(Slip Opinion) OCTOBER TERM, 2023 1
Syllabus
NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
being done in connection with this case, at the time the opinion is issued.
The syllabus constitutes no part of the opinion of the Court but has been
prepared by the Reporter of Decisions for the convenience of the reader.
See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.
SUPREME COURT OF THE UNITED STATES
Syllabus
DIAZ v. UNITED STATES
CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR
THE NINTH CIRCUIT
No. 23–14. Argued March 19, 2024—Decided June 20, 2024
Petitioner Delilah Diaz was stopped at a port of entry on the United
States-Mexico border. Border patrol officers searched the car that
Diaz was driving and found more than 54 pounds of methamphetamine
hidden in the vehicle. Diaz was charged with importing methamphetamine in violation of 21 U. S. C. §§952 and 960, charges that required
the Government to prove that Diaz “knowingly” transported drugs. In
her defense, Diaz claimed not to know that the drugs were hidden in
the car. To rebut Diaz’s claim, the Government planned 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. Diaz objected in a pretrial motion under Federal Rule of Evidence 704(b), which provides that “[i]n 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.” The court ruled that Agent Flood could
not testify in absolute terms about whether all couriers knowingly
transport drugs, but could testify that most couriers know they are
transporting drugs. At trial, Agent Flood testified that most couriers
know that they are transporting drugs. The jury found Diaz guilty,
and Diaz appealed, challenging Agent Flood’s testimony under Rule
704(b). The Court of Appeals held that because Agent Flood did not
explicitly opine that Diaz knowingly transported methamphetamine,
his testimony did not violate Rule 704(b).
Held: Expert testimony that “most people” in a group have a particular
mental state is not an opinion about “the defendant” and thus does not
violate Rule 704(b). Pp. 4–11.
(a) Federal Rule of Evidence 704(a) sets out a general rule that “[a]n
2 DIAZ v. UNITED STATES
Syllabus
opinion is not objectionable just because it embraces an ultimate issue.” The provision at issue, Rule 704(b), is an exception to that general rule. Rule 704 departed from the once-prevailing common-law
practice that a witness could not state their conclusions on any ultimate issue, i.e., issues that the jury must resolve to decide the case.
See United States v. Spaulding, 293 U. S. 498, 506. When adopted in
1975, Rule 704 had no exceptions: All ultimate-issue opinions were
permitted. Years later, Rule 704(b) was adopted to proscribe only expert opinions in a criminal case that are about whether a defendant
has “a mental state or condition” that is “an element of the crime
charged or of a defense.” Pp. 4–7.
(b) In this case, Agent Flood did not express an opinion about
whether Diaz herself knowingly transported methamphetamine. Instead, he testified about the knowledge of most drug couriers. That
opinion does not necessarily describe Diaz’s mental state. Because
Agent Flood did not express an opinion about whether Diaz herself
knowingly transported methamphetamine, his testimony did not violate Rule 704(b).
Diaz’s counterarguments are unpersuasive. She first argues that
Agent Flood functionally stated an opinion about whether she knowingly transported drugs when he opined that most couriers know that
they are transporting drugs. But an opinion about most couriers is not
an opinion about all couriers. Agent Flood asserted that Diaz was part
of a group of persons that may or may not have a particular mental
state. The ultimate issue of Diaz’s mental state was thus left to the
jury’s judgment. Diaz next relies on dictionary definitions of “about”
to argue that Rule 704(b)’s phrase “state an opinion about” includes all
testimony that “concerns” whether the defendant had a particular
state of mind. That text’s surrounding context, however, makes clear
that Rule 704(b) addresses only conclusions as to the defendant’s mental state. Rule 704(a) further confirms the narrow scope of testimony
prohibited by Rule 704(b). Because Rule 704(b) is an “exception” to
Rule 704(a), Rule 704(b) can only be understood to cover a subset of
the testimony that Rule 704(a) expressly allows, which is opinion testimony that includes ultimate issues. Diaz’s reading would have the
exception swallow the rule. Pp. 7–11.
Affirmed.
THOMAS, J., delivered the opinion of the Court, in which ROBERTS, C. J.,
and ALITO, KAVANAUGH, BARRETT, and JACKSON, JJ., joined. JACKSON, J.,
filed a concurring opinion. GORSUCH, J., filed a dissenting opinion, in
which SOTOMAYOR and KAGAN, JJ., joined.
Cite as: 602 U. S. ____ (2024) 1
Opinion of the Court
NOTICE: This opinion is subject to formal revision before publication in the
United States Reports. Readers are requested to notify the Reporter of
Decisions, Supreme Court of the United States, Washington, D. C. 20543,
[email protected], of any typographical or other formal errors.
SUPREME COURT OF THE UNITED STATES
_________________
No. 23–14
_________________
DELILAH GUADALUPE DIAZ, PETITIONER v.
UNITED STATES
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE NINTH CIRCUIT
[June 20, 2024]
JUSTICE THOMAS delivered the opinion of the Court.
Federal Rule of Evidence 704(b) prohibits expert witnesses from stating opinions “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.” In
this drug-trafficking prosecution, petitioner argued that
she lacked the mental state required to convict because she
was unaware that drugs were concealed in her car when she
drove it across the United States-Mexico border. At trial,
the Government’s expert witness opined that most drug
couriers know that they are transporting drugs. Because
the expert witness did not state an opinion about whether
petitioner herself had a particular mental state, we conclude that the testimony did not violate Rule 704(b). We
therefore affirm.
I
In August 2020, Delilah Diaz, a United States citizen, attempted to enter the United States from Mexico. When
Diaz drove into the port of entry, a border patrol officer
asked her to roll down the car’s rear driver-side window.
Diaz responded that the window was manual, so the officer
2 DIAZ v. UNITED STATES
Opinion of the Court
left his inspection booth and tried to roll down the window
himself. The officer “felt some resistance” and then heard
“a crunch-like sound in the door.” App. 25. Aware from
experience that car doors are a common hiding spot for contraband, the officer investigated further with a “buster,” a
handheld tool that measures an object’s density. After the
buster detected an abnormal density in the doors, officers
brought in a narcotics detection canine and sent the car
through an X-ray machine. They discovered 56 packages of
methamphetamine tucked inside the car’s door panels and
underneath the carpet in the trunk. The methamphetamine weighed just over 54 pounds and had an estimated
retail value of $368,550.
Diaz was arrested and, after waiving her Miranda rights,
agreed to an interview. See Miranda v. Arizona, 384 U. S.
