NOTICE: This opinion is subject toformal revision before publication in the United States Reports.Readers are requested to notify the Reporter of Decisions, SupremeCourt of the United States, Washington, D.C. 20543,pio@supremecourt.gov, of any typographical or other formalerrors.SUPREME COURT OF THE UNITED STATES_________________No. 23–14_________________DELILAH GUADALUPE DIAZ, PETITIONER v.UNITED STATESon writ of certiorari to the united statescourt of appeals for the ninth circuit[June 20, 2024]Justice Thomas delivered the opinion of theCourt.Federal Rule of Evidence 704(b) prohibits expertwitnesses from stating opinions “about whether the defendant did ordid not have a mental state or condition that constitutes anelement of the crime charged or of a defense.” In thisdrug-trafficking prosecution, petitioner argued that she lacked themental state required to convict because she was unaware that drugswere concealed in her car when she drove it across the UnitedStates-Mexico border. At trial, the Government’s expert witnessopined that most drug couriers know that they are transportingdrugs. Because the expert witness did not state an opinion aboutwhether petitioner herself had a particular mental state, weconclude that the testimony did not violate Rule 704(b). Wetherefore affirm.IIn August 2020, Delilah Diaz, a United Statescitizen, attempted to enter the United States from Mexico. WhenDiaz drove into the port of entry, a border patrol officer askedher to roll down the car’s rear driver-side window. Diaz respondedthat the window was manual, so the officer left his inspectionbooth and tried to roll down the window himself. The officer “feltsome resistance” and then heard “a crunch-like sound in the door.”App. 25. Aware from experience that car doors are a common hidingspot for contraband, the officer investigated further with a“buster,” a handheld tool that measures an object’s density. Afterthe buster detected an abnormal density in the doors, officersbrought in a narcotics detection canine and sent the car through anX-ray machine. They discovered 56 packages of methamphetaminetucked inside the car’s door panels and underneath the carpet inthe trunk. The methamphetamine weighed just over 54 pounds and hadan estimated retail value of $368,550.Diaz was arrested and, after waiving herMiranda rights, agreed to an interview. See Mirandav. Arizona, 384 U.S.436 (1966). Diaz claimed that she had no idea drugs were hiddenin the car. The officers, however, found her story hard to believe.Diaz explained that she was driving her boyfriend’s car.Contradictorily, she also told officers that she had seen herboyfriend only “two, three times tops,” did not know his phonenumber, and did not know where he lived. Response in Opposition inNo. 3:20–cr–02546 (SD Cal.), ECF Doc. 33–1, p.13. Diaz’sstory grew even more dubious when officers questioned her about twocellphones discovered inside the car. She acknowledged that sheowned one of the phones. But, she maintained the other phone hadbeen “given to [her]” by a friend—whom she would “rather not”identify. Id., at 32, 34. And, she insisted that the phonewas “locked” and that she did not “have access to it.” Id.,at 32–33.[1]Diaz was charged with importing methamphetaminein violation of 21 U.S.C. §§952 and 960. The chargesrequired the Government to prove that Diaz “knowingly” transporteddrugs. In response, Diaz asserted what is known colloquially as a“blind mule” defense: she argued that she did not know that therewere drugs in the car. Before trial, the Government gave noticethat it would call Homeland Security Investigations Special AgentAndrew Flood as an expert witness. Agent Flood would testify aboutthe common practices of Mexican drug-trafficking organizations.Specifically, he planned to explain that drug traffickers“generally do not entrust large quantities of drugs to people whoare unaware they are transporting them.” United States’ Notice, ECFDoc. 30, p.7.Diaz objected to Agent Flood’s profferedtestimony under Federal Rule of Evidence 704(b). That Rule providesthat, “[i]n a criminal case, an expert witness must not state anopinion about whether the defendant did or did not have a mentalstate or condition that constitutes an element of the crime chargedor of a defense.” Diaz argued that if Agent Flood testified thatdrug traffickers never use unknowing couriers, that would befunctionally equivalent to an opinion about whether Diaz knowinglytransported drugs. The District Court granted Diaz’s motion in partand denied it in part. The court agreed with Diaz that Agent Floodcould not testify in absolute terms about whether all couriersknowingly transport drugs. But, insofar as Agent Flood planned totestify only that most couriers know they are transporting drugs,the court concluded that his testimony was admissible.At trial, Agent Flood testified that “in mostcirc*mstances, the driver knows they are hired ... totake the drugs from point A to point B.” App. to Pet. for Cert.15a. To use an unknowing courier, Agent Flood explained, wouldexpose the drug-trafficking organization to substantial risk. Theorganization could not guarantee where, if at all, the drugs wouldarrive. Id., at 16a, 26a. Even if the drugs reached theintended destination, the organization would then have to retrievethe drugs without detection. Id., at 16a, 24a–25a. Accordingto Agent Flood, drug-trafficking organizations are often unwillingto take those chances with hundreds of thousands of dollars on theline. Agent Flood acknowledged on cross-examination thatdrug-trafficking organizations sometimes use unknowingcouriers.The jury found Diaz guilty, and the DistrictCourt sentenced her to 84 months’ imprisonment. On appeal, Diazagain challenged Agent Flood’s testimony under Rule 704(b). TheCourt of Appeals held that Rule 704(b) prohibits only “an ‘explicitopinion’ on the defendant’s state of mind.” 2023 WL 314309, *2(CA9, Jan. 19, 2023). Because Agent Flood did not opine aboutwhether Diaz knowingly transported methamphetamine, the courtconcluded that the testimony did not violate Rule 704(b).Ibid.We granted certiorari, 601 U.S. ___(2023), and now affirm.IIFederal Rule of Evidence 704 addresses“Opinion[s] on an Ultimate Issue.” Rule 704(a) sets out a generalrule that “[a]n opinion is not objectionable just because itembraces an ultimate issue.” Rule 704(b) adds one caveat:“Exception: In a criminal