Amicus Curiae Brief — Delilah Guadalupe Diaz, Petitioner v. United States

No. 23-14

In the Supreme Court of the United States

DELILAH GUADALUPE DIAZ,

Petitioner,

v.

UNITED STATES OF AMERICA,

Respondent.

ON WRIT OF CERTIORARI TO THE UNITED STATES

COURT OF APPEALS FOR THE NINTH CIRCUIT

BRIEF OF AMICUS CURIAE NATIONAL ASSOCIATION OF CRIMINAL DEFENSE LAWYERS

IN SUPPORT OF PETITIONER

SEAN HECKER

DAVID PATTON

SEAN KELLY

KAPLAN HECKER & FINK LLP

350 Fifth Avenue

63rd Floor

New York, NY 10118

CARMEN IGUINA GONZÁLEZ

Counsel of Record

JOSHUA MATZ

KAPLAN HECKER & FINK LLP

1050 K Street NW|Suite 1040

Washington, DC 20001

(212) 763-0883

ciguinagonzalez@kaplanhecker.com

JOSHUA L. DRATEL

LAW OFFICES OF

DRATEL & LEWIS

29 Broadway, Suite 1412

New York, NY 10006

Counsel for Amicus Curiae

i

TABLE OF CONTENTS

TABLE OF AUTHORITIES ...................................... iii

INTEREST OF AMICUS CURIAE.............................1

INTRODUCTION & SUMMARY OF

ARGUMENT ................................................................1

ARGUMENT ................................................................4

I.

RULES 702 AND 704(b) ARE FAILING TO

ACHIEVE THEIR CORE PURPOSE

WHEN IT COMES TO “OFFICER

EXPERTS” ........................................................4

A. The Evolution of the Expert Witness and

Rise of the “Officer Expert” ..............................5

B. The Rise of Guardrails on Expert

Testimony Under Rules 702 and 704(b) ..........7

1. Rule 702 safeguards the jury from

irrelevant and unreliable evidence .............8

2. Rule 704(b) protects the jury from unhelpful

expert

testimony

that

intrudes on its role as finder of fact ..........10

C. The Guardrails Imposed by Rules 702 and

704(b) Are Malfunctioning in Too Many

Cases Involving Officer Experts .....................11

1. Courts are failing their gatekeeping

duties when officer experts testify ............11

2. Courts under-enforce Rule 704(b)’s

limits on expert testimony ........................13

ii

II.

THE REFLEXIVE ACCEPTANCE OF

“OFFICER EXPERT” TESTIMONY ON

ISSUES OF MENS REA RAISES

PRACTICAL AND CONSTITUTIONAL

CONCERNS ....................................................15

A. Courts and Criminal Defendants Lack

Effective Means of Testing Officer Expert

Testimony ........................................................16

1. Courts cannot meaningfully test

experience-based officer testimony ...........16

2. Often, defendants can neither crossexamine nor rebut experience-based

“officer expert” testimony ..........................20

B. Officer experts often present an aura of

trustworthiness but lack neutrality ...............25

C. The Near Automatic Admission of Officer

Expert Testimony Leads to Constitutional

Concerns ..........................................................26

1. Right to Confrontation ..............................27

2. Right to a Fair Trial ..................................28

III.