436 (1966). Diaz claimed that she had no idea drugs were
hidden in the car. The officers, however, found her story
hard to believe. Diaz explained that she was driving her
boyfriend’s car. Contradictorily, she also told officers that
she had seen her boyfriend only “two, three times tops,” did
not know his phone number, and did not know where he
lived. Response in Opposition in No. 3:20–cr–02546 (SD
Cal.), ECF Doc. 33–1, p. 13. Diaz’s story grew even more
dubious when officers questioned her about two cellphones
discovered inside the car. She acknowledged that she
owned one of the phones. But, she maintained the other
phone had been “given to [her]” by a friend—whom she
would “rather not” identify. Id., at 32, 34. And, she insisted
that the phone was “locked” and that she did not “have access to it.” Id., at 32–33.1
Diaz was charged with importing methamphetamine in
——————
1 Between her conviction and sentencing, Diaz confessed that she had
fabricated the boyfriend story. She also admitted that she had previously
smuggled drugs into the United States and had volunteered to make the
drug run that led to her arrest.
Cite as: 602 U. S. ____ (2024) 3
Opinion of the Court
violation of 21 U. S. C. §§952 and 960. The charges required the Government to prove that Diaz “knowingly”
transported drugs. In response, Diaz asserted what is
known colloquially as a “blind mule” defense: she argued
that she did not know that there were drugs in the car. Before trial, the Government gave notice that it would call
Homeland Security Investigations Special Agent Andrew
Flood as an expert witness. Agent Flood would testify about
the common practices of Mexican drug-trafficking organizations. Specifically, he planned to explain that drug traffickers “generally do not entrust large quantities of drugs to
people who are unaware they are transporting them.”
United States’ Notice, ECF Doc. 30, p. 7.
Diaz objected to Agent Flood’s proffered testimony under
Federal Rule of Evidence 704(b). That Rule provides that,
“[i]n 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.” Diaz argued that if Agent
Flood testified that drug traffickers never use unknowing
couriers, that would be functionally equivalent to an opinion about whether Diaz knowingly transported drugs. The
District Court granted Diaz’s motion in part and denied it
in part. The court agreed with Diaz that Agent Flood could
not testify in absolute terms about whether all couriers
knowingly transport drugs. But, insofar as Agent Flood
planned to testify only that most couriers know they are
transporting drugs, the court concluded that his testimony
was admissible.
At trial, Agent Flood testified that “in most circumstances, the driver knows they are hired . . . to take the
drugs from point A to point B.” App. to Pet. for Cert. 15a.
To use an unknowing courier, Agent Flood explained, would
expose the drug-trafficking organization to substantial risk.
The organization could not guarantee where, if at all, the
drugs would arrive.
Id., at 16a, 26a. Even if the drugs
4 DIAZ v. UNITED STATES
Opinion of the Court
reached the intended destination, the organization would
then have to retrieve the drugs without detection. Id., at
16a, 24a–25a. According to Agent Flood, drug-trafficking
organizations are often unwilling to take those chances
with hundreds of thousands of dollars on the line. Agent
Flood acknowledged on cross-examination that drug-trafficking organizations sometimes use unknowing couriers.
The jury found Diaz guilty, and the District Court sentenced her to 84 months’ imprisonment. On appeal, Diaz
again challenged Agent Flood’s testimony under Rule
704(b). The Court of Appeals held that Rule 704(b) prohibits only “an ‘explicit opinion’ on the defendant’s state of
mind.”
2023 WL 314309, *2 (CA9, Jan. 19, 2023). Because
Agent Flood did not opine about whether Diaz knowingly
transported methamphetamine, the court concluded that
the testimony did not violate Rule 704(b).
Ibid.
We granted certiorari,
601 U. S. ___ (2023), and now affirm.
II
Federal Rule of Evidence 704 addresses “Opinion[s] on an
Ultimate Issue.” Rule 704(a) sets out a general rule that
“[a]n opinion is not objectionable just because it embraces
an ultimate issue.” Rule 704(b) adds one caveat:
“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.”
Rule 704 departed from the once-prevailing common-law
practice. Prior to Rule 704, many States applied what was
known as the “ultimate issue” rule. That rule categorically
barred witnesses from “stat[ing] their conclusions on” any
“ultimate issue”—i.e., issues that the jury must resolve to
decide the case. United States v. Spaulding, 293 U. S. 498,
Cite as: 602 U. S. ____ (2024) 5
Opinion of the Court
506 (1935); see 7 J. Wigmore, Evidence §1921, p. 18 (3d ed.
1940) (Wigmore) (explaining that an ultimate issue is “the
exact question which the jury are required to decide” (internal quotation marks omitted)). For example, in a medical
malpractice suit, an ultimate issue may be “whether [the]
plaintiff ’s condition resulted solely from malpractice.” De
Groot v. Winter, 261 Mich. 660, 671,
247 N. W. 69 (1933).
In a murder case, by way of comparison, an ultimate issue
may be who fired the gun that killed the victim. See State
v. Carr,
196 N. C. 129, 131–132,
144 S. E. 698, 700 (1928).
Under the common-law rule, a witness could not provide his
answer to those ultimate issues. Witnesses remained free,
however, to offer related testimony, even testimony that directly helped the jury resolve an ultimate issue. See, e.g.,
Furlong v. Carraher,
108 Iowa 492, 495,
79 N. W. 277, 278
(1899) (holding that witness could not testify about deceased’s mental capacity to enter will, but could testify to
her “condition of . . . mind at the time the will was executed”); De Groot,
261 Mich., at 671,
247 N. W., at 69 (holding that witness could testify that plaintiff ’s condition could
have resulted from malpractice); Hill v. State,
134 Tex.
Crim. 163, 169,
114 S. W. 2d 1180, 1183 (1938) (holding that
witness could testify in a murder case on how “the bruises
and wounds on the deceased’s body could have been
caused”).
The logic underpinning the ultimate-issue rule was that
it prevented witnesses from taking over the jury’s role. See
1 K. Broun, McCormick on Evidence 80 (7th ed. 2013)
(McCormick) (explaining that the rule’s “stated justification” was to exclude testimony that “usurps the function” or
“invades the province of the jury” (internal quotation marks
and footnote omitted)). If a witness gave an opinion “covering the very question which was to be settled by the jury,”
some feared that the jury would be left with “no other duty
but that of recording the finding of [the] witnes[s].” Chicago
& Alton R. Co. v. Springfield & N. W. R. Co.,
67 Ill. 142, 145
6 DIAZ v. UNITED STATES
Opinion of the Court
(1873).
Although the ultimate-issue rule’s exact origins are unclear, legal scholars agree that several States had adopted
it by the late 1800s. See W. Stoebuck, Opinions on Ultimate
Facts: Status, Trends, and a Note of Caution, 41 Denver L.