case, an expertwitness must not state an opinion about whether the defendant didor did not have a mental state or condition that constitutes anelement of the crime charged or of a defense. Those matters are forthe trier of fact alone.”Rule 704 departed from the once-prevailingcommon-law practice. Prior to Rule 704, many States applied whatwas known as the “ultimate issue” rule. That rule categoricallybarred witnesses from “stat[ing] their conclusions on” any“ultimate issue”—i.e., issues that the jury mustresolve 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 “theexact question which the jury are required to decide” (internalquotation marks omitted)). For example, in a medical malpracticesuit, an ultimate issue may be “whether [the] plaintiff’scondition resulted solely from malpractice.” De Groot v.Winter, 261 Mich. 660, 671, 247 N.W. 69 (1933). In a murdercase, by way of comparison, an ultimate issue may be who fired thegun that killed the victim. See State v. Carr, 196N.C. 129, 131–132, 144 S.E. 698, 700 (1928). Under thecommon-law rule, a witness could not provide his answer to thoseultimate issues. Witnesses remained free, however, to offer relatedtestimony, even testimony that directly helped the jury resolve anultimate issue. See, e.g., Furlong v.Carraher, 108 Iowa 492, 495, 79 N.W. 277, 278 (1899)(holding that witness could not testify about deceased’s mentalcapacity to enter will, but could testify to her “condition of... mind at the time the will was executed”); DeGroot, 261 Mich., at 671, 247 N.W., at 69 (holding thatwitness could testify that plaintiff’s condition could haveresulted from malpractice); Hill v. State, 134 Tex.Crim. 163, 169, 114 S.W.2d 1180, 1183 (1938) (holding that witnesscould testify in a murder case on how “the bruises and wounds onthe deceased’s body could have been caused”).The logic underpinning the ultimate-issue rulewas that it prevented witnesses from taking over the jury’s role.See 1 K. Broun, McCormick on Evidence 80 (7th ed. 2013) (McCormick)(explaining that the rule’s “stated justification” was to excludetestimony that “usurps the function” or “invades the province ofthe jury” (internal quotation marks and footnote omitted)). If awitness gave an opinion “covering the very question which was to besettled by the jury,” some feared that the jury would be left with“no other duty but that of recording the finding of [the]witnes[s].” Chicago & Alton R. Co. v. Springfield& N.W. R. Co., 67 Ill. 142, 145 (1873).Although the ultimate-issue rule’s exact originsare unclear, legal scholars agree that several States had adoptedit 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 onthe battlefield before Troy was no more dense than the oneenshrouding the origins of the [ultimate-issue] rule”). The rulewas short lived though, and courts and commentators came to doubtit* propriety within a matter of decades. See ibid. Manyrejected the idea that ultimate-issue testimony usurps the jury’srole, since a witness’s “credibility” and “the soundness of hisjudgment” “always remain for the jury’s determination.”Goldfoot v. Lofgren, 135 Ore. 533, 541, 296 P. 843,847 (1931). Others labeled the rule “impracticable andmisconceived” because it excluded “the most necessary testimony” onissues where “the jury should have help if it is needed.” 7 Wigmore18–19. By the 1940s, “a trend [had] emerged to abandon” the rulealtogether. 1 McCormick 80. It soon became unclear whether, and towhat extent, the ultimate-issue rule carried any force. SeeStoebuck 236.Rule 704 made clear that the ultimate-issue ruledid not apply in federal courts. When Rule 704 was originallyadopted in 1975, it had no exceptions: All ultimate-issue opinionswere permitted. 88Stat. 1937.About nine years later, in the wake of the JohnHinckley, Jr., trial, Congress created the exception now found inRule 704(b). On March 30, 1981, Hinckley attempted to assassinatePresident Ronald Reagan, shooting and wounding the President andthree other men. See L. Caplan, The Insanity Defense and the Trialof John W. Hinckley, Jr. 7–9 (1984). At his criminal trial,Hinckley claimed that he was insane. Both the prosecution anddefense offered competing expert opinions on the ultimate issue ofHinckley’s sanity. See R. Bonnie, J. Jeffries, & P. Low, A CaseStudy in the Insanity Defense 54 (4th ed. 2021). To the surprise ofmany, Hinckley was found not guilty by reason of insanity. Seeid., at 133; R. Slovenko, The Insanity Defense in the Wakeof the Hinckley Trial, 14 Rutgers L.J. 373 (1982). Congressadopted Rule 704(b) shortly thereafter to carve out an “exception”to Rule 704’s blanket rule admitting ultimate-issue opinions. AsRule 704(b) now reads, “[i]n a criminal case, an expert witnessmust not state an opinion about whether the defendant did or didnot have a mental state or condition that constitutes an element ofthe crime charged or of a defense.”By its terms, Rule 704(b)’s exception covers anarrow set of opinions. The exception does not apply in civil casesor affect lay witness testimony. And, it exclusively addressesmental states and conditions that are “element[s] of the crimecharged or of a defense.” Rule 704(b) thus proscribes only expertopinions in a criminal case that are about a particular person(“the defendant”) and a particular ultimate issue (whether thedefendant has “a mental state or condition” that is “an element ofthe crime charged or of a defense”).IIIRule 704(b) applies only to opinions about thedefendant. Because Agent Flood did not express an opinion aboutwhether Diaz herself knowingly transported methamphetamine, histestimony did not violate Rule 704(b).Agent Flood instead testified about theknowledge of most drug couriers. Specifically, he explainedthat “in most circ*mstances, the driver knows they are hired... to take the drugs from point A to point B.” App. toPet. for Cert. 15a. That opinion does not necessarily describeDiaz’s mental state. After all, Diaz may or may not be like mostdrug couriers. Diaz herself made this point at trial. She arguedthat another person, an alleged boyfriend, had deceived her intocarrying the drugs.