ALLOWING OFFICER EXPERTS TO

TESTIFY ABOUT CLASSWIDE MENS

REA WOULD POSE ADDITIONAL

RISKS ..............................................................30

CONCLUSION ..........................................................32

iii

CASES

TABLE OF AUTHORITIES

Ake v. Oklahoma,

470 U.S. 68 (1985) .................................................. 25

Buck v. Davis,

580 U.S. 100 (2017) ................................................ 30

California v. Green,

399 U.S. 149 (1970) ................................................ 23

Carella v. California,

491 U.S. 263 (1989) (per curiam) .................... 11, 28

Crawford v. Washington,

541 U.S. 36 (2004) ............................................ 27, 28

Daubert v. Merrell Dow Pharms., Inc.,

509 U.S. 579 (1993) ...... 4, 8, 9, 11, 16, 17, 18, 20, 23

Estelle v. Williams,

425 U.S. 501 (1976) ................................................ 28

General Electric Co. v. Joiner,

522 U.S. 136 (1997) .................................................. 9

Kumho Tire Co. v. Carmichael,

526 U.S. 137 (1999) ................................ 9, 16, 17, 19

Melendez-Diaz v. Massachusetts,

557 U.S. 305 (2009) .................................................. 4

Sandstrom v. Montana,

442 U.S. 510 (1979) ................................................ 28

Southern Union Co. v. United States,

567 U.S. 343 (2012) ................................................ 14

Tome v. United States,

513 U.S. 150 (1995) ................................................ 10

iv

United States v. Abel,

469 U.S. 45 (1984) .................................................. 26

United States v. Alvarez,

837 F.2d 1024 (11th Cir. 1988) .............................. 25

United States v. Augustin,

661 F.3d 1105 (11th Cir. 2011) .............................. 13

United States v. Campos,

217 F.3d 707 (9th Cir. 2000) ............................ 18, 27

United States v. Davis,

397 F.3d 173 (3d Cir. 2005) ................................... 18

United States v. DiDomenico,

985 F.2d 1159 (2d Cir. 1993) ................................. 11

United States v. Dukagjini,

326 F.3d 45 (2d Cir. 2003) ............................... 20, 21

United States v. Gonzalez-Rodriguez,

621 F.3d 354 (5th Cir. 2010) .................................. 13

United States v. Hayat,

710 F.3d 875 (9th Cir. 2013) .................................. 31

United States v. Holguin,

51 F.4th 841 (9th Cir. 2022) ............ 6, 16, 17, 21, 23

United States v. Lightfoot,

228 F.2d 861 (7th Cir. 1956) .................................. 31

United States v. Lopez-Lopez,

282 F.3d 1 (1st Cir. 2002) ...................................... 18

United States v. Maher,

454 F.3d 13 (1st Cir. 2006) .................................... 28

United States v. Medina-Copete,

757 F.3d 1092 (10th Cir. 2014) .............................. 23

v

United States v. Mejia,

545 F.3d 179 (2d Cir. 2008) ............. 7, 27, 28, 29, 31

United States v. Mitchell,

996 F.2d 419 (D.C. Cir. 1993) ................................ 13

United States v. Mrabet,

No. 23 Cr. 69, 2023 WL 8179685

(S.D.N.Y. Nov. 27, 2023) ........................................ 19

United States v. Nixon,

418 U.S. 683 (1974) .................................................. 4

United States v. Parra,

402 F.3d 752 (7th Cir. 2005) .................................. 18

United States v. Valencia-Lopez,

971 F.3d 891 (9th Cir. 2020) .................................. 19

United States v. Young,

745 F.2d 733 (2d Cir. 1984) ................................... 25

Weisgram v. Marley Co.,

528 U.S. 440 (2000) ............................................ 8, 11

RULES

Fed. R. Evid. 702 ..................... 2-5, 7-11, 13, 15, 16, 18

Fed. R. Evid. 702(c) ..................................................... 9

Fed. R. Evid. 702, Advisory Committee Notes to the

2000 amend. ........................................................... 10

Fed. R. Evid. 702, Advisory Committee Notes to the

2023 amend. ........................................................... 10

Fed. R. Evid. 704(b)

...................... 1-5, 7, 10, 11, 13, 14, 15, 19, 23, 26-32

vi

OTHER AUTHORITIES

David L. Faigman et al., Mod. Sci. Evidence

(2023-2024 Ed.) ...................................................... 18

Brian R. Gallini, To Serve and Protect? Officers As

Expert Witnesses in Federal Drug Prosecutions,

19 Geo. Mason L. Rev. 363 (2012). .............. 7, 12, 29

Stephen Garvey et al., Juror First Votes in

Criminal Trials, 1 J. Empirical Legal Stud. 371

(2004) ...................................................................... 26

Tal Golan, Revisiting the History of Scientific

Expert Testimony, 73 Brook. L. Rev. 879 (2008)..... 5

Samuel Gross et al., Expert Information and

Expert Evidence: A Preliminary Taxonomy, 34

Seton Hall L. Rev. 141 (2003).......................... 17, 26

Jennifer L. Groscup et al., The Effects of Daubert

on the Admissibility of Expert Testimony in

State and Federal Criminal Cases, 8 Psychol.

Pub. Pol’y & L. 339 (2002) ..................................... 12

Learned Hand, Historical and Practical

Considerations Regarding Expert Testimony, 15

Harv. L. Rev. 40 (1901) ........................................ 4, 5

Dana R. Hassin, How Much is Too Much? Rule

704(b) Opinions on Personal Use vs. Intent to

Distribute, 55 U. Miami L. Rev. 667 (2001) .......... 14

Edward J. Imwinkelried, The Next Step After

Daubert:

