Diaz v. United States

                   PRELIMINARY PRINT

             Volume 602 U. S. Part 1
                             Pages 526–555




       OFFICIAL REPORTS
                                    OF


   THE SUPREME COURT
                               June 20, 2024


Page Proof Pending Publication


                   REBECCA A. WOMELDORF
                           reporter of decisions




    NOTICE: This preliminary print is subject to formal revision before
  the bound volume is published. Users are requested to notify the Reporter
  of Decisions, Supreme Court of the United States, Washington, D.C. 20543,
  pio@supremecourt.gov, of any typographical or other formal errors.
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                                 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 offcers 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 Govern-
  ment to prove that Diaz “knowingly” transported drugs. In her de-
  fense, Diaz claimed not to know that the drugs were hidden in the car.
  To rebut Diaz's claim, the Government planned to call Homeland Secu-
  rity Investigations Special Agent Andrew Flood as an expert witness
  to testify that drug traffckers 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

Page Proof Pending Publication
  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 testifed that most couriers know that
  they are transporting drugs. The jury found Diaz guilty, and Diaz ap-
  pealed, 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. 531–538.
    (a) Federal Rule of Evidence 704(a) sets out a general rule that “[a]n
 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., is-
 sues 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
                       Cite as: 602 U. S. 526 (2024)                    527

                                 Syllabus

  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. 531–534.
     (b) In this case, Agent Flood did not express an opinion about whether
  Diaz herself knowingly transported methamphetamine. Instead, he
  testifed 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 trans-
  ported methamphetamine, his testimony did not violate Rule 704(b).
     Diaz's counterarguments are unpersuasive. She frst 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 defnitions 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
Page Proof Pending Publication
  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 confrms the narrow scope of testimony pro-
  hibited 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 testi-
  mony that Rule 704(a) expressly allows, which is opinion testimony that
  includes ultimate issues. Diaz's reading would have the exception swal-
  low the rule. Pp. 534–537.
Affrmed.

   Thomas, J., delivered the opinion of the Court, in which Roberts, C. J.,
and Alito, Kavanaugh, Barrett, and Jackson, JJ., joined. Jackson,
J., fled a concurring opinion, post, p. 538. Gorsuch, J., fled a dissenting
opinion, in which Sotomayor and Kagan, JJ., joined, post, p. 542.

  Jeffrey L. Fisher argued the cause for petitioner. With
him on the briefs were Pamela S. Karlan, Easha Anand,
L. Nicole Allan, and Carlton F. Gunn.
  Matthew Guarnieri argued the cause for the United
States. With him on the brief were Solicitor General Pre-
528                    DIAZ v. UNITED STATES

                          Opinion of the Court

logar, Acting Assistant Attorney General Argentieri, Dep-
uty Solicitor General Feigin, and Javier A. Sinha.*
  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 con-
stitutes an element of the crime charged or of a defense.”
In this drug-traffcking 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 affrm.
Page Proof Pending Publication
                                    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 offcer
asked her to roll down the car's rear driver-side window.
Diaz responded that the window was manual, so the offcer
left his inspection booth and tried to roll down the window
himself. The offcer “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-

   *Briefs of amici curiae urging reversal were fled for the National Asso-
ciation of Criminal Defense Lawyers by Carmen Iguina González, Joshua
Matz, Sean Hecker, David Patton, and Joshua L. Dratel; and for the Na-
tional Association of Federal Defenders by Vincent J. Brunkow, Cathi
Shusky, and Shelley Fite.
   Matthew Coles fled a brief for John Monahan et al. as amici curiae
urging affrmance.
                      Cite as: 602 U. S. 526 (2024)                  529

                          Opinion of the Court

traband, the offcer investigated further with a “buster,” a
handheld tool that measures an object's density. After the
buster detected an abnormal density in the doors, offcers
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 offcers, however, found her story
hard to believe. Diaz explained that she was driving her
boyfriend's car. Contradictorily, she also told offcers 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
Page Proof Pending Publication
Cal.), ECF Doc. 33–1, p. 13. Diaz's story grew even more
dubious when offcers 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 in-
sisted 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 vi-
olation 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 colloqui-
ally as a “blind mule” defense: she argued that she did not
know that there were drugs in the car. Before trial, the
  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.
530                DIAZ v. UNITED STATES

