Volume 111

Introduction

Witnesses with the requisite knowledge or expertise often present, as an opinion, their answer to a case’s “ultimate issue.” They may opine, say, that a product was unreasonably dangerous in a product liability suit, or that a patent was infringed in a patent infringement suit, or that damages of a certain amount are appropriate, even if the jury is tasked with answering that same question.¹ 1.6 Michael H. Graham, Handbook of Federal Evidence § 704:1 (9th ed. 2023). There is no comprehensive list of all possible ultimate issues. See 29 Charles Alan Wright & Victor Gold, Federal Practice and Procedure § 6284, at 469 (2d ed. 2016).Show More This principle is unambiguously announced in Rule 704(a) of the Federal Rules of Evidence. It reads: “An opinion is not objectionable just because it embraces an ultimate issue.”² 2.Fed. R. Evid. 704(a) (emphasis added).Show More

The Rules feature only one caveat, articulated in Rule 704(b): “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.”³ 3.Fed. R. Evid. 704(b).Show More In other words, if the contents of a criminal defendant’s mind are an ultimate issue, no expert may share an opinion on that topic. “Those matters are for the trier of fact alone.”⁴ 4.Id.Show More Rule 704(b) recognizes the risk of an expert “intruding” on the jury’s exclusive prerogative to assess a defendant’s mental state.⁵ 5.United States v. DiDomenico, 985 F.2d 1159, 1164 (2d Cir. 1993).Show More

Last Term, in Diaz v. United States,⁶ 6.144 S. Ct. 1727, 1730 (2024).Show More the Supreme Court considered a category of opinion testimony which tiptoes up to that forbidden line. Delilah Diaz was crossing the United States-Mexico border when a border patrol officer found roughly 55 pounds of methamphetamine hidden in two concealed spaces in her car.⁷ 7.Brief for the United States at 6, Diaz, 144 S. Ct. 1727 (No. 23-14).Show More Shortly after arrest, she disclaimed knowledge of the drugs, attributing them to a supposed boyfriend.⁸ 8.Id.Show More Her story was full of implausibilities,⁹ 9.Id. at 6–7.Show More though, and she was charged with knowingly and intentionally importing methamphetamine.¹⁰ 10.Id. at 8.Show More Still, she proceeded to trial with the defense that she was a “blind mule,” or an unknowing courier of drugs.¹¹ 11.See Brief for Petitioner at 4-7, Diaz, 144 S. Ct. 1727 (No. 23-14).Show More Her mental state—that is, whether she knew about the drugs inside her vehicle—was the only live issue for the jury.

At trial, the prosecution called HSI Special Agent Andrew Flood as an expert on drug trafficking operations. Agent Flood carefully avoided testifying directly to Diaz’s mental state. Instead, he testified that “most” drug couriers are aware of the drugs in their presence.¹² 12.Petition for a Writ of Certiorari at 17, Diaz,144 S. Ct. 1727 (No. 23-14).Show More Put differently, he testified to the typical mental state—indeed, to the mens rea, or guilty mind—of a “class” of persons to which Diaz belonged. The trial court admitted his testimony, unpersuaded by Diaz’s protests that Agent Flood’s testimony was the “functional equivalent” of testimony regarding her mental state.¹³ 13.Id. app. at 31a–33a; Joint Appendix at JA10, Diaz,144 S. Ct. 1727 (No. 23-14).Show More

The U.S. Court of Appeals for the Ninth Circuit affirmed.¹⁴ 14.United States v. Diaz, No. 21-50238, 2023 WL 314309 (9th Cir. Jan. 19, 2023), aff’d, 144 S. Ct. 1727, 1730 (2024).Show More It held that testimony regarding the infrequency of unknowing drug couriers is admissible, provided that the expert does not express an “‘explicit opinion’ on the defendant’s state of mind.”¹⁵ 15.Id. at *2 (citation omitted).Show More The Ninth Circuit acknowledged, however, that the Fifth Circuit had reached the opposite conclusion in a line of similar cases.¹⁶ 16.Id.Show More The Supreme Court granted certiorari to resolve the deepening circuit split.¹⁷ 17.See Petition for a Writ of Certiorari, supra note 12, at 8–13 (cataloguing the circuit split).Show More

The Supreme Court also affirmed, albeit on slightly different grounds. The majority opinion, written by Justice Thomas,¹⁸ 18.Diaz, 144 S. Ct. at 1735. Justice Thomas wrote the majority opinion. Id. at 1730. Chief Justice Roberts joined that opinion, as did Justices Alito, Kavanaugh, Barrett, and Jackson. Id. Justice Jackson penned a concurrence. Id. at 1736 (Jackson, J., concurring). And Justice Gorsuch, joined by Justices Sotomayor and Kagan, dissented. Id. at 1738 (Gorsuch, J., dissenting).Show More focused on the meaning of the word “about.”¹⁹ 19.See Fed. R. Evid. 704(b) (“[A]n expert witness must not state an opinion about whether the defendant did or did not have a mental state . . . .” (emphasis added)).Show More It declined Diaz’s suggestion that the Court adopt the Oxford English Dictionary definition, which lists “concerning” and “in reference to” as equivalent terms.²⁰ 20.Brief for Petitioner, supra note 11, at 18.Show More Instead, it opted to interpret the word in its context. It found that “[t]he words surrounding ‘about’ make clear that Rule 704(b) . . . does not preclude testimony ‘about’ mental-state ultimate issues in the abstract.”²¹ 21.Diaz, 144 S. Ct. at 1735.Show More Rule 704(b) only “targets conclusions ‘about whether’ a certain fact is true.”²² 22.Id.Show More