Developing

A

Similarly

Epistemological Approach to Ensuring the

Reliability of Nonscientific Expert Testimony, 15

Cardozo L. Rev. 2271 (1994) .................................... 6

vii

Mark J. Kadish, The Drug Courier Profile: In

Planes, Trains, and Automobiles; and Now in

the Jury Box, 46 Am. U. L. Rev. 747 (1997) .... 14, 29

Anna Lvovsky, Rethinking Police Expertise, 131

Yale L.J. 475 (2021) ......................................... 12, 13

Anna Lvovsky, The Judicial Presumption of Police

Expertise, 130 Harv. L. Rev. 1995 (2017).......... 6, 13

Jennifer L. Mnookin, Idealizing Science and

Demonizing Experts: An Intellectual History of

Expert Evidence, 52 Vill. L. Rev. 763 (2007) ..... 6, 26

Joëlle Anne Moreno, What Happens When Dirty

Harry Becomes an (Expert) Witness for the

Prosecution?,

79 Tul. L. Rev. 1 (2004).................... 7, 11, 12, 24, 26

NAT’L RESEARCH COUNCIL OF THE NAT’L ACADS.,

STRENGTHENING FORENSIC SCIENCE IN THE

UNITED STATES (2009) ............................................ 12

Ross Andrew Oliver, Testimonial Hearsay As the

Basis for Expert Opinion: The Intersection of the

Confrontation Clause and Federal Rule of

Evidence 703 After Crawford v. Washington, 55

Hastings L.J. 1539 (2004) ...................................... 28

Wes R. Porter, Repeating, Yet Evading Review:

Ad-mitting Reliable Expert Testimony in

Criminal Cases Still Depends Upon Who Is

Asking, 36 Rutgers L. Rec. 48 (2009) .............. 24, 25

Anne Bowen Poulin, Experience-Based Opinion

Testimony: Strengthening the Lay Opinion Rule,

39 Pepp. L. Rev. 551 (2012) ............................. 12, 17

viii

JED S. RAKOFF, WHY THE INNOCENT PLEAD GUILTY

AND THE GUILTY GO FREE (2021) ........................... 12

Jeffrey M. Schumm, Precious Little Guidance to

the “Gatekeepers” Regarding Admissibility of

Nonscien-tific Evidence: An Analysis of Kumho

Tire Co. v. Carmichael, 27 Fl. St. U. L. Rev. 865

(2000) ................................................................ 17, 18

Seth Stoughton, Evidentiary Rulings As Police

Reform, 69 U. Miami L. Rev. 429 (2015)......... 24, 25

Seth W. Stoughton, Principled Policing: Warrior

Cops and Guardian Officers, 51 Wake Forest L.

Rev. 611 (2016)....................................................... 13

Scott E. Sundby, The Jury as Critic: An Empirical

Look at How Capital Juries Perceive Expert and

Lay Testimony, 83 Va. L. Rev. 1109 (1997);.......... 24

Hon. Jack B. Weinstein, Science, and the

Challenges of Expert Testimony in the

Courtroom, 77 Or. L. Rev. 1005 (1998) ................. 18

Wright & Miller, 29 Fed. Prac. & Proc. Evid.

(2d ed. 2023). ...................................................... 8, 30

1

INTREST OF AMICUS CURIAE 1

The National Association of Criminal Defense

Lawyers (“NACDL”) is a nonprofit voluntary professional bar association that works on behalf of criminal

defense attorneys to ensure justice and due process for

those accused of crime or misconduct. Its members include private criminal defense lawyers, public defenders, military defense counsel, law professors, and

judges with experience in both federal and state courts

throughout the United States. NACDL is dedicated to

advancing the proper, efficient, and just administration of justice. NACDL files numerous amicus briefs

each year in the U.S. Supreme Court and other federal

and state courts to provide amicus assistance in cases

that present issues of broad importance to criminal defendants, criminal defense lawyers, and the criminal

justice system as a whole.

INTRODUCTION &

SUMMARY OF ARGUMENT

This case turns on the interpretation of Federal

Rule of Evidence 704(b). Specifically, the Court must

decide whether that rule allows a law enforcement officer—functioning as an expert witness in a drug trafficking case—to testify that most defendants caught

with drugs at the border know that they are transporting drugs. The answer to this question is “no”: law enforcement officers cannot properly testify as experts

about the mens rea of similarly situated defendants as

1 No counsel for a party authored this brief in whole or in part,

and no party or counsel for a party made a monetary contribution

intended to fund its preparation or submission. No person other

than amicus or its counsel made a monetary contribution to the

preparation or submission of this brief.

2

a class. That conclusion follows from the plain text of

Rule 704(b), as Petitioner explains. See Pet’r Br. 1730. Simply put, Rule 704(b) forbids any expert opinion

“about whether” a defendant has a mental state; expert testimony about classwide mens rea unquestionably concerns the mens rea of the defendant and is

therefore prohibited by the Rule. See id. at 20.

That reading of the Rule resolves this case. The

parties’ dispute, however, also reflects different views

of how the plain meaning of the Rule applies to expert

testimony by law enforcement officers. Properly understanding the nature of such testimony—and how it

developed and the difficulties it poses in criminal

cases—is essential to an appreciation of why Petitioner’s plain text interpretation of Rule 704(b) is well

grounded in the realities of trial.

