"If You Most Know, You Know”: Expert Testimony That “Most” Drug Couriers Know They’re Carrying Drugs Allowed to Prove Defendant Knew She Was Carrying Drugs

If you know, you know. The government’s less familiar version — if most know, the defendant knows — just got a boost from the Supreme Court of the United States, which recently held that Federal Rule of Evidence 704(b) did not prohibit the government’s expert witness from testifying that “most” drug couriers know they are transporting drugs. The majority opinion, written by Justice Thomas and joined by five other members of the Court, concluded that Rule 704(b)’s language precludes only opinion testimony “about whether [defendant] herself had a particular mental state.” In a concurring opinion, Justice Jackson observed that the majority opinion opens the door to defendants “to elicit expert testimony on the likelihood that the defendant had a particular mental state based on the defendant’s membership in a particular group.” Justice Gorsuch, joined by Justices Sotomayor and Kagan, dissented, arguing that opinion testimony about what most people in a group know amounts to testimony “about whether the defendant did or did not have” a certain “mental state” that violates Rule 704(b). Both the concurring and dissenting opinions noted that the majority decision did not address authority outside of Rule 704(b) that might prohibit such opinion testimony, and the dissent argued that such authorities should have prohibited the opinion at issue. The case is Diaz v. United States, No. 23-14, 602 U.S. ___ (June 20, 2024).

Background Facts

The defendant attempted to enter the United States from Mexico with 56 packages of methamphetamine, weighing over 54 pounds, hidden in her car doors. She claimed that she did not know the drugs were hidden in the car. She also claimed that she was driving her boyfriend’s car, but that she had only seen her boyfriend two or three times, did not know his phone number, and did not know where he lived. The car also contained two cell phones, one of which she admitted was hers, but the other she claimed belonged to a friend she would not identify.

The defendant was charged with importing methamphetamine, in violation of 21 U.S.C. §§ 952 and 960. Those charges required the government to prove that she acted “knowingly.” The defendant argued that she had not acted “knowingly” because she did not know the drugs were in the car doors.

At trial, the government proffered a Homeland Security Investigations special agent, who “testified that ‘in most circumstances, the driver knows they are hired . . . to take the drugs from point A to point B.’” In support of this opinion, the special agent explained that using an “unknowing courier . . . would expose the drug-trafficking organization to substantial risk” and that “drug-trafficking organizations are often unwilling to take those chances with hundreds of thousands of dollars on the line.” However, the special agent “acknowledged on cross-examination that drug-trafficking organizations sometimes use unknowing couriers.”

The jury found the defendant guilty. On appeal, the defendant challenged the special agent’s testimony under Rule 704(b).

Opinion Evidence That Is Not Directly About the Defendant’s Mental State Does Not Violate Rule 704(b)

Federal Rule of Evidence 704 deals with opinions on an ultimate issue. The previously prevailing “ultimate issue” rule precluded testimony on issues the jury must resolve to decide the case. By the 1940s, the “ultimate issue” rule had fallen into disfavor, with “[m]any reject[ing] the idea that ultimate-issue testimony usurps the jury’s role, since a witness’s ‘credibility’ and ‘the soundness of his judgment’ ‘always remain for the jury’s determination’” and other noting that the rule “excluded ’the most necessary testimony’ on issues where ‘the jury should have help if it is needed.’”

In 1975, Rule 704 was first adopted and made clear that the ultimate-issue rule did not apply in federal courts: “[i]t had no exceptions: All ultimate-issue opinions were permitted.” That changed in the wake of the trial of John Hinckley for the attempted assassination of President Ronald Reagan, where dueling experts offered opinions on the ultimate issue of Hinckley’s sanity. In 1984, shortly after the jury’s determination that Hinckley was not guilty by reason of insanity, Congress adopted Rule 704(b) as an exception to the rule allowing ultimate-issue opinions. Rule 704(b) provides:

Exception. In a criminal case, an expert witness must not 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 or of a defense. Those matters are for the trier of fact alone.

Fed. R. Evid. 704(b).

