers—referred to as “blind mules”—do not exist. Id.

At trial, Flood testified that in his experience, drug traffickers do not entrust large quantities of drugs to drivers unaware of those drugs. Id. at 10. He added that “in most circumstances, the driver knows they are hired . . . to take the drugs from point A to point B.” Id. at 10–11. On cross-examination, he stated that drug traffickers can use unknowing couriers but rarely do so. Id. at 11.

The court told the jury it was free to give the expert testimony as much or little weight as it deemed fit. Id. at 12. The jury found Diaz guilty, and the judge sentenced her to eighty-four months in prison. Id. at 13. That sentence was below the guideline’s range because Diaz admitted after the verdict that she was paid to carry drugs and knew they were in her car; as a result, 18 U.S.C. § 3553(f) permitted the court to ignore the statutory minimum sentence of ten years.

The United States Court of Appeals for the Ninth Circuit affirmed. United States v. Diaz at 2. The court applied an abuse-of-discretion standard and held that under Ninth Circuit precedent, Flood’s testimony was permissible because he did not “provide an ‘explicit opinion’ on the defendant’s state of mind.” Id. at 6–7.

The United States Supreme Court granted Diaz’s petition for certiorari on November 13, 2023.

Analysis 

THE TEXT OF RULE 704(b)

Diaz argues that the Ninth Circuit erred because the text of Federal Rule of Evidence 704(b) does not apply only to an explicit opinion on a defendant’s mental state. Brief for Petitioner, Diaz at 17–18. Diaz contends that the words “about whether” give the rule a broader sweep. Id. at 18. Diaz asserts that the plain meaning of those words also includes testimony that “concerns” or “refers to” whether the defendant had a certain mental state. Id. at 18–19. Diaz adds that even the United States did not adopt the Ninth Circuit’s test in its brief in opposition to certiorari. Id. at 19.

Diaz urges the Court to adopt a fair reading of Rule 704(b). Id. at 21. By reading the rule narrowly, Diaz posits, the Ninth Circuit allows experts to give “class[ ]wide mens rea testimony” by stating that people in the defendant’s position “generally possess the requisite state of mind.” Id. at 21, 24. Diaz claims that experts therefore could—and in the Ninth Circuit often do—testify in absolute terms that drug couriers always knowingly transport drugs or that drug traffickers never use unknowing couriers. Id. at 22–23. Diaz cites the United States Court of Appeals for the Fifth Circuit, which has said that such testimony is functionally equivalent to testifying that the defendant had a certain mental state. Id. at 24. Rule 704(b) expressly dictates, Diaz points out, that the defendant’s mental state is “for the trier of fact alone.” Id. at 28.

The United States counters that Rule 704(b) permits various forms of testimony that help the jury decide whether the defendant had a certain mental state, so long as the testimony does not “state a direct opinion on” that mental state. Brief for Respondent, the United States at 19–20. The United States asserts that experts are therefore only restricted from explicitly concluding that the defendant had a certain mental state. Id. at 22–23. The United States adds that courts should assess whether experts have crossed that line by looking at their testimony as a whole. Id. at 28. The United States argues that though it is true that when an expert first testifies that a class of persons always have a certain mental state, the expert may not then identify the defendant as a member of that class, testimony about the class alone does not identify the defendant as a member. Id. at 25, 28–29.

The United States warns that the scope of Diaz’s interpretation of Rule 704(b) is broad. Id. at 24. The United States cites the United States Court of Appeals for the D.C. Circuit, which has said that Rule 704 would be “swallow[ed]” if expert testimony is impermissible anytime it helps the jury conclude whether the defendant had a certain mental state. Id. Diaz herself, the United States points out, called an expert witness who testified about how plausible it was that people in her position knew drugs were hidden in the car. Id. at 27.

THE PURPOSE OF RULE 704(b)

Diaz contends that the purpose of Rule 704(b) is to allow the trier of fact to assess the defendant’s mental state on an individual level. Brief for Petitioner at 32. Diaz argues that such an individual perspective is central to how criminal law works because defendants are deemed blameworthy based on their individual mental state and not general assumptions. Id. at 32. Diaz asserts that the best evidence of a defendant’s mental state is his or her own actions and statements. Id. at 33. Diaz maintains that class wide mens rea testimony enables the jury to infer that the defendant had a certain mental state based on general assumptions. Id. at 34. Diaz argues that this allows the jury to forgo making its own individualized assessment. Id.