Amicus Curiae therefore submits this brief to explain that the problems with classwide mens rea testimony stem as much from the testimony itself as from

who is offering it: the law enforcement officer testifying based on his or her training and experience.

As we describe in Part I, the widespread use of law

enforcement officers as experts is a relatively new, but

now pervasive, phenomenon. And the use of officer experts to testify on issues of mens rea is a still more recent variation of that trend.

Such testimony raises unique concerns that courts

have struggled to address. In principle, Federal Rules

of Evidence 702 and 704(b) provide helpful guidance

as judges perform their gatekeeping duty to ensure

that only relevant, reliable expert testimony is admitted without usurping the jury’s factfinding role. But

experience reveals a concerning breakdown in courts’

3

gatekeeping function when law enforcement officers

take the stand as experts. Too often, courts rely on

nothing more than “training and experience” as proxies for reliability, thus failing to subject officer expert

testimony to the scrupulous testing that the Rules of

Evidence demand. That breakdown in the application

of Rule 702 makes Rule 704(b) all the more important

as a safeguard: not only to protect the role of the jury

as trier of facts, but also the rights of the accused. The

Ninth Circuit’s reading of Rule 704(b), however, eviscerates this protection, allowing law enforcement officers to testify on matters going directly to defendants’ mens rea and usurp the jury’s role.

As we explain in Part II, the importance of adhering to Rule 704(b)’s text is further supported by the

practical and constitutional concerns posed by officer

expert testimony based only on training and experience. Such testimony—particularly when it concerns

mens rea—is effectively immune to reliability testing

from courts or adversarial challenge from defendants.

When officer experts’ opinions are based on their personal experiences investigating crime, unmoored from

any specific methodology or discernible principles,

courts struggle to measure the unmeasurable, unduly

deferring to officer experts’ ipse dixit. Defendants, in

turn, find it nearly impossible to cross examine such

officer experts or to present rebuttal witnesses. And

these practical concerns give rise to constitutional

ones, impeding defendants’ ability to confront witnesses against them and experience a fair trial. A textualist reading of Rule 704(b) appreciably mitigates

these concerns by defining (and limiting) the scope of

proper officer expert testimony on mens rea.

4

The need for that interpretation is bolstered by the

potential for abuse of officer experts in other settings.

We explain this point in Part III, which demonstrates

how a ruling for Respondent here could have sweeping

implications throughout criminal law.

Accordingly, for the reasons set out by Petitioner,

and for the reasons set out by Amicus Curiae below,

the Court should hold that expert testimony from law

enforcement officers on classwide mens rea has no

rightful place in our administration of criminal justice.

ARGUMENT

I.

RULES 702 AND 704(b) ARE FAILING TO

ACHIEVE THEIR CORE PURPOSE WHEN

IT COMES TO “OFFICER EXPERTS”

“No one will deny that the law should in some way

effectively use expert knowledge wherever it will aid

in settling disputes. The only question is as to how it

can do so best.” Learned Hand, Historical and Practical Considerations Regarding Expert Testimony, 15

Harv. L. Rev. 40, 40 (1901). Courts play an essential

role in deciding that question, principally through the

interpretation and application of the rules of evidence.

Fundamentally, the Federal Rules of Evidence

serve as a bulwark against the unchecked admission

of irrelevant and unreliable expert evidence. See

Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579,

590-92 (1993). And in criminal cases, they serve a particularly important purpose: helping courts carry out

their “primary constitutional duty … to do justice in

criminal prosecutions.” United States v. Nixon, 418

U.S. 683, 707 (1974); see also Melendez-Diaz v. Massachusetts, 557 U.S. 305, 340 (2009) (Kennedy, J.,

5

dissenting) (describing the rules governing expert witnesses as “part of the protections for the accused”).

Here, the Court addresses this objective in the context of a specific type of expert testimony: the law enforcement officer testifying on issues of mens rea based

on his or her training and experience. In this Part, we

first describe the rise of this relatively new phenomenon. Next, we explain how, in principle, courts are

well-equipped with Rules 702 and 704(b) to address

the issues it presents. Finally, we explain how, in practice, some courts are not using those tools as required—instead allowing officer experts broad discretion to testify, based on personalized experiences immune from objective testing, on issues well beyond

their expertise (including the mens rea of defendants

as a class). Allowing such testimony not only does violence to the text and purpose of the rules, but also intrudes on the jury’s fundamental role as finder of fact.

A.

