Diaz v. United States

Summary

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 Federal Rule of Evidence 704(b).

Text

PRELIMINARY PRINT 
Volume 602 U. S. Part 1 
Pages 526–555 
OFFICIAL REPORTS 
OF 
THE SUPREME COURT 
June 20, 2024 
REBECCA A. WOMELDORF 
reporter of decisions 
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526 OCTOBER TERM, 2023 
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 
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

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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 
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. 
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. 
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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 
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. 
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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-
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

Cite as: 602 U. S. 526 (2024) 531 
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 
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.” 
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532 DIAZ v. UNITED STATES 
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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 
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 
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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 
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, 
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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 
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. 
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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 
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 left to decide: Is Diaz 
like the majority of couriers? Or, is Diaz one of the less-
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”). 
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. 
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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-
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), 
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. 
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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 
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 
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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 
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 
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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 
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. 
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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 
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 
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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-
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-
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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-
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. 
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. 
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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-
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 
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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-
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 
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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-
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. 
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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-
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 
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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. 
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 
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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-
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

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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-
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-
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 
Page Proof Pending Publication

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 
that it will ultimately prove immaterial in practice, I re-
spectfully dissent. 
Page Proof Pending Publication

Page Proof Pending Publication
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 
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

Retrieval chunks

• #1

  PRELIMINARY PRINT Volume 602 U. S. Part 1 Pages 526–555 OFFICIAL REPORTS OF THE SUPREME COURT June 20, 2024 REBECCA A. WOMELDORF reporter of decisions Page Proof Pending Publication NOTICE: This preliminary print is subject to formal revision before the bound volume is published. Users are requested to noti…

• #2

  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 practic…

• #3

  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 di…

• #4

  Matthew Coles fled a brief for John Monahan et al. as amici curiae urging affrmance. Page Proof Pending Publication 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 abn…

• #5

  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. …

• #6

  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…

• #7

  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) (Mc- Cormick) (explaining that the…

• #8

  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 ultim…

• #9

  “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 st…

• #10

  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 peo…

• #11

  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…

• #12

  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- lar person (`the defendant') and a particular ultimate issue (whether the defendant has `a mental state or condition' that is `an el…

• #13

  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 T…

• #14

  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 men…

• #15

  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 tes…

• #16

  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, …

• #17

  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.” Wechs…

• #18

  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- ple, an expert may still testify that the defendant suffered from some diagnosable illness or syndrome at the time of the char…

• #19

  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.…

• #20

  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 …

• #21

  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 Page Proof Pending Publication Cite as: 602 U. S. 526 (2024) 549 Gorsuch, J., dissenting pains to emphasize, “[t]hose matters are for the trier …

• #22

  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…

• #23

  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 …

• #24

  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 i…

• #25

  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'…

• #26

  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 outsid…

• #27

  punctuation. The following additional edits were made: None