(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., 

.


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 methamphet-
  amine in violation of 

 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 Home-
  land 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 Evi-
  dence 704(b), which provides that “[i]n a criminal case, an expert wit-
  ness 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 is-
    sue.” The provision at issue, Rule 704(b), is an exception to that gen-
    eral rule. Rule 704 departed from the once-prevailing common-law
    practice that a witness could not state their conclusions on any ulti-
    mate issue, i.e., issues that the jury must resolve to decide the case.
    See United States v. Spaulding, 

. 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 ex-
    pert 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. In-
    stead, 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 vio-
    late Rule 704(b).
       Diaz’s counterarguments are unpersuasive. She first argues that
    Agent Flood functionally stated an opinion about whether she know-
    ingly 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 men-
    tal 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 tes-
    timony 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: 

 (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 wit-
nesses from stating opinions “about whether the defendant
did or did not have a mental state or condition that consti-
tutes 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 con-
clude that the testimony did not violate Rule 704(b). We
therefore affirm.
                            I
  In August 2020, Delilah Diaz, a United States citizen, at-
tempted 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 con-
traband, 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 methampheta-
mine 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, 

 (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 ac-
cess 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: 

 (2024)            3

                      Opinion of the Court

violation of 

 and 960. The charges re-
quired 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. Be-
fore 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 organiza-
tions. Specifically, he planned to explain that drug traffick-
ers “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 opin-
ion 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 circum-
stances, 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. 

 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. 

 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-traffick-
ing organizations sometimes use unknowing couriers.
   The jury found Diaz guilty, and the District Court sen-
tenced 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) prohib-
its only “an ‘explicit opinion’ on the defendant’s state of
mind.” 

, *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). 

   We granted certiorari, 

 (2023), and now af-
firm.
                              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 consti-
    tutes 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, 

,
                  Cite as: 

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                      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” (inter-
nal 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, 

, 

 (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, 

, 131–132, 

 (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 di-
rectly helped the jury resolve an ultimate issue. See, e.g.,
Furlong v. Carraher, 

, 

(1899) (holding that witness could not testify about de-
ceased’s mental capacity to enter will, but could testify to
her “condition of . . . mind at the time the will was exe-
cuted”); De Groot, 

, 

 (hold-
ing that witness could testify that plaintiff ’s condition could
have resulted from malpractice); Hill v. State, 

, 

 (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 justifica-
tion” 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 “cover-
ing 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., 

, 145
6                  DIAZ v. UNITED STATES

                     Opinion of the Court

(1873).
   Although the ultimate-issue rule’s exact origins are un-
clear, 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 

 Many rejected the idea that
ultimate-issue testimony usurps the jury’s role, since a wit-
ness’s “credibility” and “the soundness of his judgment” “al-
ways remain for the jury’s determination.” Goldfoot v.
Lofgren, 

, 

 (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 car-
ried 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. 

.
   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 assassi-
nate President Ronald Reagan, shooting and wounding the
President and three other men. See L. Caplan, The Insan-
ity 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 compet-
ing expert opinions on the ultimate issue of Hinckley’s san-
ity. See R. Bonnie, J. Jeffries, & P. Low, A Case Study in
                 Cite as: 

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                     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 

; 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 par-
ticular person (“the defendant”) and a particular ultimate
issue (whether the defendant has “a mental state or condi-
tion” that is “an element of the crime charged or of a de-
fense”).
                             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 methamphet-
amine, 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 cir-
cumstances, 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 cou-
riers. 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 re-
covering 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. 

. 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 ex-
pert 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 in-
volved 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 know-
ingly transported the drugs. The jury alone drew that con-
clusion. 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 el-
ement 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: 

 (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 con-
flates an opinion about most couriers with one about all cou-
riers. 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 defend-
ant’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 de-
fendant was part of a group of people that all have a partic-
ular 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 ex-
pert 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 par-
ticular mental state. Of all drug couriers—a group that in-
cludes 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 less-
numerous-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 testi-
mony 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 Pe-
titioner 18–19; post, at 6–7. But, a word’s meaning is in-
formed by its surrounding context. See Smith v. United
States, 

 (1993). A crucial part of that con-
text is the other words in the sentence. See FCC v. AT&T
Inc., 

 (2011). The words surrounding
“about” make clear that Rule 704(b) addresses a far nar-
rower 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 refer-
ence, 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 conclu-
sions as to the defendant’s mental state.
   Rule 704(a) further confirms the narrow scope of testi-
mony 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 prac-
tice by permitting testimony that “embraces an ultimate is-
sue.” 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 opin-
ions.
   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: 

 (2024)                 11

                      Opinion of the Court

broader than Rule 704(a)—or even the original ultimate-is-
sue rule. Even though the ultimate-issue rule and Rule
704(a) address opinions that include the ultimate issue it-
self, 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 de-
fendant” and thus does not violate Rule 704(b). Accord-
ingly, the judgment of the Court of Appeals is affirmed.