The Evolution of the Expert Witness

and Rise of the “Officer Expert”

The use of witnesses with specialized knowledge is

not a recent development. Before trial by jury was the

norm, experts were used in cases in one of two ways:

(1) through “a jury of persons especially fitted to judge

the peculiar facts upon which the particular issue at

bar turns,” and (2) independent experts enlisted to

make recommendations directly to the judge. See

Hand, supra, at 40-42. But as the adversarial trial system developed (and elevated the role of lawyers), so

too did the rise of the modern expert. See Tal Golan,

Revisiting the History of Scientific Expert Testimony,

73 Brook. L. Rev. 879, 881-86 (2008).

6

Over time, the “self-informing” jury gave way to a

passive jury; complex questions involving science and

technology entered courtrooms; and the “skilled witness” appeared as a regular courtroom fixture. See

Jennifer L. Mnookin, Idealizing Science and Demonizing Experts: An Intellectual History of Expert Evidence, 52 Vill. L. Rev. 763, 768-71 (2007). “By the

1950s, expert witnesses were a mainstay in American

courts, from doctors to forensic analysts to pollsters

showing market trends in unfair trade practice suits.”

Anna Lvovsky, The Judicial Presumption of Police Expertise, 130 Harv. L. Rev. 1995, 2017 (2017).

Yet notwithstanding these developments, widespread reliance on the “olde constable” as an expert

witness in criminal cases is of a much more recent vintage. “Scholars trace the rise of law enforcement expert testimony to the 1950s and 60s,” United States v.

Holguin, 51 F.4th 841, 867 (9th Cir. 2022) (Berzon, J.,

concurring in part, dissenting in part), when “reformers recast policemen as trained investigators, [and]

judges began to recognize officers as professional experts on the patterns of urban crime—including on

matters previously deemed either commonsensical or

requiring scientific expertise,” Lvovsky, Presumption,

supra, at 2016.

Since the beginning of this trend, the use of officer

experts has increased drastically. In the 1960s, for instance, police officers or FBI agents accounted for only

6% of experts called by prosecutors. See Edward J. Imwinkelried, The Next Step After Daubert: Developing

A Similarly Epistemological Approach to Ensuring the

Reliability of Nonscientific Expert Testimony, 15

Cardozo L. Rev. 2271, 2279 (1994). By the 1980s,

7

police officers had emerged as “a new type of ‘skilled

witness’” in many more contexts. United States v.

Mejia, 545 F.3d 179, 189 (2d Cir. 2008) (cleaned up).

Today, “[t]he most common prosecution expert witness is a police officer or a federal agent.” Joëlle Anne

Moreno, What Happens When Dirty Harry Becomes an

(Expert) Witness for the Prosecution?, 79 Tul. L. Rev.

1, 4 (2004). In short, there has been a sea change in

this field. The officer expert is now called to testify

about, inter alia, the nature and structure of organized

crime or criminal gangs, the meaning of code words or

jargon, and (as here) issues of mens rea for defendants

as a class or “classwide mens rea.” See Mejia, 545 F.3d

at 189-90; Brian R. Gallini, To Serve and Protect? Officers As Expert Witnesses in Federal Drug Prosecutions, 19 Geo. Mason L. Rev. 363, 375-85 (2012).

B.

The Rise of Guardrails on Expert Testimony Under Rules 702 and 704(b)

The rise of the officer expert raises important questions about the proper application of the Rules of Evidence. Most central to that issue are two rules: Rule

702, governing the admission of expert testimony generally, and Rule 704(b), setting limits to the admission

of expert evidence going to the mental state or condition of the defendant. These two Rules—interpreted

correctly and applied faithfully—provide essential

guardrails on officer experts, including with respect to

testimony on issues of defendants’ mens rea. We will

describe those rules in greater detail before turning to

an explanation of their significance when it comes to

the challenges posed by officer experts and properly

circumscribing their testimony.

8

1. Rule 702 safeguards the jury from irrelevant and unreliable evidence

We start with Rule 702, which has two counterbalancing policy goals. See Wright & Miller, 29 Fed. Prac.

& Proc. Evid. § 6262 (2d ed. 2023). On the one hand,

this rule seeks to “promote the trier of fact’s search for

truth by helping it to understand other evidence or accurately determine the facts in dispute.” Id. On the

other hand, it aims to “preserve the trier of fact’s traditional powers to decide the meaning of evidence and

the credibility of witnesses by placing limits on the admissibility of expert opinion.” Id. In achieving a balance between these goals, perhaps the most important

limit imposed by Rule 702 is the requirement that expert testimony presented to the jury meet “exacting

standards of reliability.” Weisgram v. Marley Co., 528

U.S. 440, 455 (2000) (emphasis added).