[2] Duringopening statements, Diaz’s counsel explained that Diaz met herboyfriend while she was “broken-hearted over the death of hermother” and recovering from “a debilitating back injury.” TrialTr., ECF Doc. 112, pp.140–141. Diaz’s boyfriend “tookadvantage” of those circ*mstances to lure Diaz to Mexico.Id., at 140. As her counsel described it, the boyfriend thenloaned Diaz a car that was secretly loaded with drugs for her driveback to the United States. Diaz supported that story during hercase in chief. She presented an automobile mechanics expert whotestified that there was “no way for someone to suspect or knowthat there was drugs hidden within th[e] car.” Trial Tr., ECF Doc.113, p.62. Diaz also challenged the Government’s contrarytheory. On cross-examination, Diaz’s counsel highlighted that AgentFlood was not involved in Diaz’s case and that the Governmentit*elf was aware of cases involving unknowing couriers.The jury was thus well aware that unknowingcouriers exist and that there was evidence to suggest Diaz could beone of them. It simply concluded that the evidence as a wholepointed to a different conclusion: that Diaz knowingly transportedthe drugs. The jury alone drew that conclusion. While Agent Floodprovided evidence to support one theory, his testimony was justthat—evidence for the jury to consider or reject when decidingwhether Diaz in fact knew about the drugs in her car. Because AgentFlood did not give an opinion “about whether” Diaz herself “did ordid not have a mental state or condition that constitutes anelement of the crime charged or of a defense,” his testimony didnot 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 knowinglytransported drugs when he opined that couriers generally transportdrugs knowingly. Brief for Petitioner 24 (internal quotation marksomitted); see also post, at 7–8 (opinion of Gorsuch, J.).That argument mistakenly conflates an opinion about mostcouriers with one about all couriers. A hypothetical helpsexplain why this distinction matters under Rule 704(b). Take forexample an expert who testifies at an arson trial that all peoplein the defendant’s shoes set fires maliciously (the mental staterequired for common-law arson). Although the expert never spoke thedefendant’s name, the expert nonetheless violated Rule 704(b). Thatis because the expert concluded that the defendant was part of agroup of people that all have a particular mental state. The phrase“all people in the defendant’s shoes” includes, of course, thedefendant himself. So, when the expert testified that all people inthe defendant’s shoes always set fires with malicious intent, theexpert also opined that the defendant had that mental state. Theexpert thus stated an opinion on the defendant’s mental state, anultimate issue reserved for the jury, in violation of Rule704(b).Here, by contrast, Agent Flood asserted thatDiaz was part of a group of persons that may or may not havea particular mental state. Of all drug couriers—a group thatincludes Diaz—he opined that the majority knowingly transportdrugs. The jury was then left to decide: Is Diaz like the majorityof couriers? Or, is Diaz one of theless-numerous-but-still-existent couriers who unwittingly transportdrugs? The ultimate issue of Diaz’s mental state was left to thejury’s judgment. As a result, Agent Flood’s testimony did notviolate Rule 704(b).Diaz and the dissent next zero in on the word“about” in Rule 704(b). They rely on dictionary definitions of“about” to argue that Rule 704(b)’s prohibition includes alltestimony that “‘concerns’ or is ‘in reference to’ whetherthe defendant possessed a particular state of mind.” Brief forPetitioner 18–19; post, at 6–7. But, a word’s meaning isinformed by its surrounding context. See Smith v. UnitedStates, 508 U.S.223, 233 (1993). A crucial part of that context is the otherwords in the sentence. See FCC v. AT&T Inc.,562 U.S.397, 405 (2011). The words surrounding “about” make clear thatRule 704(b) addresses a far narrower category of testimony thanDiaz and the dissent posit. To begin, the Rule targets“opinion[s].” In other words, the testimony must be more than ageneral reference, and it must reach a particular conclusion. SeeBlack’s Law Dictionary 1244 (rev. 4th ed. 1968) (defining opinionevidence as “what the witness thinks, believes, or infers in regardto facts in dispute”). Moreover, the Rule does not precludetestimony “about” mental-state ultimate issues in the abstract.Instead, it targets conclusions “about whether” a certain fact istrue: “[T]he defendant did or did not have a mental state orcondition.” The language as a whole thus conveys that Rule 704(b)is limited to conclusions as to the defendant’s mental state.Rule 704(a) further confirms the narrow scope oftestimony prohibited by Rule 704(b). Recall that the originalultimate-issue rule excluded opinions on the ultimate issue itself.See supra, at 4–5. Rule 704(a) abolished that practice bypermitting testimony that “embraces an ultimate issue.” See 5Oxford English Dictionary 169 (2d ed. 1989) (defining “embrace” as“[t]o include, contain, comprise”). Because Rule 704(b) is an“exception” to Rule 704(a), it can only be understood to cover asubset of the testimony that Rule 704(a) expressly allows. Inshort, since Rule 704(a) permits opinion testimony that includesultimate issues, Rule 704(b) must exclude only a subset of thosesame opinions.The reading offered by Diaz and the dissentwould have the exception swallow the rule. If Rule 704(b) were asbroad as they suggest, it would be a standalone prohibition broaderthan Rule 704(a)—or even the original ultimate-issue rule. Eventhough the ultimate-issue rule and Rule 704(a) address opinionsthat include the ultimate issue itself, Rule 704(b) would prohibitall opinions even related to the ultimate issue of a defendant’smental state. Rule 704’s text does not support such an expansion.The Rule as a whole makes clear that an opinion is “about” theultimate issue of the defendant’s mental state only if it includesa conclusion on that precise topic, not merely if it concerns orrefers to that topic.IVAn expert’s conclusion that “most people” in agroup have a particular mental state is not an opinion about “thedefendant” and thus does not violate Rule 704(b). Accordingly, thejudgment of the Court of Appeals is affirmed.It is so ordered.
Notes
1Between her convictionand sentencing, Diaz confessed that she had fabricated theboyfriend story. She also admitted that she had previously smuggleddrugs into the United States and had volunteered to make the drugrun that led to her arrest.
2Though Diaz lateradmitted the boyfriend never existed, she maintained her storythroughout the trial. N.1, supra.