                      Opinion of the Court

Government gave notice that it would call Homeland Secu-
rity Investigations Special Agent Andrew Flood as an expert
witness. Agent Flood would testify about the common
practices of Mexican drug-traffcking organizations. Spe-
cifcally, he planned to explain that drug traffckers “gener-
ally 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 testifed that drug traffckers never use unknowing
couriers, that would be functionally equivalent to an opinion
about whether Diaz knowingly transported drugs. The Dis-
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trict 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 testifed 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-traffcking organization to substantial risk. The orga-
nization could not guarantee where, if at all, the drugs would
arrive. Id., at 16a, 26a. Even if the drugs reached the in-
tended destination, the organization would then have to re-
trieve the drugs without detection. Id., at 16a, 24a–25a.
According to Agent Flood, drug-traffcking organizations are
often unwilling to take those chances with hundreds of thou-
sands of dollars on the line. Agent Flood acknowledged on
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                      Opinion of the Court

cross-examination that drug-traffcking organizations some-
times 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) 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 affrm.

                                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
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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,
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.”
532                DIAZ v. UNITED STATES

                      Opinion of the Court

DeGroot 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 fred 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 di-
rectly 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 de-
ceased's mental capacity to enter will, but could testify to
her “condition of . . . mind at the time the will was exe-
cuted”); DeGroot, 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
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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) (Mc-
Cormick) (explaining that the rule's “stated justifcation” was
to exclude testimony that “usurps the function” or “invades
the province of the jury” (internal quotation marks and foot-
note 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 fnding of [the] witnes[s].” Chicago &
Alton R. Co. v. Springfeld & N. W. R. Co., 67 Ill. 142, 145
(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 battlefeld before Troy was no more
                   Cite as: 602 U. S. 526 (2024)            533

                      Opinion of the Court

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 mat-
ter of decades. See ibid. 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. Lof-
gren, 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
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adopted in 1975, it had no exceptions: All ultimate-issue opin-
ions 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 assassi-
nate 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 the Insanity
Defense 54 (4th ed. 2021). To the surprise of many, Hinck-
ley 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 “ex-
ception” to Rule 704's blanket rule admitting ultimate-issue
opinions. As Rule 704(b) now reads, “[i]n a criminal case,
534                  DIAZ v. UNITED STATES

                        Opinion of the Court

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 con-
dition” 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
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whether Diaz herself knowingly transported methamphet-
amine, his testimony did not violate Rule 704(b).
  Agent Flood instead testifed about the knowledge of most
drug couriers. Specifcally, 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
couriers. Diaz herself made this point at trial. She argued
that another person, an alleged boyfriend, had deceived her
into 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 recov-
ering 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

  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. 526 (2024)           535

                     Opinion of the Court

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 testifed 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 conclu-
sion. While Agent Flood provided evidence to support one
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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 know-
ingly transported drugs when he opined that couriers gen-
erally transport drugs knowingly. Brief for Petitioner 24
(internal quotation marks omitted); see also post, at 549
(opinion of Gorsuch, J.). That argument mistakenly con-
fates 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 testifes at an arson trial that all people in the defend-
ant's shoes set fres maliciously (the mental state required
536                DIAZ v. UNITED STATES

                      Opinion of the Court

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 particu-
lar mental state. The phrase “all people in the defendant's
shoes” includes, of course, the defendant himself. So, when
the expert testifed that all people in the defendant's shoes
always set fres 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 ulti-
mate 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
includes Diaz—he opined that the majority knowingly trans-
port drugs. The jury was then lef t to decide: Is Diaz
like the majority of couriers? Or, is Diaz one of the less-
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numerous-but-still-existent couriers who unwittingly trans-
port drugs? The ultimate issue of Diaz's mental state was
left to the jury's judgment. As a result, Agent Flood's testi-
mony 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 defnitions of “about”
to argue that Rule 704(b)'s prohibition includes all testimony
that “ `concerns' or is `in reference to' whether the defendant
possessed a particular state of mind.” Brief for Petitioner
18–19; post, at 548. 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
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                      Opinion of the Court

a particular conclusion. See Black's Law Dictionary 1244
(rev. 4th ed. 1968) (defning “opinion evidence” as “what the
witness thinks, believes, or infers in regard to facts in dis-
pute”). Moreover, the Rule does not preclude testimony
“about” mental-state ultimate issues in the abstract. In-
stead, 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 confrms 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 531–532. Rule 704(a) abolished that
practice by permitting testimony that “embraces an ultimate
issue.” See 5 Oxford English Dictionary 169 (2d ed. 1989)
(defning “embrace” as “[t]o include, contain, comprise”).
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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
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.
538                DIAZ v. UNITED STATES

                     Jackson, J., concurring

                              IV
  An expert's conclusion that “most people” in a group have
a particular mental state is not an opinion about “the defend-
ant” and thus does not violate Rule 704(b). Accordingly, the
judgment of the Court of Appeals is affrmed.
                                               It is so ordered.