Parsing this language, the majority opinion determined that “Rule 704(b) thus proscribes only expert opinions in a criminal case that are about a particular person (‘the defendant’) and a particular ultimate issue (whether the defendant has ‘a mental state or condition’ that is ‘an element of the crime charged or of a defense’).”

The Court determined that the special agent’s testimony did not run afoul of Rule 704(b) because he only “testified about the knowledge of most drug couriers,” which did “not necessarily describe [the defendant’s] mental state.” It held that testimony that all couriers know they are carrying drugs would violate Rule 704(b) because that testimony necessarily means that the defendant (a courier) knew she was carrying drugs. The special agent’s testimony, by contrast, was not testimony about the particular defendant’s state of mind because she may or may not have been in the majority of couriers who know that they are carrying drugs, and the issue remained for the jury to decide: “Is [the defendant] like the majority of couriers? Or, is [she] one of the less-numerous-but-still-existent couriers who unwittingly transport drugs?”

Responding to the defendant’s (and dissent’s) argument that the special agent’s testimony about what most couriers know was testimony “about” what the defendant knew, the Court held that Rule 704(b) “does not preclude testimony ‘about’ mental-state ultimate issues in the abstract,” but only “targets conclusions ‘about whether’ a certain fact is true: ‘[T]he defendant did or not have a mental state or condition.’” The Court noted that the defendant’s broad interpretation of Rule 704(b) would not just prohibit ultimate-issue evidence about a defendant’s mental state, but “would prohibit all opinions even related to the ultimate issue of a defendant’s mental state.”

Justice Jackson joined fully in the Court’s decision but wrote separately to emphasize that “[b]oth the Government and the defense are permitted, consistent with Rule 704(b), to elicit testimony ‘on the likelihood’ that the defendant had a particular mental state, ‘based on the defendant’s membership in a particular group.’” As an example, she noted that the defendant had introduced the opinion of an automobile specialist that a driver of the defendant’s “particular car would almost certainly not know that it contained drugs,” which testimony is allowed under the Court’s opinion. She further noted that, under the Court’s opinion, the defendant would also be allowed to offer “expert testimony on the prevalence and characteristics of unknowing drug couriers.”

In addition, Justice Jackson’s concurring opinion stated that, under the Court’s opinion, Rule 704(b) does not prohibit expert opinion “contextualizing a defendant’s mental health condition, including by explaining the likelihood that those with a particular condition would have a particular mental state.” So, while an expert could not testify that a defendant’s schizophrenia was so severe that it prevented him from appreciating the wrongfulness of his conduct, the expert could “‘tell the jury that when people with schizophrenia as severe as [a] defendant’s commit acts of violence, it is generally because they do not appreciate the wrongfulness of their conduct.’”

Evidence Like That Allowed in Diaz Still Must Comply With Other Evidentiary Rules to Be Admissible

As interpreted by the Court, Rule 704(b) precludes ultimate-issue opinion testimony on a very narrow topic: whether the defendant herself had a particular mental state. It does not prohibit all opinion testimony that bears on that topic, including expert testimony that most people in the defendant’s position would have the requisite mental state for conviction. That, however, is different from a conclusion that such testimony is always allowed. Indeed, the concurring opinion acknowledged the dangers of such testimony and noted that “there are also safeguards outside of Rule 704(b) to prevent the misuse of expert testimony” that the Court’s opinion does not address or displace.

And, while the dissent took issue with the Court’s narrow view of what it means for an opinion to be “about whether the defendant did or did not have a mental state or condition,” much of the dissenting opinion addressed concerns more directly implicated by evidentiary rules addressing the relevance of the opinion offered and whether that opinion was offered by a qualified expert using a reliable methodology to offer an opinion that would be helpful to the jury:

The government comes away with a powerful new tool in its pocket. Prosecutors can now put an expert on the stand—someone who apparently has the convenient ability to read minds—and let him hold forth on what “most” people like the defendant think when they commit a legally proscribed act. Then, the government need do no more than urge the jury to find that the defendant is like “most” people and convict. What authority exists for allowing that kind of charade in federal criminal trials is anybody’s guess, but certainly it cannot be found in Rule 704.

The last sentence, in particular, suggests a belief that the special agent’s opinion was not just prohibited by Rule 704(b), but was not authorized by any authority either within or outside of Rule 704.