SUPREME COURT OF THE UNITED STATES_________________No. 23–14_________________DELILAH GUADALUPE DIAZ, PETITIONER v.UNITED STATESon writ of certiorari to the united statescourt of appeals for the ninth circuit[June 20, 2024]Justice Jackson, concurring.I join the Court’s opinion in full. Federal Ruleof Evidence 704(b) forbids expert witnesses in criminal trials fromoffering their “opinion about whether the defendant did or did nothave a mental state or condition that constitutes an element of thecrime charged or of a defense.” As the Court explains, Rule 704(b)is narrow. Against the backdrop of Federal Rules that authorizeadmission of all relevant evidence, it prohibits “only expertopinions ... about a particular person (‘thedefendant’) and a particular ultimate issue (whether the defendanthas ‘a mental state or condition’ that is ‘an element of the crimecharged or of a defense’).” Ante, at 7. But, as narrow as itis, Rule 704(b) strikes a very important balance: It allows forpotentially highly probative expert testimony to be submitted tothe jury, while leaving “[t]he ultimate issue of [the defendant’s]mental state ... to the jury’s judgment.” Ante,at 9.I write separately to emphasize that, asCongress designed it, Rule 704(b) is party agnostic. Neither theGovernment nor the defense can call an expert to offer her opinionabout whether the defendant had or did not have a particular mentalstate at the time of the offense. See ante, at 7. But acorollary is also true. Both the Government and the defense arepermitted, consistent with Rule 704(b), to elicit expert testimony“on the likelihood” that the defendant had a particular mentalstate, “based on the defendant’s membership in a particular group.”Brief for John Monahan etal. as Amici Curiae 1(Evidence Professors Brief). Indeed, the type of mental-stateevidence that Rule 704(b) permits can prove essential not only forprosecutors, but for defendants as well.IThis very case illustrates the significance ofmental-state evidence to both parties in a criminal trial. TheGovernment expert opined (based on his almost 30 years ofexperience as a special agent) that, “in most circ*mstances,” drugcouriers know that they are transporting drugs. App. to Pet. forCert. 10a, 15a. Diaz challenged this testimony, and, today, theCourt holds that the Government did not violate Rule 704(b). Seeante, at 7. Notably, however, the Government was not theonly party that relied on this type of mental-state evidence duringthe trial. Diaz called an automobile specialist who testified thata driver of her particular car would almost certainly notknow that it contained drugs. See Supp. Excerpts of Record in No.21–50238 (CA9), pp. 139–159. That type of evidence is permissibleunder the interpretation of Rule 704(b) the Court adoptstoday.[1]* Moreover, as thedissent observes, Diaz might have opted to introduce other types ofexpert evidence related to the mental-state element. Seepost, at 10 (opinion of Gorsuch,J.). For example, Diazcould have offered expert testimony on the prevalence andcharacteristics of unknowing drug couriers. See Tr. of Oral Arg.24; see also Brief for National Association of Federal Defenders asAmicus Curiae 5–16 (NAFD Brief) (describing numerouscases involving so-called “blind mules”).For the reasons described in today’s opinion,none of that evidence would deprive the jury of its ability todecide the last link in the inferential chain: whether Diaz herselfhad the requisite mens rea. But, at the same time, havingall of this testimony might have helped the jury determine whetherthe Government had met—or failed to meet—its burden of proving thatDiaz knew of the drugs found in her car. Thus, far from disservingour criminal justice system, see post, at 10, the type ofmental-state evidence that Rule 704(b) permits can be of criticalassistance to lay factfinders tasked with determining a defendant’smental state as an element of the alleged crime (or defense).Other examples provide further proof. Considerexpert evidence on mental health conditions. Congress crafted Rule704(b) to prohibit experts from opining on a particular defendant’smental state at the time of an offense, but it did not precludeexperts from contextualizing a defendant’s mental health condition,including by explaining the likelihood that those with a particularcondition would have a particular mental state. For example, asDiaz acknowledges, the interpretation of Rule 704(b) the Courtadopts today “allow[s] psychiatrists who testify as experts to... tell the jury that when people with schizophreniaas severe as [a] defendant’s commit acts of violence, it isgenerally because they do not appreciate the wrongfulness of theirconduct.” Brief for Petitioner 21–22; see also Brief for UnitedStates 35–36. That type of expert evidence would not result in thespectacle of dueling experts on the defendant’s actual mentalstate, which Congress sought to eliminate when it codified Rule704(b). See ante, at 6–7; see also S.Rep. No. 98–225,p. 230 (1983). Instead, given the biases, stereotypes, and unevenknowledge that many people have about mental health conditions,such expert evidence could help jurors better understand adefendant’s condition and thereby call into question a mensrea that might otherwise be too easily assumed. 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 todomestic abuse. “A number of myths and misconceptions about[battered woman syndrome] victims affect our criminal justicesystem,” and it is clear that those mistaken views “affect jurors.”Linn v. State, 929 N.W.2d 717, 742 (Iowa 2019); seealso id., at 742–746 (summarizing relevant evidence). Rule704(b) allows experts to testify about the typical mental states ofthose with battered woman syndrome, helping jurors to betterunderstand how those experiencing it respond to aggression or reactto 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’sattempts both to disprove the mens rea in a number ofserious crimes and to support a range of defenses, including duressand self-defense. See, e.g., United States v.Lopez, 913 F.3d 807, 819–824 (CA9 2019); UnitedStates v. Nwoye, 824 F.3d 1129, 1136–1138 (CADC2016).IIAll that said, I fully acknowledge that thereare serious and well-known risks of overreliance on experttestimony—risks that are especially acute in criminal trials. SeeNAFD Brief 21–22, 24–25; see also United States v.Alvarez, 837 F.2d 1024, 1030 (CA11 1988) (“When the expertis a government law enforcement agent testifying on behalf of theprosecution about participation in prior and similar cases, thepossibility that the jury will give undue weight to the expert’stestimony is greatly increased”). But there are also safeguardsoutside of Rule 704(b) to prevent the misuse of expert testimony.Nothing in the Court’s opinion today should be read to displacethose important checks and limitations.This means, of course, that when faced withflawed or faulty testimony concerning the mental states of groupsor categories of individuals, parties can utilize the traditionaltools in a lawyer’s toolkit, like vigorous cross-examination andcareful refutation in closing argument. Parties can also seek toemploy other Rules of Evidence that might require exclusion—thosethat guard against irrelevant or unduly prejudicial testimony, forexample, and those that require courts to bar unqualified oroverreaching experts. See Fed. Rules Evid. 401, 402, 403, 702; seealso, e.g., United States v. Finley,301 F.3d 1000, 1014–1015 (CA9 2002) (“Expert testimony thatcompels the jury to conclude that the defendant did or did notpossess the requisite mens rea does not ‘assist the trier offact’ under Rule 702 because such testimony encroaches on thejury’s vital and exclusive function to make credibilitydeterminations”); United States v. Lipscomb,14 F.3d 1236, 1242 (CA7 1994) (describing safeguards that canbe used to prevent testimony from law enforcement experts fromunduly prejudicing a defendant).District court judges also have a role to play.They should be protective of Congress’s intent to preserve thejury’s core duty, by providing specific admonitions andinstructions when expert testimony about a relevant mental state isintroduced. See Evidence Professors Brief 27–29; see also UnitedStates v. Smart, 98 F.3d 1379, 1388–1389 (CADC 1996) (requiring that districtcourts sometimes use jury instructions to prevent expert testimonyfrom violating Rule 704(b)).With this understanding of both the importantuses and the potential misuses of Rule 704(b), I join the Court’sopinion.