In Daubert v. Merrell Dow Pharmaceuticals, this

Court read Rule 702 as establishing “a standard of evidentiary reliability,” which it described as a requirement that expert “testimony must be supported by appropriate validation—i.e., ‘good grounds,’ based on

what is known.” 509 U.S. at 590. The Court was clear

that courts were entrusted as the gatekeepers to ensure that this reliability requirement was met: “under

the Rules the trial judge must ensure that any and all

scientific testimony or evidence admitted is not only

relevant, but reliable.” Id. at 589. And the Daubert

Court provided some “general observations” of factors

that could guide the reliability determination, including whether the evidence “can be (and has been)

tested,” “whether the theory or technique has been

subjected to peer review and publication,” “the known

9

or potential rate of error,” and the level of “acceptance”

of the technique in question. Id. at 593-94.

A few years after the Daubert ruling—in General

Electric Co. v. Joiner—this Court again emphasized

“the ‘gatekeeper’ role of the trial judge in screening

such [expert] evidence,” and noted that “nothing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that

is connected to existing data only by the ipse dixit of

the expert.” 522 U.S. 136, 142, 146 (1997).

Finally, tripling down on the point, the Court in

Kumho Tire Co. v. Carmichael clarified that the “general principles” announced in Daubert applied to all

“expert matters described in Rule 702,” “whether the

testimony reflects scientific, technical, or other specialized knowledge.” 526 U.S. 137, 149 (1999). Therefore, before the introduction of any form of expert evidence, the trial judge “must determine whether the

testimony has a reliable basis in the knowledge and

experience of the relevant discipline.” Id. (cleaned up).

In addition, this Court emphasized that courts’ “gatekeeping duty” to ensure relevance and reliability in expert testimony is designed “to make certain that an

expert, whether basing testimony upon professional

studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.”

Id. at 152.

In 2000, following this so-called Daubert trilogy of

cases, Congress amended Rule 702 to require the proponent of expert testimony to show, inter alia, that

“the testimony is the product of reliable principles and

methods.” Fed. R. Evid. 702(c). And in 2023, Congress

10

again amended Rule 702, not to impose any new procedures, but in meaningful part “to emphasize that

each expert opinion must stay within the bounds of

what can be concluded from a reliable application of

the expert’s basis and methodology.” Fed. R. Evid. 702,

Advisory Committee Notes to the 2023 amend.

The Advisory Committee Notes—which this Court

has recognized are “a useful guide in ascertaining the

meaning of the Rules,” Tome v. United States, 513 U.S.

150, 160 (1995)—do not exempt officer experts from

this “exacting” reliability requirement. Instead, the

Notes explain that officer expert testimony based on

“extensive experience” should be admitted only “[s]o

long as the principles and methods are reliable and

applied reliably to the facts of the case.” Fed. R. Evid.

702, Advisory Committee Notes to the 2000 amend.

2. Rule 704(b) protects the jury from

unhelpful expert testimony that intrudes on its role as finder of fact

Rule 704(b), which is at the center of this case, adds

yet another layer of protection against encroachments

on the role of the jury as the finder of fact. It provides:

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.

Fed. R. Evid. 704(b).

As the Second Circuit held, “the rule recognizes

that expert testimony concerning a defendant’s mental state poses a uniquely heightened danger of

11

intruding on the jury’s function.” United States v. DiDomenico, 985 F.2d 1159, 1164 (2d Cir. 1993). “[W]ithout it, juries could simply follow the expert’s commentary about defendant’s mens rea despite its independent duty to evaluate mens rea.” Gallini, supra, at 366.

And so, Congress made clear where the responsibility

to assess issues of mens rea lies: “Those matters are

for the trier of fact alone.” Fed. R. Evid. 704(b).

C.

The Guardrails Imposed by Rules 702

and 704(b) Are Malfunctioning in Too

Many Cases Involving Officer Experts

Together, Rules 702 and 704(b)—underwritten by

the Daubert trilogy—guard against the introduction of

unreliable expert testimony that could run afoul of the

maxim that “the truth-finding task [is] assigned solely

to juries.” Carella v. California, 491 U.S. 263, 265

(1989) (per curiam). Unfortunately, those guardrails

too often fall when it comes to the admission of officer

expert testimony in criminal cases—especially on matters of mens rea.

At bottom, the problem is twofold. First, courts are

not performing their gatekeeping function to subject

such experience-based testimony to “exacting standards of reliability.” Weisgram, 528 U.S. at 455. Second,

and relatedly, courts are misinterpreting Rule 704(b)

and failing to limit officer experts from testifying only

to matters within their purported expertise.