Ctr. J. 226, 226–227 (1964) (Stoebuck) (“The mist the gods
drew about them on the battlefield before Troy was no more
dense than the one enshrouding the origins of the [ultimate-issue] rule”). The rule was short lived though, and courts
and commentators came to doubt its propriety within a
matter of decades. See ibid. Many rejected 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.” Goldfoot v.
Lofgren,
135 Ore. 533, 541,
296 P. 843, 847 (1931). Others
labeled the rule “impracticable and misconceived” because
it excluded “the most necessary testimony” on issues where
“the jury should have help if it is needed.” 7 Wigmore 18–
19. By the 1940s, “a trend [had] emerged to abandon” the
rule altogether. 1 McCormick 80. It soon became unclear
whether, and to what extent, the ultimate-issue rule carried any force. See Stoebuck 236.
Rule 704 made clear that the ultimate-issue rule did not
apply in federal courts. When Rule 704 was originally
adopted in 1975, it had no exceptions: All ultimate-issue
opinions were permitted.
88 Stat. 1937.
About nine years later, in the wake of the John Hinckley,
Jr., trial, Congress created the exception now found in Rule
704(b). On March 30, 1981, Hinckley attempted to assassinate President Ronald Reagan, shooting and wounding the
President and three other men. See L. Caplan, The Insanity Defense and the Trial of John W. Hinckley, Jr. 7–9
(1984). At his criminal trial, Hinckley claimed that he was
insane. Both the prosecution and defense offered competing expert opinions on the ultimate issue of Hinckley’s sanity. See R. Bonnie, J. Jeffries, & P. Low, A Case Study in
Cite as: 602 U. S. ____ (2024) 7
Opinion of the Court
the Insanity Defense 54 (4th ed. 2021). To the surprise of
many, Hinckley was found not guilty by reason of insanity.
See id., at 133; R. Slovenko, The Insanity Defense in the
Wake of the Hinckley Trial, 14 Rutgers L. J. 373 (1982).
Congress adopted Rule 704(b) shortly thereafter to carve
out an “exception” to Rule 704’s blanket rule admitting
ultimate-issue opinions. As Rule 704(b) now reads, “[i]n 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.”
By its terms, Rule 704(b)’s exception covers a narrow set
of opinions. The exception does not apply in civil cases or
affect lay witness testimony. And, it exclusively addresses
mental states and conditions that are “element[s] of the
crime charged or of a defense.” 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”).
III
Rule 704(b) applies only to opinions about the defendant.
Because Agent Flood did not express an opinion about
whether Diaz herself knowingly transported methamphetamine, his testimony did not violate Rule 704(b).
Agent Flood instead testified about the knowledge of most
drug couriers. Specifically, he explained that “in most circumstances, the driver knows they are hired . . . to take the
drugs from point A to point B.” App. to Pet. for Cert. 15a.
That opinion does not necessarily describe Diaz’s mental
state. After all, Diaz may or may not be like most drug couriers. Diaz herself made this point at trial. She argued that
another person, an alleged boyfriend, had deceived her into
8 DIAZ v. UNITED STATES
Opinion of the Court
carrying the drugs.2 During opening statements, Diaz’s
counsel explained that Diaz met her boyfriend while she
was “broken-hearted over the death of her mother” and recovering from “a debilitating back injury.” Trial Tr., ECF
Doc. 112, pp. 140–141. Diaz’s boyfriend “took advantage” of
those circumstances to lure Diaz to Mexico. Id., at 140. As
her counsel described it, the boyfriend then loaned Diaz a
car that was secretly loaded with drugs for her drive back
to the United States. Diaz supported that story during her
case in chief. She presented an automobile mechanics expert who testified that there was “no way for someone to
suspect or know that there was drugs hidden within th[e]
car.” Trial Tr., ECF Doc. 113, p. 62. Diaz also challenged
the Government’s contrary theory. On cross-examination,
Diaz’s counsel highlighted that Agent Flood was not involved in Diaz’s case and that the Government itself was
aware of cases involving unknowing couriers.
The jury was thus well aware that unknowing couriers
exist and that there was evidence to suggest Diaz could be
one of them. It simply concluded that the evidence as a
whole pointed to a different conclusion: that Diaz knowingly transported the drugs. The jury alone drew that conclusion. While Agent Flood provided evidence to support
one theory, his testimony was just that—evidence for the
jury to consider or reject when deciding whether Diaz in fact
knew about the drugs in her car. Because Agent Flood did
not give an opinion “about whether” Diaz herself “did or did
not have a mental state or condition that constitutes an element of the crime charged or of a defense,” his testimony
did not violate Rule 704(b).
Diaz’s counterarguments, echoed by the dissent, are not
persuasive. Diaz and the dissent argue that Agent Flood
“functional[ly]” stated an opinion about whether Diaz
——————
2 Though Diaz later admitted the boyfriend never existed, she main-
tained her story throughout the trial. N. 1, supra.
Cite as: 602 U. S. ____ (2024) 9
Opinion of the Court
knowingly transported drugs when he opined that couriers
generally transport drugs knowingly. Brief for Petitioner
24 (internal quotation marks omitted); see also post, at 7–8
(opinion of GORSUCH, J.). That argument mistakenly conflates an opinion about most couriers with one about all couriers. A hypothetical helps explain why this distinction
matters under Rule 704(b). Take for example an expert
who testifies at an arson trial that all people in the defendant’s shoes set fires maliciously (the mental state required
for common-law arson). Although the expert never spoke
the defendant’s name, the expert nonetheless violated Rule
704(b). That is because the expert concluded that the defendant was part of a group of people that all have a particular mental state. The phrase “all people in the defendant’s
shoes” includes, of course, the defendant himself. So, when
the expert testified that all people in the defendant’s shoes
always set fires with malicious intent, the expert also
opined that the defendant had that mental state. The expert thus stated an opinion on the defendant’s mental state,
an ultimate issue reserved for the jury, in violation of Rule
704(b).
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. Of all drug couriers—a group that includes Diaz—he opined that the majority knowingly
transport drugs. The jury was then left to decide: Is Diaz
like the majority of couriers? Or, is Diaz one of the lessnumerous-but-still-existent couriers who unwittingly
transport drugs? The ultimate issue of Diaz’s mental state
was left to the jury’s judgment. As a result, Agent Flood’s
testimony did not violate Rule 704(b).