Notes
1*Before this Court, Diazforthrightly admits that such evidence would be impermissible underthe dissent’s interpretation of Rule 704(b). See Reply Brief 8; Tr.of Oral Arg. 27.
SUPREME COURT OF THE UNITED STATES_________________No. 23–14_________________DELILAH GUADALUPE DIAZ, PETITIONER v.UNITED STATESon writ of certiorari to the united statescourt of appeals for the ninth circuit[June 20, 2024]Justice Gorsuch, with whom Justice Sotomayorand Justice Kagan join, dissenting.Federal Rule of Evidence 704(b) prohibits anexpert witness from offering an opinion “about whether thedefendant did or did not have [the] mental state” needed to convicther of a crime. “Those matters,” the Rule instructs, “are for thetrier of fact alone.” Following the government’s lead, the Courttoday carves a new path around that command. There’s no Rule 704(b)problem, the Court holds, as long as the government’s expert limitshimself to testifying that most people like the defendanthave the mental state required to secure a conviction.The upshot? The government comes away with apowerful new tool in its pocket. Prosecutors can now put an experton the stand—someone who apparently has the convenient ability toread minds—and let him hold forth on what “most” people like thedefendant think when they commit a legally proscribed act. Then,the government need do no more than urge the jury to find that thedefendant is like “most” people and convict. What authority existsfor allowing that kind of charade in federal criminal trials isanybody’s guess, but certainly it cannot be found in Rule 704.IDelilah Diaz’s conviction for drug traffickingturned on her state of mind. In that, hers was an everyday case.Often in our criminal justice system, the difference betweenfreedom and years in prison turns on just that question. Perhaps ithas always been so. The government’s duty to prove that thedefendant it seeks to convict had a culpable state of mind whencommitting a proscribed act is as ancient as it is fundamental toour system of justice. At common law, “a complete crime” generallyrequired “both a will” (or mens rea) “and an act” (oractus reus). 4 W. Blackstone, Commentaries on the Laws ofEngland 21 (1769) (Blackstone). That same view “took deep and earlyroot in American soil” where, to this day, a crime ordinarilyarises “only from concurrence of an evil-meaning mind with anevil-doing hand.” Morissette v. United States,342 U.S.246, 251–252 (1952); see 1 J. Bishop, Commentaries on theCriminal Law §291, p. 163 (6th ed. 1877) (Bishop). So ingrained isthis view that courts have long presumed criminal statutes demandproof of mens rea even when they are “silent” on thesubject. Morissette, 342 U.S., at 252; seeStaples v. United States, 511U.S. 600, 605 (1994).Why does our law generally insist not just on abad act but also a culpable state of mind? A significant part of ithas to do with respect for the individual and his liberty in a freesociety. “Criminal liability imports a condemnation, the gravestwe,” as a Nation, “permit ourselves to make.” H. Wechsler, AmericanLaw Institute II–A Thoughtful Code of Substantive Law, 45 J. Crim.L. & C. 524, 528 (1955) (Wechsler); see also 4 Blackstone20–21; 1 Bishop §287, at 161. Of course, our law recognizesgradations of mens rea, ranging from purpose and knowledgeto recklessness and negligence. See, e.g., ALI, Model PenalCode §2.02 (1985); United States v. Bailey, 444 U.S.394, 404 (1980). But to subject a presumptively free individualto serious punishments for acts undertaken without proof of any ofthat would be “the badge of tyranny, the plainest illustration ofinjustice.” Wechsler 528. The principle “that an injury canamount to a crime only when inflicted” with some accompanyingmens rea is, we have said, “as universal and persistent inmature systems of law as belief in freedom of the human will and aconsequent ability and duty of the normal individual to choosebetween good and evil.” Morissette, 342 U.S., at250.At trial, deciding whether a criminal defendantacted with a culpable mental state is a job for the jury. No matterhow “clear the proof” or “incontrovertible” the inference,the question whether a defendant possessed a culpable mensrea “must always be submitted to the jury.” Id., at 274(internal quotation marks omitted). Always, too, the governmentbears the burden of proving the requisite mens rea. Never,we have held, may the government seek to “shift the burden of proofto 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 mustpresume a defendant’s state of mind from any particular set offacts, no matter how compelling they may be. Francis v.Franklin, 471 U.S.307, 316 (1985).Reflecting the centrality of mens rea tocriminal punishment and the jury’s role in finding it, Rule 704(b)of the Federal Rules of Evidence provides that, “[i]n a criminalcase, an expert witness must not state an opinion about whether thedefendant did or did not have a mental state or condition thatconstitutes an element of the crime charged or of a defense.” Asthe Rule continues: “Those matters are for the trier of factalone.”By all accounts, the immediate impetus for theRule was the trial of John Hinckley for the attempted assassinationof President Ronald Reagan. Ante, at 6. In that case,experts didn’t just offer competing views on whether Hinckleysuffered from a medically diagnosable mental illness. They wentmuch further. The trial descended into a battle between experts whoclaimed to know exactly what Hinckley was (or was not) thinking atthe moment he pulled the trigger. Ibid.In the trial’s aftermath, Congress continued torecognize the value of expert mental health evidence. So, forexample, an expert may still testify that the defendant sufferedfrom some diagnosable illness or syndrome at the time of thecharged act and discuss its symptoms. Cf. ante, at 3–4(Jackson, J., concurring) (discussing schizophrenia and batteredwoman syndrome). From testimony like that, a jury might infer thatthe defendant did not have the requisite mental state to convict.But in Rule 704(b) Congress declared that task belonged to the juryalone, and allowing a parade of witnesses to speculate about whatdid or did not transpire in the head of a particular defendant at aparticular moment in the past did not reflect well on federaljudicial proceedings and did not aid the jury.Rule 704(b) may have been a new addition to theFederal Rules of Evidence, but it reflects a much older tradition.For centuries, courts have grappled with the role expert witnessesshould play at trial. See, e.g., 1 S. Greenleaf, Evidence§440, p. 489 (1842); Folkes v. Chadd, 3 Dougl. 