                                                   It is so ordered.
                  Cite as: 

 (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 offer-
ing their “opinion about whether the defendant did or did
not have a mental state or condition that constitutes an el-
ement 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 evi-
dence, it prohibits “only expert opinions . . . about a partic-
ular 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 proba-
tive 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 de-
signed it, Rule 704(b) is party agnostic. Neither the Gov-
ernment nor the defense can call an expert to offer her opin-
ion 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 defend-
ant had a particular mental state, “based on the defendant’s
membership in a particular group.” Brief for John Mo-
nahan 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 Govern-
ment expert opined (based on his almost 30 years of experi-
ence 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 Govern-
ment was not the only party that relied on this type of men-
tal-state evidence during the trial. Diaz called an automo-
bile 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 character-
istics 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: 

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                     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 likeli-
hood that those with a particular condition would have a
particular mental state. For example, as Diaz acknowl-
edges, 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 be-
cause they do not appreciate the wrongfulness of their con-
duct.” 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, stereo-
types, 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, 

, 1579–1580 (CA11 1993) (en banc).
   Or consider defendants who have been subject to domes-
tic abuse. “A number of myths and misconceptions about
[battered woman syndrome] victims affect our criminal jus-
tice system,” and it is clear that those mistaken views “af-
fect jurors.” Linn v. State, 

 (Iowa
2019); see also 

 at 742–746 (summarizing relevant evi-
dence). Rule 704(b) allows experts to testify about the typ-
ical 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, Fed-
eral Practice and Procedure §6285 (2d ed. Supp. 2023).
Such evidence can play a pivotal role in a defendant’s at-
tempts 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, 

, 819–824 (CA9 2019); United States v. Nwoye, 

, 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, 

 (CA11 1988) (“When the expert is a gov-
ernment 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 lim-
itations.
   This means, of course, that when faced with flawed or
                  Cite as: 

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                     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, 

, 1014–1015 (CA9 2002) (“Expert
testimony that compels the jury to conclude that the de-
fendant did or did not possess the requisite mens rea does
not ‘assist the trier of fact’ under Rule 702 because such tes-
timony encroaches on the jury’s vital and exclusive function
to make credibility determinations”); United States v. Lip-
scomb, 

 (CA7 1994) (describing safe-
guards that can be used to prevent testimony from law en-
forcement 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 in-
troduced. See Evidence Professors Brief 27–29; see also
United States v. Smart, 

, 1388–1389 (CADC
1996) (requiring that district courts sometimes use jury in-
structions 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 opin-
ion.
                 Cite as: 

 (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 wit-
ness 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 re-
quired to secure a conviction.
   The upshot? The government comes away with a power-
ful new tool in its pocket. Prosecutors can now put an ex-
pert on the stand—someone who apparently has the con-
venient 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 defend-
ant 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. Of-
ten 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 an-
cient as it is fundamental to our system of justice. At com-
mon 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) (Black-
stone). That same view “took deep and early root in Amer-
ican soil” where, to this day, a crime ordinarily arises “only
from concurrence of an evil-meaning mind with an evil-do-
ing hand.” Morissette v. United States, 

, 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, 

; see Staples v.
United States, 

 (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 reckless-
ness and negligence. See, e.g., ALI, Model Penal Code §2.02
(1985); United States v. Bailey, 

 (1980).
But to subject a presumptively free individual to serious
punishments for acts undertaken without proof of any of
                  Cite as: 

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                     GORSUCH, J., dissenting

that would be “the badge of tyranny, the plainest illustra-
tion of injustice.” Wechsler 528. The principle “that an in-
jury 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.” Moris-
sette, 

.
   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.” 

 (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, 

 (1977); see Mullaney v. Wil-
bur, 

 (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, 

 (1985).
   Reflecting the centrality of mens rea to criminal punish-
ment and the jury’s role in finding it, Rule 704(b) of the Fed-
eral 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 condi-
tion 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, ex-
perts 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 be-
tween experts who claimed to know exactly what Hinckley
was (or was not) thinking at the moment he pulled the trig-
ger. 