Diaz and the dissent next zero in on the word “about” in
Rule 704(b). They rely on dictionary definitions of “about”
to argue that Rule 704(b)’s prohibition includes all testimony that “ ‘concerns’ or is ‘in reference to’ whether the de-
10 DIAZ v. UNITED STATES
Opinion of the Court
fendant possessed a particular state of mind.” Brief for Petitioner 18–19; post, at 6–7. But, a word’s meaning is informed by its surrounding context. See Smith v. United
States, 508 U. S. 223, 233 (1993). A crucial part of that context is the other words in the sentence. See FCC v. AT&T
Inc.,
562 U. S. 397, 405 (2011). The words surrounding
“about” make clear that Rule 704(b) addresses a far narrower category of testimony than Diaz and the dissent
posit. To begin, the Rule targets “opinion[s].” In other
words, the testimony must be more than a general reference, and it must reach a particular conclusion. See Black’s
Law Dictionary 1244 (rev. 4th ed. 1968) (defining opinion
evidence as “what the witness thinks, believes, or infers in
regard to facts in dispute”). Moreover, the Rule does not
preclude testimony “about” mental-state ultimate issues in
the abstract. Instead, it targets conclusions “about
whether” a certain fact is true: “[T]he defendant did or did
not have a mental state or condition.” The language as a
whole thus conveys that Rule 704(b) is limited to conclusions as to the defendant’s mental state.
Rule 704(a) further confirms the narrow scope of testimony prohibited by Rule 704(b). Recall that the original
ultimate-issue rule excluded opinions on the ultimate issue
itself. See supra, at 4–5. Rule 704(a) abolished that practice by permitting testimony that “embraces an ultimate issue.” See 5 Oxford English Dictionary 169 (2d ed. 1989)
(defining “embrace” as “[t]o include, contain, comprise”).
Because Rule 704(b) is an “exception” to Rule 704(a), it can
only be understood to cover a subset of the testimony that
Rule 704(a) expressly allows. In short, since Rule 704(a)
permits opinion testimony that includes ultimate issues,
Rule 704(b) must exclude only a subset of those same opinions.
The reading offered by Diaz and the dissent would have
the exception swallow the rule. If Rule 704(b) were as broad
as they suggest, it would be a standalone prohibition
Cite as: 602 U. S. ____ (2024) 11
Opinion of the Court
broader than Rule 704(a)—or even the original ultimate-issue rule. Even though the ultimate-issue rule and Rule
704(a) address opinions that include the ultimate issue itself, Rule 704(b) would prohibit all opinions even related to
the ultimate issue of a defendant’s mental state. Rule 704’s
text does not support such an expansion. The Rule as a
whole makes clear that an opinion is “about” the ultimate
issue of the defendant’s mental state only if it includes a
conclusion on that precise topic, not merely if it concerns or
refers to that topic.
IV
An expert’s conclusion that “most people” in a group have
a particular mental state is not an opinion about “the defendant” and thus does not violate Rule 704(b). Accordingly, the judgment of the Court of Appeals is affirmed.
It is so ordered.
Cite as: 602 U. S. ____ (2024) 1
JACKSON, J., concurring
SUPREME COURT OF THE UNITED STATES
_________________
No. 23–14
_________________
DELILAH GUADALUPE DIAZ, PETITIONER v.
UNITED STATES
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE NINTH CIRCUIT
[June 20, 2024]
JUSTICE JACKSON, concurring.
I join the Court’s opinion in full. Federal Rule of Evidence
704(b) forbids expert witnesses in criminal trials from offering their “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.” As the Court
explains, Rule 704(b) is narrow. Against the backdrop of
Federal Rules that authorize admission of all relevant evidence, it prohibits “only expert opinions . . . 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’).”
Ante, at 7. But, as narrow as it is, Rule 704(b) strikes a very
important balance: It allows for potentially highly probative expert testimony to be submitted to the jury, while
leaving “[t]he ultimate issue of [the defendant’s] mental
state . . . to the jury’s judgment.” Ante, at 9.
I write separately to emphasize that, as Congress designed it, Rule 704(b) is party agnostic. Neither the Government nor the defense can call an expert to offer her opinion about whether the defendant had or did not have a
particular mental state at the time of the offense. See ante,
at 7. But a corollary is also true. Both the Government and
the defense are permitted, consistent with Rule 704(b), to
2 DIAZ v. UNITED STATES
JACKSON, J., concurring
elicit expert testimony “on the likelihood” that the defendant had a particular mental state, “based on the defendant’s
membership in a particular group.” Brief for John Monahan et al. as Amici Curiae 1 (Evidence Professors Brief ).
Indeed, the type of mental-state evidence that Rule 704(b)
permits can prove essential not only for prosecutors, but for
defendants as well.
I
This very case illustrates the significance of mental-state
evidence to both parties in a criminal trial. The Government expert opined (based on his almost 30 years of experience as a special agent) that, “in most circumstances,” drug
couriers know that they are transporting drugs. App. to
Pet. for Cert. 10a, 15a. Diaz challenged this testimony, and,
today, the Court holds that the Government did not violate
Rule 704(b). See ante, at 7. Notably, however, the Government was not the only party that relied on this type of mental-state evidence during the trial. Diaz called an automobile specialist who testified that a driver of her particular
car would almost certainly not know that it contained
drugs. See Supp. Excerpts of Record in
No. 21–50238 (CA9), pp. 139–159. That type of evidence is
permissible under the interpretation of Rule 704(b) the
Court adopts today.* Moreover, as the dissent observes,
Diaz might have opted to introduce other types of expert
evidence related to the mental-state element. See post, at
10 (opinion of GORSUCH, J.). For example, Diaz could have
offered expert testimony on the prevalence and characteristics of unknowing drug couriers. See Tr. of Oral Arg. 24;
see also Brief for National Association of Federal Defenders
as Amicus Curiae 5–16 (NAFD Brief ) (describing numerous
cases involving so-called “blind mules”).
——————
*Before this Court, Diaz forthrightly admits that such evidence would
be impermissible under the dissent’s interpretation of Rule 704(b). See
Reply Brief 8; Tr. of Oral Arg. 27.
Cite as: 602 U. S. ____ (2024) 3
JACKSON, J., concurring
For the reasons described in today’s opinion, none of that
evidence would deprive the jury of its ability to decide the
last link in the inferential chain: whether Diaz herself had
the requisite mens rea. But, at the same time, having all of
this testimony might have helped the jury determine
whether the Government had met—or failed to meet—its
burden of proving that Diaz knew of the drugs found in her
car. Thus, far from disserving our criminal justice system,
see post, at 10, the type of mental-state evidence that Rule
704(b) permits can be of critical assistance to lay factfinders
tasked with determining a defendant’s mental state as an
element of the alleged crime (or defense).