1. Courts are failing their gatekeeping

duties when officer experts testify

Reputable empirical evidence indicates that the

Rules and the Daubert principles have “had little or no

impact in real criminal cases.” Moreno, supra, at 17;

12

see also Jennifer L. Groscup et al., The Effects of Daubert on the Admissibility of Expert Testimony in State

and Federal Criminal Cases, 8 Psychol. Pub. Pol’y &

L. 339, 344-45 (2002). 2 And nowhere is that more evident than in the admission of officer expert testimony.

In far too many cases, the standards of reliability

otherwise governing the admission of expert testimony apparently “disappear” “whenever the expert is

a police officer.” Moreno, supra, at 18; see also Gallini,

supra, at 365 (“Federal courts rarely undertake the

analysis required to determine whether the [officer]

expert’s testimony has a sound methodological basis.”). Instead, “[l]aw enforcement officers are routinely permitted to testify as experts based on their

law enforcement experience.” Anne Bowen Poulin, Experience-Based Opinion Testimony: Strengthening the

Lay Opinion Rule, 39 Pepp. L. Rev. 551, 554 (2012).

As several scholars have concluded, officers testifying as experts face little to no judicial scrutiny, since

most courts give officer experts significant deference

in criminal matters. See generally, e.g., Anna Lvovsky,

Rethinking Police Expertise, 131 Yale L.J. 475, 485-91

2 This includes a failure to subject forensic evidence used in crim-

inal trials to the same standards of reliability as those required

in civil cases. See NAT’L RESEARCH COUNCIL OF THE NAT’L ACADS.,

STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES 106

(2009) (“Review of reported judicial opinions reveals that, at least

in criminal cases, forensic science evidence is not routinely scrutinized pursuant to the standard of reliability enunciated in

Daubert.”); see also JED S. RAKOFF, WHY THE INNOCENT PLEAD

GUILTY AND THE GUILTY GO FREE 57-70 (2021) (noting that most

techniques categorized as forensic science “are unscientific, involve a great deal of disguised guesswork, and too frequently result in false convictions”).

13

(2021); Lvovsky, Presumption, supra, at 2023-36; Seth

W. Stoughton, Principled Policing: Warrior Cops and

Guardian Officers, 51 Wake Forest L. Rev. 611, 649

(2016).

The result is a pattern of hyper-deference—rather

than gatekeeping under Rule 702—when officers are

held out as experts in the criminal setting. Such deference is not only at odds with the text and purpose of

the Rules of Evidence, but also imperils defendants’

rights and undermines the historic function of juries.

2. Courts under-enforce Rule 704(b)’s

limits on expert testimony

In addition to unduly lax scrutiny under Rule 702,

many courts have under-enforced the limits of Rule

704(b)—most drastically (and impactfully) in the context of drug-related cases where whether the defendant is guilty or not turns on knowledge or intent.

To be sure, courts pay occasional lip service to the

principle that Rule 704(b) requires the drawing of “a

very fine line” in the use of officer experts testifying

about issues of defendants’ mens rea. United States v.

Augustin, 661 F.3d 1105, 1124 (11th Cir. 2011); United

States v. Mitchell, 996 F.2d 419, 422-23 (D.C. Cir.

1993) (“Rule 704(b) invites the drawing of rather subtle lines”); see also United States v. Gonzalez-Rodriguez, 621 F.3d 354, 364 (5th Cir. 2010) (noting “a fine

but critical line” between permissible expert testimony on drug operations and impermissible testimony

that is the “functional equivalent” of an opinion on

mens rea).

In practice, though, courts in jurisdictions like the

Ninth Circuit are misinterpreting Rule 704(b) and

14

allowing prosecutors to step completely over the line

by permitting officer experts to testify about the mens

rea of defendants as a class. See, e.g., Mark J. Kadish,

The Drug Courier Profile: In Planes, Trains, and Automobiles; and Now in the Jury Box, 46 Am. U. L. Rev.

747, 776 (1997); see also Dana R. Hassin, How Much

is Too Much? Rule 704(b) Opinions on Personal Use vs.

Intent to Distribute, 55 U. Miami L. Rev. 667, 675-76

(2001). In cases where Rule 704(b) should be paramount, it is often sidelined.

* * *

The rise of officer experts is a comparatively modern development. The use of such experts to testify

about mens rea is a still more decidedly recent twist

on that trend. Given widespread judicial failure to

properly interpret and apply the Rules of Evidence to

such expert testimony, the time has come for this

Court to reaffirm “the jury’s historic role as a bulwark

between the State and the accused.” Southern Union

Co. v. United States, 567 U.S. 343, 350 (2012) (cleaned

up). Petitioner’s interpretation of Rule 704(b) achieves

that end by clarifying the important line that Rule

704(b) draws as to matters of mens rea specifically:

“Those matters are for the trier of fact alone.” In contrast, the position pressed by Respondent risks allowing experts to do exactly what the Rules say they cannot—and, thus, risks undermining core protections for

criminal defendants, as well as the most central purpose of criminal juries.