  Justice Jackson, concurring.
   I join the Court's opinion in full. Federal Rule of Evi-
dence 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 evi-
dence, it prohibits “only expert opinions . . . about a particu-
Page Proof Pending Publication
lar 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 534. 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 536.
   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 men-
tal state at the time of the offense. See ante, at 534. But
a corollary is also true. Both the Government and the de-
fense are permitted, consistent with Rule 704(b), to elicit ex-
pert testimony “on the likelihood” that the defendant had a
particular mental state, “based on the defendant's member-
ship in a particular group.” Brief for John Monahan et al.
as Amici Curiae 1 (Evidence Professors Brief). Indeed, the
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                        Jackson, J., concurring

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 signifcance 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 534. Notably, however, the Gov-
ernment was not the only party that relied on this type of
mental-state evidence during the trial. Diaz called an auto-
mobile specialist who testifed 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),
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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 551–552 (opinion of Gor-
such, J.). For example, Diaz could have offered expert tes-
timony 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 Cu-
riae 5–16 (NAFD Brief) (describing numerous cases involv-
ing so-called “blind mules”).
   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

  *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.
540                DIAZ v. UNITED STATES

                     Jackson, J., concurring

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 552, the type of mental-state evidence that Rule
704(b) permits can be of critical assistance to lay factfnders
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 men-
tal health condition, including by explaining the likelihood
that those with a particular condition would have a partic-
ular 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
Page Proof Pending Publication
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 codifed
Rule 704(b). See ante, at 533–534; 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 bet-
ter understand a defendant's condition and thereby call into
question a mens rea that might otherwise be too easily as-
sumed. See, e. g., United States v. Brown, 32 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 [bat-
tered woman syndrome] victims affect our criminal justice
                   Cite as: 602 U. S. 526 (2024)           541

                     Jackson, J., concurring

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 ju-
rors 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
Page Proof Pending Publication
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 govern-
ment 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 fawed or
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
542                 DIAZ v. UNITED STATES

                      Gorsuch, J., dissenting

courts to bar unqualifed 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 testi-
mony 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 en-
croaches 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 specifc admonitions and in-
structions 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
Page Proof Pending Publication
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.

  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.
                   Cite as: 602 U. S. 526 (2024)           543

                     Gorsuch, J., dissenting

  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 conven-
ient 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 fnd that the defendant is like
“most” people and convict. What authority exists for allow-
ing that kind of charade in federal criminal trials is any-
body's guess, but certainly it cannot be found in Rule 704.

                                I
   Delilah Diaz's conviction for drug traffcking turned on her
state of mind. In that, hers was an everyday case. Often
in our criminal justice system, the difference between free-
dom and years in prison turns on just that question. Per-
haps it has always been so. The government's duty to prove
Page Proof Pending Publication
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, Commen-
taries 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 con-
currence 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 signifcant part of it
has to do with respect for the individual and his liberty in a
544                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

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 punish-
ments for acts undertaken without proof of any of that would
be “the badge of tyranny, the plainest illustration of injus-
tice.” Wechsler 528. The principle “that an injury can
amount to a crime only when inficted” with some accompa-
nying 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 individ-
Page Proof Pending Publication
ual 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 (inter-
nal 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 de-
fendant'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).
   Refecting the centrality of mens rea to criminal punish-
ment and the jury's role in fnding it, Rule 704(b) of the Fed-
                   Cite as: 602 U. S. 526 (2024)            545

                     Gorsuch, J., dissenting

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 condition
that constitutes an element of the crime charged or of a de-
fense.” 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 533. In that case,
experts didn't just offer competing views on whether Hin-
ckley suffered from a medically diagnosable mental illness.
They 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 trig-
ger. Ibid.
   In the trial's aftermath, Congress continued to recognize
the value of expert mental health evidence. So, for exam-
Page Proof Pending Publication
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 540–541
(Jackson, J., concurring) (discussing schizophrenia and bat-
tered 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 de-
clared 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 refect 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 refects a much older tradition.
For centuries, courts have grappled with the role expert wit-
nesses should play at trial. See, e. g., 1 S. Greenleaf, Evi-
dence § 440, p. 489 (1842); Folkes v. Chadd, 3 Dougl. 157, 158–
159, 99 Eng. Rep. 589, 590 (K. B. 1782). For a long stretch,
546                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

many courts barred experts from offering opinions on so-
called ultimate issues like mens rea. See 3 J. Wigmore, Evi-
dence §§ 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.