   In the trial’s aftermath, Congress continued to recognize
the value of expert mental health evidence. So, for exam-
ple, 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 requi-
site 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 partic-
ular moment in the past did not reflect well on federal judi-
cial 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, Ev-
idence §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, Ev-
idence §§1920, 1921 (1904); United States v. Spaulding, 

 (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: 

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                     GORSUCH, J., dissenting

                                II
   The government violated that Rule in this case. Proceed-
ings began when prosecutors charged Ms. Diaz with import-
ing a controlled substance into this country. See 

, 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 gov-
ernment 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 inno-
cently (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 testi-
fied 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, of-
fering 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 coun-
try. And that, the prosecutors argued, made all the differ-
ence. See App. 32a; Brief for United States in No. 21–50238
(CA9), pp. 46, 58. The Ninth Circuit endorsed the govern-
ment’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 per-
sons that includes her. App. to Pet. for Cert. 6a (quoting
United States v. Gomez, 

 (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 “explic-
itly” 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 pro-
scribed mental state when that class includes the defend-
ant. 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 hypothet-
ical 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 govern-
ment asks us to affirm its judgment on other grounds. As
the government sees it, Agent Flood’s opinion was permis-
sible for a different reason than the Ninth Circuit offered.
It was permissible, the government says, because it wasn’t
                  Cite as: 

 (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 men-
tal 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 govern-
ment’s problem. The Rule does not only prohibit an expert
from stating a definitive opinion about the defendant’s men-
tal 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 Peti-
tioner 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 ex-
pert 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 con-
vict. 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 col-
lapses 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. Probabil-
istic 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, concern-
ing, 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 in-
sists, that an expert may offer the probabilistic assessment
that “most” people like the defendant know they are carry-
ing drugs? The only difference between the two opinions is
that the first addresses the defendant “explicitly,” the sec-
ond 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 mud-
dled 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 defend-
ant’s shoes have the requisite mens rea. But what if he said,
as the government initially proffered, that drivers “gener-
ally” know? ECF Doc. 30, at 7. Or that they “almost al-
ways” 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 any-
thing goes and eviscerate Rule 704(b) in the process. Ra-
ther 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. Nor-
mally, it will require the government to resort to circum-
stantial 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: 

 (2024)            9

                    GORSUCH, J., dissenting

tribunal can search the heart, or fathom the intentions of
the mind, otherwise than as they are demonstrated by out-
ward 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 in-
volved—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 be-
longed to a friend, someone she would “rather not” identify.

. As well, the government could highlight her
statement that the phone was “locked” and she did not
“have access to it.” 

 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., 

, 589–590
(1993); Kumho Tire Co. v. Carmichael, 

(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 enforce-
ment agents) on the stand to testify that all couriers know
when they are carrying drugs. See, e.g., United States v.
Flores, 

 (CA9 2013). Not only was
that testimony improper under the government’s own cur-
rent 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, 

. So now, the govern-
ment puts on witnesses to say most couriers know. We can-
not be certain how many individuals sit in federal prison
because of the government’s past impermissible and mis-
taken “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. Allow-
ing into our proceedings speculative guesswork about a de-
fendant’s state of mind diminishes the seriousness due
them. It risks the reliability of the outcomes they produce
                   Cite as: 

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                      GORSUCH, J., dissenting

(just ask those convicted in cases where government ex-
perts opined that “all” couriers know). It undermines our
historic commitment that mens rea is a necessary compo-
nent 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 presump-
tively free person, his free will and individuality, by encour-
aging 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 conse-
quences 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 Ev-
idence may have to say about cases like this one. And, look-
ing briefly to some of those other Rules, I see reason for
hope.
  Take a few examples. Under Rule 402, any evidence pre-
sented 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 de-
fendant 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 rec-
ognize that each individual is presumed innocent and dis-
tinctly endowed with free will and choice, where the indi-
vidual 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 testi-
mony 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 intelli-
gence and their practical knowledge of men and the ways of
men.” Aetna Life Ins. Co. v. Ward, 

 (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, 

 (1949) (Jackson, J., concurring). As this
Court has recognized, too, expert opinions about the defend-
ant’s “state of mind at the crucial moment” when commit-
ting a criminal act may “easily mislead” the jury into
“thinking the opinions show more than they do.” Clark v.
Arizona, 

 (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: 

 (2024)             13

                     GORSUCH, J., dissenting

imprimatur of the [g]overnment.” United States v. Young,

 (1985). A witness like that “may induce the
jury to trust [the witness’s] judgment rather than its own
view of the evidence.” 

 at 18–19; see also United States
v. Scheffer, 

 (1998) (plurality opinion) (ex-
perts 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 testi-
mony is likely to advance the promise of a fair trial.
    Add to those Rules at least one more. As part of its “gate-
keeping” 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 un-
derstand the evidence.’ ” Kumho Tire Co, 

(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 mo-
ment 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. Testi-
mony 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., 

 (quoting L. Hand, Histor-
ical and Practical Considerations Regarding Expert Testi-
mony, 

, 54 (1901)). In a criminal trial,
expert testimony about DNA testing or the chemical com-
position of illegal drugs may sometimes help a jury under-
stand 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 cred-
ible or (as the government puts it) “transparently flimsy.”
Brief in Opposition 16. Jurors are more than up to perform-
ing that task, and they hardly need the help of some clair-
voyant.
                             *
  Persuaded that today’s decision is mistaken, but hopeful
that it will ultimately prove immaterial in practice, I re-
spectfully dissent.