Diaz next argues that Rule 704(b) should be interpreted in relation to the Constitution. Id. at 35. In particular, Diaz asserts that Rule 704(b) is supposed to protect the right to a jury trial by ensuring that the jury is the trier of fact and no one else is. Id. Diaz posits that when an expert gives class wide mens rea testimony, the expert usurps the jury’s role. Id. at 37. Diaz asserts that although a jury remains free to reject that testimony, the risk that it will not do so is “intolerable” because assessing the defendant’s mental state is the jury’s main function, and the defendant’s liberty is at stake. Id. at 38. Diaz further argues that Rule 704(b) buttresses due process rights by requiring proof beyond a reasonable doubt about the individual defendant. Id. at 39. Diaz asserts that prosecutors can avoid the burden of proof by solely relying on class wide mens rea testimony. Id. at 41.

The United States counters that the sole purpose of Rule 704(b) is to prevent experts from testifying about the “ultimate issue” of whether a defendant had a certain mental state. Id. at 31–32. The United States notes that Congress enacted Rule 704 to abolish a judge-made rule that prohibited all testimony about ultimate issues. Id. at 31. The United States claims that Congress amended that rule later after a highly publicized trial in which experts gave opposing opinions about the ultimate issue of whether the defendant qualified for an insanity defense. Id. at 31–32. The United States asserts that Rule 704(b) is therefore a targeted exception and only forbids expert testimony that “leaves no room for inference” by the jury or “compels the conclusion” that the defendant had a certain mental state. Id. at 33, 36.

The United States also rejects Diaz’s assertion that an expert usurps the jury’s role by giving class wide mens rea testimony. Id. at 39. The United States contends that when a jury assesses whether a defendant had a certain mental state, it must always rely on some evidence as it does for other elements of crimes. Id. The United States adds that criminal procedure adequately mitigates the risk that juries may rely too much on one expert’s testimony. Id. at 37. The United States points to cross examination, a defendant’s own experts, and jury instructions as examples of protections that emphasize the jury’s own role in evaluating the expert’s testimony. Id. at 37–38. Moreover, the United States disagrees with Diaz’s concerns about due process rights because the jury is not required to accept any facts based on one expert’s testimony. Id. at 40. As such, the United States argues that Diaz’s interpretation of Rule 704(b) is not necessary to preserve the role of the jury. Id. at 37.

THE EXPERT TESTIMONY IN THIS CASE

Diaz argues that Flood’s testimony violated Rule 704(b) by stating an opinion about whether Diaz had a certain mental state. Brief for Petitioner at 42. Diaz contends that because the jury knew that Diaz was a courier, it could only interpret Flood’s testimony—that all couriers know they are transporting drugs in most circumstances—to mean that Diaz knew she was transporting drugs. Id. Next, Diaz argues that Flood’s testimony about drug traffickers—that they do not entrust large quantities of drugs to drivers unaware of those drugs—went too far. Id. at 43. Diaz asserts that while testimony about drug traffickers’ modus operandi is permissible, Flood went beyond merely describing methods when he alluded to drivers’ awareness of drugs. Id. Diaz asserts that Flood impermissibly slid an opinion about drivers’ mental states into his general observation about drug traffickers. Id.

The United States counters that Flood stayed within the bounds of what Rule 704(b) permits and that the district court did not abuse its discretion. Brief for Respondent at 40. The United States notes that Flood did not testify in absolute terms, only talked about drivers’ mental states in most circumstances, and stressed that he was relying on his own experience. Id. The United States also points out that Flood acknowledged that he was not involved in this case, and that he was aware of cases in which drug traffickers use unknowing couriers. Id. at 40–41.