                               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 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.
Page Proof Pending Publication
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 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; United States'
Notice in No. 3:20–cr–02546 (SD Cal.), ECF Doc. 30, p. 7.
And in response to the government's questions, Agent Flood
testifed 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.
                   Cite as: 602 U. S. 526 (2024)            547

                     Gorsuch, J., dissenting

    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 ques-
tion 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 tes-
tify 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-
Page Proof Pending Publication
ment's maneuver, holding that Rule 704(b) prohibits only tes-
timony “ `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 (CA9 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 convic-
tion. Tr. of Oral Arg. 72–73, 76. The Rule also bars an ex-
pert 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 535–536. Likewise, the Rule
bars an expert from opining that a hypothetical person who
matches the defendant's description (say, a hypothetical wo-
man 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
548                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

“about” the defendant's mental state and cannot be offered
consistent with Rule 704(b). On this, the Court, too, agrees.
Ante, at 535–536.
                            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 affrm 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 defni-
tive. 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 in-
sists, 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-
Page Proof Pending Publication
ment's problem. The Rule does not only prohibit an expert
from stating a defnitive 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 defnitive 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
convict. Period. Lest any doubt remain, the Rule takes
                   Cite as: 602 U. S. 526 (2024)            549

                     Gorsuch, J., dissenting

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. Probabi-
listic though the testimony may be, an expert who says that
an individual defendant “most likely” had the requisite men-
tal state for conviction offers an opinion about, concerning,
regarding, or in reference to her mental state. On that, 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 frst addresses the defendant “explicitly,” the second
a class that includes her. All of which returns us to a dis-
Page Proof Pending Publication
tinction 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 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 fner 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.
550                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

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. Nor-
mally, it will require the government to resort to circumstan-
tial 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 tribu-
nal 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 ap-
Page Proof Pending Publication
pealing 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 529. 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 fimsy.” Brief in Opposition
                   Cite as: 602 U. S. 526 (2024)             551

                     Gorsuch, J., dissenting

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 cer-
tainly 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 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.
Page Proof Pending Publication
   The particular nook of the criminal law we fnd 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, 510 Fed. Appx. 594, 595 (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 535–536. Eventually, the government felt it had to back-
track 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
552                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

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 ( just ask
those convicted in cases where government experts opined
that “all” couriers know). It undermines our historic com-
mitment that mens rea is a necessary component of every
serious crime by turning the inquiry into a defendant's men-
tal state from an exacting one guided by hard facts and rea-
sonable 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 assump-
Page Proof Pending Publication
tion 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 535. 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 Evi-
dence may have to say about cases like this one. And, look-
ing briefy 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
                   Cite as: 602 U. S. 526 (2024)            553

                     Gorsuch, J., dissenting

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 not
“about” whether the defendant possessed the requisite men-
tal 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-
Page Proof Pending Publication
ognize that each individual is presumed innocent and dis-
tinctly endowed with free will and choice, where the individ-
ual 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 of-
fered 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 man-
aged that task for centuries and, as we have long recognized,
they are “ftted 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 “diffcult 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 focked together.” Krulewitch v. United States, 336
U. S. 440, 454 (1949) (Jackson, J., concurring). As this Court
554                DIAZ v. UNITED STATES

                     Gorsuch, J., dissenting

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
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) (ex-
perts like these may attain an “aura of infallibility”). For
precisely that reason, the government may be highly temp-
ted to do as it did in this case and seek to throw in an “ex-
Page Proof Pending Publication
pert” 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 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
                      Cite as: 602 U. S. 526 (2024)       555

                        Gorsuch, J., dissenting

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, Histori-
cal and Practical Considerations Regarding Expert Testi-
mony, 15 Harv. L. Rev. 40, 54 (1901)). In a criminal trial,
expert testimony about DNA testing or the chemical compo-
sition of illegal drugs may sometimes help a jury understand
facts they do not encounter in daily life. But none of 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 fimsy.” 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
Page
that it willProof
             ultimatelyPending        Publication
                       prove immaterial  in practice, I re-
spectfully dissent.
                           Reporter’s Note

  The attached opinion has been revised to refect the usual publication
and citation style of the United States Reports. The revised pagination
makes available the offcial United States Reports citation in advance of
publication. The syllabus has been prepared by the Reporter of Decisions
Page Proof Pending Publication
for the convenience of the reader and constitutes no part of the opinion of
the Court. A list of counsel who argued or fled briefs in this case, and
who were members of the bar of this Court at the time this case was
argued, has been inserted following the syllabus. Other revisions may
include adjustments to formatting, captions, citation form, and any errant
punctuation. The following additional edits were made:

None