Other examples provide further proof. Consider expert
evidence on mental health conditions. Congress crafted
Rule 704(b) to prohibit experts from opining on a particular
defendant’s mental state at the time of an offense, but it did
not preclude experts from contextualizing a defendant’s
mental health condition, including by explaining the likelihood that those with a particular condition would have a
particular mental state. For example, as Diaz acknowledges, the interpretation of Rule 704(b) the Court adopts
today “allow[s] psychiatrists who testify as experts to . . .
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.” Brief for Petitioner 21–22; see also Brief for United
States 35–36. That type of expert evidence would not result
in the spectacle of dueling experts on the defendant’s actual
mental state, which Congress sought to eliminate when it
codified Rule 704(b). See ante, at 6–7; see also S. Rep. No.
98–225, p. 230 (1983). Instead, given the biases, stereotypes, and uneven knowledge that many people have about
mental health conditions, such expert evidence could help
jurors better understand a defendant’s condition and
thereby call into question a mens rea that might otherwise
be too easily assumed. See, e.g., United States v. Brown, 32
4 DIAZ v. UNITED STATES
JACKSON, J., concurring
F. 3d 236, 239 (CA7 1994); United States v. Thigpen, 4 F. 3d
1573, 1579–1580 (CA11 1993) (en banc).
Or consider defendants who have been subject to domestic abuse. “A number of myths and misconceptions about
[battered woman syndrome] victims affect our criminal justice system,” and it is clear that those mistaken views “affect jurors.” Linn v. State,
929 N. W. 2d 717, 742 (Iowa
2019); see also
id., at 742–746 (summarizing relevant evidence). Rule 704(b) allows experts to testify about the typical mental states of those with battered woman syndrome,
helping jurors to better understand how those experiencing
it respond to aggression or react to violence. See Evidence
Professors Brief 25; see also 29 C. Wright & V. Gold, Federal Practice and Procedure §6285 (2d ed. Supp. 2023).
Such evidence can play a pivotal role in a defendant’s attempts both to disprove the mens rea in a number of serious
crimes and to support a range of defenses, including duress
and self-defense. See, e.g., United States v. Lopez,
913 F. 3d
807, 819–824 (CA9 2019); United States v. Nwoye,
824 F. 3d
1129, 1136–1138 (CADC 2016).
II
All that said, I fully acknowledge that there are serious
and well-known risks of overreliance on expert testimony—
risks that are especially acute in criminal trials. See NAFD
Brief 21–22, 24–25; see also United States v. Alvarez, 837
F. 2d 1024, 1030 (CA11 1988) (“When the expert is a government law enforcement agent testifying on behalf of the
prosecution about participation in prior and similar cases,
the possibility that the jury will give undue weight to the
expert’s testimony is greatly increased”). But there are also
safeguards outside of Rule 704(b) to prevent the misuse of
expert testimony. Nothing in the Court’s opinion today
should be read to displace those important checks and limitations.
This means, of course, that when faced with flawed or
Cite as: 602 U. S. ____ (2024) 5
JACKSON, J., concurring
faulty testimony concerning the mental states of groups or
categories of individuals, parties can utilize the traditional
tools in a lawyer’s toolkit, like vigorous cross-examination
and careful refutation in closing argument. Parties can also
seek to employ other Rules of Evidence that might require
exclusion—those that guard against irrelevant or unduly
prejudicial testimony, for example, and those that require
courts to bar unqualified or overreaching experts. See Fed.
Rules Evid. 401, 402, 403, 702; see also, e.g., United States
v. Finley, 301 F. 3d 1000, 1014–1015 (CA9 2002) (“Expert
testimony that compels the jury to conclude that the defendant did or did not possess the requisite mens rea does
not ‘assist the trier of fact’ under Rule 702 because such testimony encroaches on the jury’s vital and exclusive function
to make credibility determinations”); United States v. Lipscomb,
14 F. 3d 1236, 1242 (CA7 1994) (describing safeguards that can be used to prevent testimony from law enforcement experts from unduly prejudicing a defendant).
District court judges also have a role to play. They should
be protective of Congress’s intent to preserve the jury’s core
duty, by providing specific admonitions and instructions
when expert testimony about a relevant mental state is introduced. See Evidence Professors Brief 27–29; see also
United States v. Smart,
98 F. 3d 1379, 1388–1389 (CADC
1996) (requiring that district courts sometimes use jury instructions to prevent expert testimony from violating Rule
704(b)).
With this understanding of both the important uses and
the potential misuses of Rule 704(b), I join the Court’s opinion.
Cite as: 602 U. S. ____ (2024) 1
GORSUCH, J., dissenting
SUPREME COURT OF THE UNITED STATES
_________________
No. 23–14
_________________
DELILAH GUADALUPE DIAZ, PETITIONER v.
UNITED STATES
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE NINTH CIRCUIT
[June 20, 2024]
JUSTICE GORSUCH, with whom JUSTICE SOTOMAYOR and
JUSTICE KAGAN join, dissenting.
Federal Rule of Evidence 704(b) prohibits an expert witness from offering an opinion “about whether the defendant
did or did not have [the] mental state” needed to convict her
of a crime. “Those matters,” the Rule instructs, “are for the
trier of fact alone.” Following the government’s lead, the
Court today carves a new path around that command.
There’s no Rule 704(b) problem, the Court holds, as long as
the government’s expert limits himself to testifying that
most people like the defendant have the mental state required to secure a conviction.
The upshot? 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.
2 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
I
Delilah Diaz’s conviction for drug trafficking turned on
her state of mind. In that, hers was an everyday case. Often in our criminal justice system, the difference between
freedom and years in prison turns on just that question.
Perhaps it has always been so. The government’s duty to
prove that the defendant it seeks to convict had a culpable
state of mind when committing a proscribed act is as ancient as it is fundamental to our system of justice. At common law, “a complete crime” generally required “both a will”
(or mens rea) “and an act” (or actus reus). 4 W. Blackstone,
Commentaries on the Laws of England 21 (1769) (Blackstone). That same view “took deep and early root in American soil” where, to this day, a crime ordinarily arises “only
from concurrence of an evil-meaning mind with an evil-doing hand.” Morissette v. United States, 342 U. S. 246, 251–
252 (1952); see 1 J. Bishop, Commentaries on the Criminal
Law §291, p. 163 (6th ed. 1877) (Bishop). So ingrained is
this view that courts have long presumed criminal statutes
demand proof of mens rea even when they are “silent” on
the subject. Morissette,
342 U. S., at 252; see Staples v.
United States,
511 U. S. 600, 605 (1994).
Why does our law generally insist not just on a bad act
but also a culpable state of mind? A significant part of it
has to do with respect for the individual and his liberty in a
free society. “Criminal liability imports a condemnation,
the gravest we,” as a Nation, “permit ourselves to make.”