Discussion 

IMPACT ON CONDUCT OF CRIMINAL TRIALS

The National Association of Criminal Defense Lawyers (“NACDL”) asserts, in support of Diaz, that adopting the Ninth Circuit’s test would hinder Rule 704(b)’s purpose of protecting criminal defendants. Brief of Amicus Curiae National Association of Criminal Defense Lawyers (“NACDL”), in Support of Petitioner at 13–14. The NACDL argues that prosecutors started using police officers as expert witnesses in the 1950s and that they are now the most common experts. Id. at 6–7. The NACDL argues that because officers testify based solely on their own experience, there are no “reliable norms, standards, or otherwise objective scientific measures” that defendants can challenge when cross-examining officers. Id. at 21. The NACDL claims that despite this, courts do not scrutinize officer-experts sufficiently and rely on their training and experience, even though their experience is biased towards “finding criminality.” Id. at 12, 20, 26.

The NACDL also argues that defendants cannot present rebuttal evidence showing that officers in fact had different experiences. Id. at 24. The National Association of Federal Defenders (“NAFD”) posits, in support of Diaz, that defendants cannot present their own experts because drug traffickers are unlikely to testify, private investigators are less trustworthy, and defendants cannot subpoena witnesses “south of the border.” Brief of Amicus Curiae National Association of Federal Defenders (“NAFD”), in Support of Petitioner at 21–22, 25. The NACDL argues that even when defendants can present their own experts, they are rarely permitted to do so, and juries may consider those experts “hired guns.” Brief of NACDL at 24.

Twenty-two law professors (“the Professors”) counter, in support of the United States, that adopting Diaz’s proposed test would harm criminal defendants. Brief for Amici Curiae John Monahan et al. (“Professors”), in Support of Respondent at 23–24. The Professors contend that defendants often use experts who testify about mental states of similarly situated people. Id. The Professors maintain that under Diaz’s “functional equivalent” test, such testimony would be impermissible. Id. at 24. The Professors assert that defendants would also have more trouble proving their right to self-defense because expert testimony about mental states of similarly situated people can be crucial, such as for battered women. Id. at 25. In addition, the Professors argue that Diaz’s test would create confusion as to whether her narrow construction of Rule 704(b) applies to defendants’ own experts—potentially violating their due process rights—and that such confusion would unduly burden appellate courts. Id. at 26.

The Professors posit that parties routinely use “framework evidence”: abstract, empirical data about how members of groups tend to think or behave. Id. at 5–6. The Professors thus note that this case will affect many other cases, even though the issue in this case focuses on drug trafficking. Id. at 4. The Professors assert that the best way to resolve the challenges that come with presenting science and data as evidence through expert testimony is to provide more guidance to juries, not to exclude such evidence altogether. Id. at 27–28.

DOWNSTREAM EFFECTS ON CONSTITUTIONAL RIGHTS AND JURIES

The NACDL argues that officer-expert testimony raises Confrontation Clause concerns when it “smuggle[s] in out-of-court statements” of cooperating defendants and witnesses as well as fellow officers. Brief of NACDL at 27. The NACDL notes that defendants are unable to cross-examine those individuals to test the reliability of their statements. Id. The NAFD asserts that expert testimony about drug couriers’ mental states also raises Fifth Amendment concerns because defendants who are drug couriers effectively bear the burden of proving that they did not have the mental state at issue. Brief of NAFD at 25.The NAFD posits that this creates a risk that defendants are convicted without proof of their own mental state. Id. at 16. The NAFD contends that prosecutors are capable of meeting their burden of proof without such testimony, given their success in convicting drug traffickers in the Fifth Circuit. Id. at 26, 30.

The Professors counter that Federal Rule of Evidence 403 and jury instructions adequately protect the defendant against evidence that is “unduly prejudicial relative to its probative value.” Brief for Professors at 22–23. The Professors argue that Diaz’s arguments for not trusting the jury with the kind of testimony at issue here “prov[e] too much” because those arguments question whether juries are capable of evaluating expert testimony at all. Id. at 9–11. The Professors contend that juries understand variation within groups. Id. at 7, 9. The Professors argue that, to mitigate any residual risk that juries will over-rely on framework evidence, courts should instruct juries to assess whether they think the expert testimony about a group is valid, whether the defendant is a member of that group, and to what extent such data apply to the defendant’s individual mental state. Id. at 17.

Conclusion 

Written by:

Gijs de Bra

Tedrick Au

Edited by:

Rachel Lu

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