The dissent noted — as the majority had — that there was ample evidence, aside from the challenged opinion testimony of the special agent, from which the jury easily could have concluded the defendant’s culpable mental state. The government could have pointed to the amount and value of the drugs in the car, as well as the defendant’s flimsy story about how she came to drive a car with concealed drugs across the border. “There was no need to gild the lily by calling to the stand an ‘expert’ in mindreading.”

The dissent asserted that there were reasons, aside from Rule 704(b), to exclude the special agent’s testimony and observed, as the concurrence had, that the Court’s opinion “does not address what any other Rule of Evidence may have to say about cases like this one.” According to the dissent, testimony like the special agent’s may be irrelevant and, thus, subject to exclusion under Rule 402. “[I]f the government is right that an expert opinion about the mental state of ‘most’ people like the defendant is not ‘about’ the defendant’s mental state, it is hard to see how that opinion might be relevant.” Or, even if relevant, its probative value may be outweighed by the danger of unfair prejudice to warrant exclusion under Rule 403. “[T]estimony about what ‘most’ people think bears minimal probative value when the question at issue is what this individual thinks. Nor can the kind of testimony offered here hold much probative value when juries, composed of the defendant’s peers, are well suited to resolve questions of mens rea without ‘expert’ assistance.” And such testimony may be subject to exclusion under Rule 702 and the trial court’s role as “gatekeeper” against unreliable expert testimony. “No one, at least outside the fortuneteller’s den, can yet claim the power to conjure reliably another’s past thoughts.” Because the Court’s decision only addressed Rule 704(b) and did not reach these rules, the dissent expressed optimism that the decision “will ultimately prove immaterial in practice.”

AGG’s Observations

  • The Court’s decision is limited to exclusion of testimony about a defendant’s mental state only under Rule 704(b). The opinion does not address the admissibility or exclusion of such testimony under any other rule, including Federal Rules of Evidence 401, 402, 403, or 702. Thus, even if Rule 704(b) does not prohibit the testimony, another rule might.
  • Under the Court’s opinion, defendants, too, may present opinion testimony that most people in defendant’s class do not possess the mental state that the government must prove as an element of the crime. Or, at least, such testimony is not prohibited by Rule 704(b). Of course, such testimony, as with that offered by the government, must be able to withstand challenges relating to relevance, unfair prejudice, and reliability.
  • Time will judge the dissent’s optimism that other Rules of Evidence may render the Court’s opinion about Rule 704(b) immaterial in practice. Our optimism is tempered by the fact that the expert’s mind-reading testimony survived through the trial court and United States Court of Appeals for the Ninth Circuit unchecked by the relevance and expert testimony rules. But perhaps the unavailability of Rule 704(b) and the dissent’s criticism of this type of evidence will result in more successful challenges under these other rules.
  • Even if rules other than Rule 704(b) ultimately preclude testimony like that offered in Diaz from reaching a jury, the narrow reading of Rule 704(b)’s prohibition against expert testimony on the defendant’s mental state eliminates a bright-line rule. This will likely result in more attempted use of expert testimony like that offered in Diaz by both the government and defendants. While such testimony may ultimately be excluded from the jury’s consideration under other rules (see the points above), developing that testimony, challenging that testimony, and ruling on the admissibility of that testimony will take significant resources from the parties and the trial courts.

For more information, please contact AGG Government Investigations partners Aaron Danzig, Sara Lord, or Jerad Rissler.


AGG’s Government Investigations attorneys have successfully represented companies and individuals, including public company executives, in civil and criminal investigations before the U.S. Department of Justice and U.S. Attorney’s Offices nationwide, the Securities and Exchange Commission (“SEC”), the Food and Drug Administration (“FDA”), the Environmental Protection Agency (“EPA”), the U.S. Department of Agriculture (“USDA”), and many other federal and state regulatory and enforcement agencies. We frequently represent clients in parallel civil and criminal investigations and in regulatory proceedings. We also assist our clients in developing and providing a coordinated response to the internal and public concerns that accompany these matters, including helping them respond to media interests and address reputational concerns.