Accordingly, the majority laid out a new bright-line rule. Rule 704(b) applies to the “precise topic” of the defendant’s mental state but not testimony that just “concerns or refers to that topic.”²³ 23.Id.Show More So, expert testimony about the mental state of all members of a class of defendants is impermissible, because, logically speaking, it invariably applies to the precise mental state of any member of that class.²⁴ 24.Id. at 1734.Show More Testimony regarding the mental state of most members of a class, by contrast, is permissible, because it “does not necessarily describe [the defendant’s] mental state.”²⁵ 25.Id. at 1733–34.Show More Agent Flood’s statements therefore fell outside the Rule’s ambit. Indeed, the at-issue testimony, according to the majority, amounted to nothing more than the assertion that “Diaz was part of a group of persons that may or may not have a particular mental state.”²⁶ 26.Id. at 1734.Show More This left the jury to handle the ultimate issue: was Diaz one of the few unknowing drug couriers or not?²⁷ 27.Id. at 1735.Show More

The majority’s analysis was concentrated on Rule 704, and a narrow interpretation of it at that. Rule 704, however, does not “set a standard of admissibility.”²⁸ 28.3 Christopher B. Mueller & Laird C. Kirkpatrick, Federal Evidence § 7:20, at 885 (4th ed. 2013).Show More It “merely removes a formal objection that might otherwise stand in the way.”²⁹ 29.Id.Show More As a result, the Court missed two adjacent problems with the at-issue testimony, either of which could have independently rendered it inadmissible.

Part I of this Comment examines the first of those problems: the striking resemblance between Agent Flood’s testimony and inadmissible criminal propensity evidence. In arguably his most problematic statement, Agent Flood effectively introduced crimes committed by persons similarly situated to the defendant. The only relevance of those other acts was a bare (and plainly improper) suggestion that the frequency of others’ past crimes made Diaz’s conduct more likely to be criminal, as well. Part II considers the testimony’s inherent lack of reliability. It argues that the majority intermingled two distinct categories of testimony: a) contextual scientific testimony related to a mental condition; and b) speculative testimony concerning a group of individuals’ inner thoughts. The former can be indispensable to criminal trials, while the latter is unreliable and unprovable by nature. The Court needlessly gave its stamp of approval to the parts of Agent Flood’s testimony which fell unmistakably into the latter category. Finally, Part III suggests an alternative, moderate holding. If the Court had added two caveats to its interpretation of “about”—caveats that addressed quasi-propensity evidence and “mindreading”³⁰ 30.Diaz, 144 S. Ct. at 1743 (Gorsuch, J., dissenting).Show More—it could have simultaneously preserved the admissibility of important evidence that bears on mental state and precluded prejudicial testimony like Agent Flood’s. This proposed holding, it argues, is both fairer and more faithful to the Rules.

1.  6 Michael H. Graham, Handbook of Federal Evidence § 704:1 (9th ed. 2023). There is no comprehensive list of all possible ultimate issues. See 29 Charles Alan Wright & Victor Gold, Federal Practice and Procedure § 6284, at 469 (2d ed. 2016). ↑
2.  Fed. R. Evid. 704(a) (emphasis added). ↑
3.  Fed. R. Evid. 704(b). ↑
4.  Id. ↑
5.  United States v. DiDomenico, 985 F.2d 1159, 1164 (2d Cir. 1993). ↑
6.  144 S. Ct. 1727, 1730 (2024). ↑
7.  Brief for the United States at 6, Diaz, 144 S. Ct. 1727 (No. 23-14). ↑
8.  Id. ↑
9.  Id. at 6–7. ↑
10.  Id. at 8. ↑
11.  See Brief for Petitioner at 4-7, Diaz, 144 S. Ct. 1727 (No. 23-14). ↑
12.  Petition for a Writ of Certiorari at 17, Diaz, 144 S. Ct. 1727 (No. 23-14). ↑
13.  Id. app. at 31a–33a; Joint Appendix at JA10, Diaz, 144 S. Ct. 1727 (No. 23-14). ↑
14.  United States v. Diaz, No. 21-50238, 2023 WL 314309 (9th Cir. Jan. 19, 2023), aff’d, 144 S. Ct. 1727, 1730 (2024). ↑
15.  Id. at *2 (citation omitted). ↑
16.  Id. ↑
17.  See Petition for a Writ of Certiorari, supra note 12, at 8–13 (cataloguing the circuit split). ↑
18.  Diaz, 144 S. Ct. at 1735. Justice Thomas wrote the majority opinion. Id. at 1730. Chief Justice Roberts joined that opinion, as did Justices Alito, Kavanaugh, Barrett, and Jackson. Id. Justice Jackson penned a concurrence. Id. at 1736 (Jackson, J., concurring). And Justice Gorsuch, joined by Justices Sotomayor and Kagan, dissented. Id. at 1738 (Gorsuch, J., dissenting). ↑
19.  See Fed. R. Evid. 704(b) (“[A]n expert witness must not state an opinion about whether the defendant did or did not have a mental state . . . .” (emphasis added)). ↑
20.  Brief for Petitioner, supra note 11, at 18. ↑
21.  Diaz, 144 S. Ct. at 1735. ↑
22.  Id. ↑
23.  Id. ↑
24.  Id. at 1734. ↑
25.  Id. at 1733–34. ↑
26.  Id. at 1734. ↑
27.  Id. at 1735. ↑
28.  3 Christopher B. Mueller & Laird C. Kirkpatrick, Federal Evidence § 7:20, at 885 (4th ed. 2013). ↑
29.  Id. ↑

30.  Diaz, 144 S. Ct. at 1743 (Gorsuch, J., dissenting). ↑