H. Wechsler, American Law Institute II–A Thoughtful
Code of Substantive Law, 45 J. Crim. L. & C. 524, 528
(1955) (Wechsler); see also 4 Blackstone 20–21; 1 Bishop
§287, at 161. Of course, our law recognizes gradations of
mens rea, ranging from purpose and knowledge to recklessness and negligence. See, e.g., ALI, Model Penal Code §2.02
(1985); United States v. Bailey,
444 U. S. 394, 404 (1980).
But to subject a presumptively free individual to serious
punishments for acts undertaken without proof of any of
Cite as: 602 U. S. ____ (2024) 3
GORSUCH, J., dissenting
that would be “the badge of tyranny, the plainest illustration of injustice.” Wechsler 528. The principle “that an injury can amount to a crime only when inflicted” with some
accompanying mens rea is, we have said, “as universal and
persistent in mature systems of law as belief in freedom of
the human will and a consequent ability and duty of the
normal individual to choose between good and evil.” Morissette, 342 U. S., at 250.
At trial, deciding whether a criminal defendant acted
with a culpable mental state is a job for the jury. No matter
how “clear the proof ” or “incontrovertible” the inference,
the question whether a defendant possessed a culpable
mens rea “must always be submitted to the jury.”
Id., at
274 (internal quotation marks omitted). Always, too, the
government bears the burden of proving the requisite mens
rea. Never, we have held, may the government seek to
“shift the burden of proof to the defendant.” Patterson v.
New York,
432 U. S. 197, 215 (1977); see Mullaney v. Wilbur,
421 U. S. 684 (1975). Nor may a court instruct a jury
that it must presume a defendant’s state of mind from any
particular set of facts, no matter how compelling they may
be. Francis v. Franklin,
471 U. S. 307, 316 (1985).
Reflecting the centrality of mens rea to criminal punishment and the jury’s role in finding it, Rule 704(b) of the Federal Rules of Evidence provides that, “[i]n 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.” As the Rule continues: “Those matters are for
the trier of fact alone.”
By all accounts, the immediate impetus for the Rule was
the trial of John Hinckley for the attempted assassination
of President Ronald Reagan. Ante, at 6. In that case, experts didn’t just offer competing views on whether Hinckley
suffered from a medically diagnosable mental illness. They
4 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
went much further. The trial descended into a battle between experts who claimed to know exactly what Hinckley
was (or was not) thinking at the moment he pulled the trigger. Ibid.
In the trial’s aftermath, Congress continued to recognize
the value of expert mental health evidence. So, for example, an expert may still testify that the defendant suffered
from some diagnosable illness or syndrome at the time of
the charged act and discuss its symptoms. Cf. ante, at 3–4
(JACKSON, J., concurring) (discussing schizophrenia and
battered woman syndrome). From testimony like that, a
jury might infer that the defendant did not have the requisite mental state to convict. But in Rule 704(b) Congress
declared that task belonged to the jury alone, and allowing
a parade of witnesses to speculate about what did or did not
transpire in the head of a particular defendant at a particular moment in the past did not reflect well on federal judicial proceedings and did not aid the jury.
Rule 704(b) may have been a new addition to the Federal
Rules of Evidence, but it reflects a much older tradition.
For centuries, courts have grappled with the role expert
witnesses should play at trial. See, e.g., 1 S. Greenleaf, Evidence §440, p. 489 (1842); Folkes v. Chadd, 3 Dougl. 157, 158–
159, 99 Eng. Rep. 589, 590 (K. B. 1782). For a long stretch,
many courts barred experts from offering opinions on so-called ultimate issues like mens rea. See 3 J. Wigmore, Evidence §§1920, 1921 (1904); United States v. Spaulding,
293
U. S. 498, 506 (1935). The Federal Rules of Evidence are no
longer so strict, see Fed. Rule Evid. 704(a), except in one
respect: mens rea. On that particular issue, Congress has
concluded that jurors need no help from experts. They are
fully capable of drawing reasonable inferences from the
facts and deciding whether the defendant acted with the
requisite mens rea. And in criminal trials that is their job
alone.
Cite as: 602 U. S. ____ (2024) 5
GORSUCH, J., dissenting
II
The government violated that Rule in this case. Proceedings began when prosecutors charged Ms. Diaz with importing a controlled substance into this country. See 21 U. S. C.
§§952, 960(a)(1). At the trial that followed, Ms. Diaz did not
dispute that she had transported drugs across the border.
The only question concerned her mens rea. If, as the government charged, she transported the drugs “knowingly,”
she faced a potential sentence of up to life in prison. See
§§960(a)(1), (b)(1)(H). If, however, Ms. Diaz acted with
some lesser mens rea (say, negligence), or perhaps innocently (as what some call a “blind mule”), she was entitled
to an acquittal.
To help prove that Ms. Diaz “knowingly” imported drugs,
the government called to the stand Andrew Flood, one of its
own employees, an agent with the Department of Homeland
Security. Ms. Diaz had made no admissions to him about
her mental state, nor had Agent Flood even interviewed
her. Instead, prosecutors called Agent Flood as an expert
on the minds of drug couriers (yes, really). App. 17; Notice
in No. 3:20–cr–02546 (SD Cal.), ECF Doc. 30, p. 7. And in
response to the government’s questions, Agent Flood testified that, “in most circumstances, the driver knows they are
hired . . . to take the drugs from point A to point B.” App.
to Pet. for Cert. 15a.
That was a violation of Rule 704(b), plain as day. Just
walk through its terms. The government called Agent
Flood as an “expert witness” to address the question
“whether the defendant did or did not have . . . a mental
state . . . that constitutes an element of the crime charged.”
After all, whether Ms. Diaz acted “knowingly” was the only
question at trial, all that separated her from a conviction.
And Agent Flood proceeded to do just as he was asked, offering an “opinion about” that very question.
To be sure, prosecutors thought they had a clever way
around the problem. They did not ask Agent Flood to testify
6 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
explicitly about Ms. Diaz’s mental state. Instead, they
asked the agent to testify about the mental state of people
exactly like Ms. Diaz, drivers bringing drugs into the country. And that, the prosecutors argued, made all the difference. See App. 32a; Brief for United States in No. 21–50238
(CA9), pp. 46, 58. The Ninth Circuit endorsed the government’s maneuver, holding that Rule 704(b) prohibits only
testimony “ ‘explicit[ly]’ ” about the defendant’s mental
state, not testimony about the mental state of a class of persons that includes her. App. to Pet. for Cert. 6a (quoting
United States v. Gomez, 725 F. 3d 1121, 1128 (2013)).