157,158–159, 99 Eng. Rep. 589, 590 (K. B. 1782). For a long stretch,many courts barred experts from offering opinions on so-calledultimate issues like mens rea. See 3 J. Wigmore, Evidence§§1920, 1921 (1904); United States v. Spaulding,293 U.S.498, 506 (1935). The Federal Rules of Evidence are no longer sostrict, see Fed. Rule Evid. 704(a), except in one respect: mensrea. On that particular issue, Congress has concluded thatjurors need no help from experts. They are fully capable of drawingreasonable inferences from the facts and deciding whether thedefendant acted with the requisite mens rea. And in criminaltrials that is their job alone.IIThe government violated that Rule in thiscase. Proceedings began when prosecutors charged Ms. Diaz withimporting a controlled substance into this country. See 21U.S.C. §§952, 960(a)(1). At the trial that followed,Ms. Diaz did not dispute that she had transported drugs across theborder. The only question concerned her mens rea. If, as thegovernment charged, she transported the drugs “knowingly,” shefaced a potential sentence of up to life in prison. See§§960(a)(1), (b)(1)(H). If, however, Ms. Diaz acted with somelesser mens rea (say, negligence), or perhaps innocently (aswhat some call a “blind mule”), she was entitled to anacquittal.To help prove that Ms. Diaz “knowingly” importeddrugs, the government called to the stand Andrew Flood, one of itsown employees, an agent with the Department of Homeland Security.Ms. Diaz had made no admissions to him about her mental state, norhad Agent Flood even interviewed her. Instead, prosecutors calledAgent Flood as an expert on the minds of drug couriers (yes,really). App. 17; Notice in No. 3:20–cr–02546 (SD Cal.), ECF Doc.30, p. 7. And in response to the government’s questions, AgentFlood testified that, “in most circ*mstances, the driver knows theyare hired ... to take the drugs from point A to pointB.” App. to Pet. for Cert. 15a.That was a violation of Rule 704(b), plain asday. Just walk through its terms. The government called Agent Floodas an “expert witness” to address the question “whether thedefendant 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 questionat trial, all that separated her from a conviction. And Agent Floodproceeded to do just as he was asked, offering an “opinion about”that very question.To be sure, prosecutors thought they had aclever way around the problem. They did not ask Agent Flood totestify explicitly about Ms. Diaz’s mental state. Instead, theyasked the agent to testify about the mental state of people exactlylike Ms. Diaz, drivers bringing drugs into the country. And that,the prosecutors argued, made all the difference. See App. 32a;Brief for United States in No. 21–50238 (CA9), pp. 46, 58. TheNinth Circuit endorsed the government’s maneuver, holding that Rule704(b) prohibits only testimony “‘explicit[ly]’” aboutthe defendant’s mental state, not testimony about the mental stateof a class of persons that includes her. App. to Pet. for Cert. 6a(quoting United States v. Gomez, 725 F.3d 1121, 1128(2013)).Before us, however, even the government disavowsthe full implications of that reasoning. Now, it concedes, the Ruledoes more than bar an expert from testifying “explicitly” that thedefendant had the mental state required for conviction. Tr. of OralArg. 72–73, 76. The Rule also bars an expert from testifying that aclass of persons (say, all people carrying drugs over the border)has the legally proscribed mental state when that class includesthe defendant. Brief for United States 36; ante, at 9.Likewise, the Rule bars an expert from opining that a hypotheticalperson who matches the defendant’s description (say, a hypotheticalwoman who drives a car full of drugs across the border) will havethe mental state required for conviction. Tr. of Oral Arg. 67. Allthose opinions, the government now acknowledges, are “about” thedefendant’s mental state and cannot be offered consistent with Rule704(b). On this, the Court, too, agrees. Ante, at 9.IIISo what is left? Instead of vacating andremanding the case to the Ninth Circuit to correct its error, thegovernment asks us to affirm its judgment on other grounds. As thegovernment sees it, Agent Flood’s opinion was permissible for adifferent reason than the Ninth Circuit offered. It waspermissible, the government says, because it wasn’tdefinitive. So, yes, an expert cannot testify thatall persons in a class that includes the defendant have aculpable mental state. Brief for United States 36. But, thegovernment insists, everything changes when an expert offers (asAgent Flood offered) only a probabilistic assessment thatmost such persons do.I cannot see how that gambit begins to solve thegovernment’s problem. The Rule does not only prohibit an expertfrom stating a definitive opinion about the defendant’smental state (or, as the government concedes, the mental state of aclass that includes her). It prohibits an expert from offeringany opinion on the subject. Return, once more, to the Rule’sterms. It bars an expert from stating an opinion “aboutwhether the defendant” had “a mental state ... thatconstitutes an element of the crime charged.” (Emphasis added.) Theword “about” means “[c]oncerning, regarding, with regard to, inreference to; in the matter of.” Oxford English Dictionary (3d ed.,June 2024); see Brief for Petitioner 18; see also American HeritageDictionary 5 (def. 4a) (5th ed. 2011). So whether an expert’sopinion happens to be definitive or probabilistic makes nodifference. An expert may not state any opinion concerning,regarding, or in reference to whether the defendant, whilecommitting a charged criminal act, had the requisite mental stateto convict. Period. Lest any doubt remain, the Rule takes pains toemphasize, “[t]hose matters are for the trier of fact alone.”Consider, too, how the government’s presenttheory collapses into the one it has disavowed. Just imagine ifa*gent Flood had explicitly addressed Ms. Diaz and said she “mostlikely knew” she was carrying drugs. Would that testimony bepermissible under Rule 704(b)? Of course not. Probabilistic thoughthe testimony may be, an expert who says that an individualdefendant “most likely” had the requisite mental state forconviction offers an opinion about, concerning, regarding, or inreference to her mental state. On that, no dispute exists. So howcan it be, as the government insists, that an expert may offer theprobabilistic assessment that “most” people like the defendant knowthey are carrying drugs? The only difference between the twoopinions is that the first addresses the defendant “explicitly,”the second a class that includes her. All of which returns us to adistinction that the government itself seems to acknowledge theRule does not tolerate.Observe, as well, where today’s tiptoeing aroundthe Rule promises to lead. The Court adopts the government’smuddled view that an expert cannot offer a probabilistic opinionabout the mental state of the defendant explicitly but can offer aprobabilistic opinion about the mental state of a group thatincludes the defendant. So what happens next? In this case, AgentFlood said “most” people in the defendant’s shoes have therequisite mens rea. But what if he said, as the governmentinitially proffered, that drivers “generally” know? ECF Doc. 30, at7. Or that they “almost always” know? Or perhaps an expert puts afiner point on it: “In my experience, 99% of drug couriers know.”When cases like those come to us, likely one of two things willhappen. We will draw some as-yet unknown line and say an expert’sprobabilistic testimony went too far. Or we will hold anything goesand eviscerate Rule 704(b) in the process. Rather than face eitherof those prospects, how much easier it would be to follow where theRule’s text leads.IVThe government’s approach, adopted by theCourt today, is no more necessary than it is appropriate. Yes,proving a defendant’s mental state at trial can require work.Normally, it will require the government to resort tocirc*mstantial evidence and inference. After all, defendants inlife do not confess their inner thoughts on the stand nearly asoften as they do in courtroom dramas. But there is nothing newabout any of that. See 4 Blackstone 21 (“no temporal tribunal cansearch the heart, or fathom the intentions of the mind, otherwisethan as they are demonstrated by outward actions”). Nor is it anysecret that the government has a long track record of success inproving mens rea the old-fashioned way by presentingcirc*mstantial evidence and appealing to reasonable inferences.This case illustrates how it can be—andregularly is—done. To persuade the jury that Ms. Diaz knew aboutthe drugs, the government could point to the amounts involved—54pounds of drugs worth over $360,000. Ante, at 2. It couldalso point to the holes in her story. She claimed the car was herboyfriend’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. ECFDoc. 33–1, at 13, 32. The government could point out, too, thatwhen cell phones were found in the car, Ms. Diaz maintained one ofthem belonged to a friend, someone she would “rather not” identify.Id., at 34. As well, the government could highlight herstatement that the phone was “locked” and she did not “have accessto it.” Id., at 32–33. And the government could then ask ajury to infer from all these facts that Ms. Diaz knew exactly whatshe was doing. As it argues to us, the government was free to argueto a jury, asking it to conclude that Ms. Diaz’s story was“transparently flimsy.” Brief in Opposition 16. Day in and day out,the government secures convictions for the knowing importation ofdrugs in just this way. Tr. of Oral Arg. 84. There was no need togild the lily by calling to the stand an “expert” in mindreading.And there is certainly no cause for this Court to sanction thepractice.To the contrary, there are sound reasons whyRule 704(b) operates as it does. The problem of junk science in thecourtroom is real and well documented. See Daubert v.Merrell Dow Pharmaceuticals, Inc., 509U.S. 579, 589–590 (1993); Kumho Tire Co. v.Carmichael, 526 U.S.137, 147 (1999); see also P. Huber, Galileo’s Revenge: JunkScience in the Courtroom 15–17 (1991). And perhaps no “science” ismore junky than mental telepathy. After Hinckley’s trial, Congressrecognized as much when it adopted Rule 704(b) to remove fromfederal courtrooms experts who claim to know what was inside aman’s head at a particular moment in the past when he committed aparticular act.The particular nook of the criminal law we findourselves in today illustrates the soundness of Congress’sapproach. Not long ago, the government tried—often successfully—toput “experts” (really, like Agent Flood, its own law enforcementagents) on the stand to testify that all couriers know whenthey are carrying drugs. See, e.g., United States v.Flores, 510 Fed. Appx. 594, 595 (CA9 2013). Not only wasthat testimony improper under the government’s own currentunderstanding of Rule 704(b). See Part II, supra;ante, at 9. Eventually, the government felt it had tobacktrack after being confronted with too much evidence that somecouriers simply have no idea they are being used to carry drugs.Flores, 510 Fed. Appx., at 595. So now, thegovernment puts on witnesses to say most couriers know. Wecannot be certain how many individuals sit in federal prisonbecause of the government’s past impermissible and mistaken“expert” testimony that all couriers know when they arecarrying drugs. About the only thing we can be sure of is that whatis good for the goose is good for the gander and that, thanks tothe Court’s opinion today, defendants will now recruit their ownwarring experts. Ones who will seek to testify (not unlike AgentFlood) that, in their experience, “most” drug couriers are kept inthe dark by cartels.None of this serves our criminal justice systemwell. A criminal conviction is “the gravest” condemnation we as asociety “permit ourselves to make.” Wechsler 528. Allowing into ourproceedings speculative guesswork about a defendant’s state of minddiminishes the seriousness due them. It risks the reliability ofthe outcomes they produce (just ask those convicted in cases