Before us, however, even the government disavows the
full implications of that reasoning. Now, it concedes, the
Rule does more than bar an expert from testifying “explicitly” that the defendant had the mental state required for
conviction. Tr. of Oral Arg. 72–73, 76. The Rule also bars
an expert from testifying that a class of persons (say, all
people carrying drugs over the border) has the legally proscribed mental state when that class includes the defendant. Brief for United States 36; ante, at 9. Likewise, the
Rule bars an expert from opining that a hypothetical person
who matches the defendant’s description (say, a hypothetical woman who drives a car full of drugs across the border)
will have the mental state required for conviction. Tr. of
Oral Arg. 67. All those opinions, the government now
acknowledges, are “about” the defendant’s mental state and
cannot be offered consistent with Rule 704(b). On this, the
Court, too, agrees. Ante, at 9.
III
So what is left? Instead of vacating and remanding the
case to the Ninth Circuit to correct its error, the government asks us to affirm its judgment on other grounds. As
the government sees it, Agent Flood’s opinion was permissible for a different reason than the Ninth Circuit offered.
It was permissible, the government says, because it wasn’t
Cite as: 602 U. S. ____ (2024) 7
GORSUCH, J., dissenting
definitive. So, yes, an expert cannot testify that all persons
in a class that includes the defendant have a culpable mental state. Brief for United States 36. But, the government
insists, everything changes when an expert offers (as Agent
Flood offered) only a probabilistic assessment that most
such persons do.
I cannot see how that gambit begins to solve the government’s problem. The Rule does not only prohibit an expert
from stating a definitive opinion about the defendant’s mental state (or, as the government concedes, the mental state
of a class that includes her). It prohibits an expert from
offering any opinion on the subject. Return, once more, to
the Rule’s terms. It bars an expert from stating an opinion
“about whether the defendant” had “a mental state . . . that
constitutes an element of the crime charged.” (Emphasis
added.) The word “about” means “[c]oncerning, regarding,
with regard to, in reference to; in the matter of.” Oxford
English Dictionary (3d ed., June 2024); see Brief for Petitioner 18; see also American Heritage Dictionary 5 (def. 4a)
(5th ed. 2011). So whether an expert’s opinion happens to
be definitive or probabilistic makes no difference. An expert may not state any opinion concerning, regarding, or in
reference to whether the defendant, while committing a
charged criminal act, had the requisite mental state to convict. Period. Lest any doubt remain, the Rule takes pains
to emphasize, “[t]hose matters are for the trier of fact
alone.”
Consider, too, how the government’s present theory collapses into the one it has disavowed. Just imagine if Agent
Flood had explicitly addressed Ms. Diaz and said she “most
likely knew” she was carrying drugs. Would that testimony
be permissible under Rule 704(b)? Of course not. Probabilistic though the testimony may be, an expert who says that
an individual defendant “most likely” had the requisite
mental state for conviction offers an opinion about, concerning, regarding, or in reference to her mental state. On that,
8 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
no dispute exists. So how can it be, as the government insists, that an expert may offer the probabilistic assessment
that “most” people like the defendant know they are carrying drugs? The only difference between the two opinions is
that the first addresses the defendant “explicitly,” the second a class that includes her. All of which returns us to a
distinction that the government itself seems to
acknowledge the Rule does not tolerate.
Observe, as well, where today’s tiptoeing around the Rule
promises to lead. The Court adopts the government’s muddled view that an expert cannot offer a probabilistic opinion
about the mental state of the defendant explicitly but can
offer a probabilistic opinion about the mental state of a
group that includes the defendant. So what happens next?
In this case, Agent Flood said “most” people in the defendant’s shoes have the requisite mens rea. But what if he said,
as the government initially proffered, that drivers “generally” know? ECF Doc. 30, at 7. Or that they “almost always” know? Or perhaps an expert puts a finer point on it:
“In my experience, 99% of drug couriers know.” When cases
like those come to us, likely one of two things will happen.
We will draw some as-yet unknown line and say an expert’s
probabilistic testimony went too far. Or we will hold anything goes and eviscerate Rule 704(b) in the process. Rather than face either of those prospects, how much easier it
would be to follow where the Rule’s text leads.
IV
The government’s approach, adopted by the Court today,
is no more necessary than it is appropriate. 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. See 4 Blackstone 21 (“no temporal
Cite as: 602 U. S. ____ (2024) 9
GORSUCH, J., dissenting
tribunal can search the heart, or fathom the intentions of
the mind, otherwise than as they are demonstrated by outward actions”). Nor is it any secret that the government
has a long track record of success in proving mens rea the
old-fashioned way by presenting circumstantial evidence
and appealing to reasonable inferences.
This case illustrates how it can be—and regularly is—
done. To persuade the jury that Ms. Diaz knew about the
drugs, the government could point to the amounts involved—54 pounds of drugs worth over $360,000. Ante, at
2. It could also point to the holes in her story. She claimed
the car was her boyfriend’s, but then said she had met him
only “three times tops,” did not know his phone number,
and did not know where he lived. ECF Doc. 33–1, at 13, 32.
The government could point out, too, that when cell phones
were found in the car, Ms. Diaz maintained one of them belonged to a friend, someone she would “rather not” identify.
Id., at 34. As well, the government could highlight her
statement that the phone was “locked” and she did not
“have access to it.”
Id., at 32–33. And the government could
then ask a jury to infer from all these facts that Ms. Diaz
knew exactly what she was doing. As it argues to us, the
government was free to argue to a jury, asking it to conclude
that Ms. Diaz’s story was “transparently flimsy.” Brief in
Opposition 16. Day in and day out, the government secures
convictions for the knowing importation of drugs in just this
way. Tr. of Oral Arg. 84. There was no need to gild the lily
by calling to the stand an “expert” in mindreading. And
there is certainly no cause for this Court to sanction the
practice.
To the contrary, there are sound reasons why Rule 704(b)
operates as it does. The problem of junk science in the
courtroom is real and well documented. See Daubert v.
Merrell Dow Pharmaceuticals, Inc.,
509 U. S. 579, 589–590
(1993); Kumho Tire Co. v. Carmichael,
526 U. S. 137, 147
(1999); see also P. Huber, Galileo’s Revenge: Junk Science
10 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
in the Courtroom 15–17 (1991). And perhaps no “science”
is more junky than mental telepathy. After Hinckley’s trial,
Congress recognized as much when it adopted Rule 704(b)
to remove from federal courtrooms experts who claim to
know what was inside a man’s head at a particular moment
in the past when he committed a particular act.
The particular nook of the criminal law we find ourselves
in today illustrates the soundness of Congress’s approach.
Not long ago, the government tried—often successfully—to
put “experts” (really, like Agent Flood, its own law enforcement agents) on the stand to testify that all couriers know
when they are carrying drugs. See, e.g., United States v.
Flores, 510 Fed. Appx. 594, 595 (CA9 2013). Not only was
that testimony improper under the government’s own current understanding of Rule 704(b). See Part II, supra; ante,
at 9. Eventually, the government felt it had to backtrack
after being confronted with too much evidence that some
couriers simply have no idea they are being used to carry
drugs. Flores,
510 Fed. Appx., at 595. So now, the government puts on witnesses to say most couriers know. We cannot be certain how many individuals sit in federal prison
because of the government’s past impermissible and mistaken “expert” testimony that all couriers know when they
are carrying drugs. About the only thing we can be sure of
is that what is good for the goose is good for the gander and
that, thanks to the Court’s opinion today, defendants will
now recruit their own warring experts. Ones who will seek
to testify (not unlike Agent Flood) that, in their experience,
“most” drug couriers are kept in the dark by cartels.
None of this serves our criminal justice system well. A
criminal conviction is “the gravest” condemnation we as a
society “permit ourselves to make.” Wechsler 528. Allowing into our proceedings speculative guesswork about a defendant’s state of mind diminishes the seriousness due
them. It risks the reliability of the outcomes they produce
Cite as: 602 U. S. ____ (2024) 11
GORSUCH, J., dissenting
(just ask those convicted in cases where government experts opined that “all” couriers know). 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.” It diminishes our respect for the presumptively free person, his free will and individuality, by encouraging the lazy assumption that he thinks like “most.” And
it reduces the vital role juries are meant to play in criminal
trials. Yes, they can still decide whether the defendant
thinks like “most” people. Ante, at 9. But that role hardly
matches Rule 704(b)’s promise that “matters” of mens rea
at trial belong to the jury “alone.”
V
In describing what I see as some of the possible consequences of the government’s approach adopted by the Court
today, I do not mean to suggest they are inevitable. Today’s
decision may go a long way toward hollowing out Rule
704(b). But it does not address what any other Rule of Evidence may have to say about cases like this one. And, looking briefly to some of those other Rules, I see reason for
hope.
Take a few examples. Under Rule 402, any evidence presented at trial must be “[r]elevant,” meaning it must have
a “tendency to make . . . more or less probable” a “fact . . . of
consequence in determining the action.” Fed. Rules Evid.
401, 402. Yet, if 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. After all,
the “fact of consequence” in cases like Ms. Diaz’s is whether
the defendant possessed the requisite mens rea. And it’s
hard to see how the government can have it both ways—
asserting in one breath that opinions like Agent Flood’s are
12 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
not “about” whether the defendant possessed the requisite
mental state to convict, while insisting in the next breath
that those opinions are relevant to (or, one might say,
“about”) the defendant’s mental state.
Rule 403 stands as another bulwark. That Rule permits
courts to “exclude relevant evidence” when its “probative
value is substantially outweighed by a danger of . . . unfair
prejudice.” Surely, in our system of justice—where we recognize that each individual is presumed innocent and distinctly endowed with free will and choice, where the individual is responsible for his culpable mental states but not
those of others—testimony 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. Juries
have managed that task for centuries and, as we have long
recognized, they are “fitted for it by their natural intelligence and their practical knowledge of men and the ways of
men.” Aetna Life Ins. Co. v. Ward, 140 U. S. 76, 88 (1891).
Meanwhile, the danger of unfair prejudice can run very
high. It can be “difficult for the individual to make his own
case stand on its own merits in the minds of the juror[s]”
when jurors are told by an expert “that birds of a feather
are flocked together.” Krulewitch v. United States,
336
U. S. 440, 454 (1949) (Jackson, J., concurring). As this
Court has recognized, too, expert opinions about the defendant’s “state of mind at the crucial moment” when committing a criminal act may “easily mislead” the jury into
“thinking the opinions show more than they do.” Clark v.
Arizona,
548 U. S. 735, 776 (2006). Even the government
candidly admits Rule 403 challenges may be proper against
such testimony. Brief for United States 30–31.
The risk of unfair prejudice can be exacerbated, too,
where, as here, the professed expert “carries with [him] the
Cite as: 602 U. S. ____ (2024) 13
GORSUCH, J., dissenting
imprimatur of the [g]overnment.” United States v. Young,
470 U. S. 1, 18 (1985). A witness like that “may induce the
jury to trust [the witness’s] judgment rather than its own
view of the evidence.”
Id., at 18–19; see also United States
v. Scheffer,
523 U. S. 303, 314 (1998) (plurality opinion) (experts like these may attain an “aura of infallibility”). For
precisely that reason, the government may be highly
tempted to do as it did in this case and seek to throw in an
“expert” on top of a seemingly strong circumstantial case—
just to be sure. But none of that means the proffered testimony is likely to advance the promise of a fair trial.
Add to those Rules at least one more. As part of its “gatekeeping” functions, a federal court must ensure that any
expert testimony it permits is reliable, grounded on widely
accepted principles, and will “ ‘assist the trier of fact to understand the evidence.’ ” Kumho Tire Co, 526 U. S., at 147
(quoting Fed. Rule Evid. 702(a) (1999)). I struggle to see
how a witness claiming to offer an opinion about another
person’s (or class of persons’) thoughts at a particular moment in the past can meet any of those standards. No one,
at least outside the fortuneteller’s den, can yet claim the
power to conjure reliably another’s past thoughts. Testimony like Agent Flood’s may be dubiously circular, too. For
each time a law enforcement agent takes the stand to say
“most people know” and that helps the government secure
another conviction, he himself is creating the very proof on
which a government expert may purport to rely in the next
trial.
Nor does testimony like that help the jury understand
“ ‘experience[s] confessedly foreign in kind to [its] own.’ ”
Kumho Tire Co., 526 U. S., at 149 (quoting L. Hand, Historical and Practical Considerations Regarding Expert Testimony,
15 Harv. L. Rev. 40, 54 (1901)). In a criminal trial,
expert testimony about DNA testing or the chemical composition of illegal drugs may sometimes help a jury understand facts they do not encounter in daily life. But none of
14 DIAZ v. UNITED STATES
GORSUCH, J., dissenting
that holds true when it comes to the job of assessing
whether a defendant’s story about her state of mind is credible or (as the government puts it) “transparently flimsy.”
Brief in Opposition 16. Jurors are more than up to performing that task, and they hardly need the help of some clairvoyant.
*
Persuaded that today’s decision is mistaken, but hopeful
that it will ultimately prove immaterial in practice, I respectfully dissent.