wheregovernment experts opined that “all” couriers know). It underminesour historic commitment that mens rea is a necessarycomponent of every serious crime by turning the inquiry into adefendant’s mental state from an exacting one guided by hard factsand reasonable inferences into a competing game of “I say so.” Itdiminishes our respect for the presumptively free person, his freewill and individuality, by encouraging the lazy assumption that hethinks like “most.” And it reduces the vital role juries are meantto play in criminal trials. Yes, they can still decide whether thedefendant thinks like “most” people. Ante, at 9. Butthat role hardly matches Rule 704(b)’s promise that “matters” ofmens rea at trial belong to the jury “alone.”VIn describing what I see as some of thepossible consequences of the government’s approach adopted by theCourt today, I do not mean to suggest they are inevitable. Today’sdecision may go a long way toward hollowing out Rule 704(b). But itdoes not address what any other Rule of Evidence may have to sayabout cases like this one. And, looking briefly to some of thoseother Rules, I see reason for hope.Take a few examples. Under Rule 402, anyevidence presented at trial must be “[r]elevant,” meaning it musthave a “tendency to make ... more or less probable” a“fact ... of consequence in determining the action.”Fed. Rules Evid. 401, 402. Yet, if the government is right that anexpert opinion about the mental state of “most” people like thedefendant is not “about” the defendant’s mental state, it ishard to see how that opinion might be relevant. After all, the“fact of consequence” in cases like Ms. Diaz’s is whether thedefendant possessed the requisite mens rea. And it’s hard tosee how the government can have it both ways—asserting in onebreath that opinions like Agent Flood’s are not “about”whether the defendant possessed the requisite mental state toconvict, while insisting in the next breath that those opinionsare relevant to (or, one might say, “about”) the defendant’smental state.Rule 403 stands as another bulwark. That Rulepermits courts to “exclude relevant evidence” when its “probativevalue is substantially outweighed by a danger of ...unfair prejudice.” Surely, in our system of justice—where werecognize that each individual is presumed innocent and distinctlyendowed with free will and choice, where the individual isresponsible for his culpable mental states but not those ofothers—testimony about what “most” people think bears minimalprobative value when the question at issue is what thisindividual thinks. Nor can the kind of testimony offered here holdmuch probative value when juries, composed of the defendant’speers, are well suited to resolve questions of mens reawithout “expert” assistance. Juries have managed that task forcenturies and, as we have long recognized, they are “fitted for itby their natural intelligence and their practical knowledge of menand the ways of men.” Aetna Life Ins. Co. v. Ward,140 U.S.76, 88 (1891).Meanwhile, the danger of unfair prejudice canrun very high. It can be “difficult for the individual to make hisown case stand on its own merits in the minds of the juror[s]” whenjurors are told by an expert “that birds of a feather are flockedtogether.” Krulewitch v. United States, 336 U.S.440, 454 (1949) (Jackson, J., concurring). As this Court hasrecognized, too, expert opinions about the defendant’s “state ofmind at the crucial moment” when committing a criminal act may“easily mislead” the jury into “thinking the opinions show morethan they do.” Clark v. Arizona, 548 U.S.735, 776 (2006). Even the government candidly admits Rule 403challenges may be proper against such testimony. Brief for UnitedStates 30–31.The risk of unfair prejudice can be exacerbated,too, where, as here, the professed expert “carries with [him] theimprimatur of the [g]overnment.” United States v.Young, 470 U.S.1, 18 (1985). A witness like that “may induce the jury to trust[the witness’s] judgment rather than its own view of the evidence.”Id., at 18–19; see also United States v.Scheffer, 523 U.S.303, 314 (1998) (plurality opinion) (experts like these mayattain an “aura of infallibility”). For precisely that reason, thegovernment may be highly tempted to do as it did in this case andseek to throw in an “expert” on top of a seemingly strongcirc*mstantial case—just to be sure. But none of that means theproffered testimony is likely to advance the promise of a fairtrial.Add to those Rules at least one more. As part ofits “gatekeeping” functions, a federal court must ensure that anyexpert testimony it permits is reliable, grounded on widelyaccepted principles, and will “‘assist the trier of fact tounderstand the evidence.’” Kumho Tire Co, 526U.S., at 147 (quoting Fed. Rule Evid. 702(a) (1999)). Istruggle to see how a witness claiming to offer an opinion aboutanother person’s (or class of persons’) thoughts at a particularmoment in the past can meet any of those standards. No one, atleast outside the fortuneteller’s den, can yet claim the power toconjure reliably another’s past thoughts. Testimony like AgentFlood’s may be dubiously circular, too. For each time a lawenforcement agent takes the stand to say “most people know” andthat helps the government secure another conviction, he himself iscreating the very proof on which a government expert may purport torely in the next trial.Nor does testimony like that help the juryunderstand “‘experience[s] confessedly foreign in kind to[its] own.’” Kumho Tire Co., 526 U.S., at 149(quoting L. Hand, Historical and Practical Considerations RegardingExpert Testimony, 15 Harv. L. Rev. 40, 54 (1901)). In a criminaltrial, expert testimony about DNA testing or the chemicalcomposition of illegal drugs may sometimes help a jury understandfacts they do not encounter in daily life. But none of that holdstrue when it comes to the job of assessing whether a defendant’sstory about her state of mind is credible or (as the governmentputs it) “transparently flimsy.” Brief in Opposition 16. Jurors aremore than up to performing that task, and they hardly need the helpof some clairvoyant.*Persuaded that today’s decision is mistaken,but hopeful that it will ultimately prove immaterial in practice, Irespectfully dissent.