15

II.

THE REFLEXIVE ACCEPTANCE OF “OFFICER EXPERT” TESTIMONY ON ISSUES

OF MENS REA RAISES PRACTICAL AND

CONSTITUTIONAL CONCERNS

As established above, too many courts have uncritically accepted expert opinion testimony from law enforcement officers on issues of mens rea. This not only

offends the text and purpose of the Rules of Evidence,

but also creates practical problems that lead to constitutional concerns. For these reasons, too, the Court

should be wary of arguments designed to blast open to

the door to such officer expert testimony.

The practical problems—illustrated in Petitioner’s

case and discussed below—are most stark where officers offer expert testimony (based on “training and experience”) about the mens rea of defendants as a

class. 3 In such cases, courts are often unable to meaningfully test the reliability of the officer expert’s testimony. Moreover, criminal defendants often struggle to

cross-examine officer experts on that particular basis,

and are unable to offer a rebuttal expert to challenge

the officer witness’s testimony. These practical problems underwrite serious constitutional concerns that

threaten defendants’ Sixth Amendment confrontation

rights and the Due Process guarantee of a fair trial.

3 There is no dispute that—subject to the proper reliability test-

ing under Rule 702 and the limits under Rule 704(b)—officer experts can testify about certain investigative techniques.

16

A.

Courts and Criminal Defendants Lack

Effective Means of Testing Officer Expert Testimony

The testimony of officer experts who opine on issues of mens rea based on training and experience can

be virtually immune from meaningful reliability testing. Courts face an uphill battle in determining its reliability, see Holguin, 51 F.4th at 867-69 (Berzon, J.,

concurring in part, dissenting in part), leading most

courts to rely on nothing more than officers’ experience. And because such testimony is offered as selfvalidating, criminal defendants cannot meaningfully

challenge it by “traditional and appropriate means”

like “[v]igorous cross-examination” and “presentation

of contrary evidence.” Daubert, 509 U.S. at 596.

1. Courts cannot meaningfully test experience-based officer testimony

Start with the difficulties faced by the court acting

as gatekeeper. As explained above, Rule 702 and the

Daubert trilogy demand that trial courts subject all

expert testimony to rigorous reliability testing. And,

importantly, courts cannot abdicate their gatekeeping

duty and accept what an expert says at face value

merely because he has been qualified as an expert. See

Kumho Tire, 526 U.S. at 158-59 (Scalia, J., concurring). If admissibility could be established by the say

so of an admittedly qualified expert, the reliability

prong would be subsumed by the qualification prong

and rendered superfluous. Qualifications do not beget

17

reliability, and courts still must perform their duty to

scrutinize expert opinions to ensure they are reliable. 4

But as a general proposition, experience-based expert testimony can present unique challenges to the

court’s gatekeeping function. See Samuel Gross et al.,

Expert Information and Expert Evidence: A Preliminary Taxonomy, 34 Seton Hall L. Rev. 141, 142 (2003)

(“[T]he problem of assessing reliability is especially

acute with respect to non-scientific expert evidence.”);

Poulin, supra, at 569-70. And these difficulties only

deepen when the expert is an officer. See Holguin, 51

F.4th at 867 (Berzon, J., concurring in part, dissenting

in part) (finding “traditional justifications for experience-based expert testimony” inapplicable to “law enforcement expertise”).

For instance, when it comes to drugs and gangs,

“[u]nlike traditional expertise, ‘[t]here is no objectively

ascertainable or empirically supportable measure of

personal experience’ with drug jargon and ‘no objective means of regulating or certifying gang experts.’”

Id. (citation omitted). Therefore, in practice, the “assessment of the reliability of criminal modus operandi

experts is almost totally void of reliability analysis employing any of the Daubert factors,” as “trial courts appear to employ precedent, or past admissions of similar testimony, as the key reliability factor.” Jeffrey M.

Schumm, Precious Little Guidance to the “Gatekeepers” Regarding Admissibility of Nonscientific

4 That is the teaching of Kumho Tire, where the Court considered

an eminently qualified tire expert but found that his testimony

flunked Rule 702 because its reliability depended only on the ipse

dixit of the expert himself. See 526 U.S. at 153-57.

18

Evidence: An Analysis of Kumho Tire Co. v. Carmichael, 27 Fl. St. U. L. Rev. 865, 882-83 (2000).

Simply put, unable to apply “exacting” reliability

testing that Rule 702 and the Daubert trilogy demand,

courts routinely rely on little more than the officers’

“training and experience” to justify the admission of

their testimony. E.g., United States v. Parra, 402 F.3d

752, 758-59, 765 (7th Cir. 2005); United States v. Davis, 397 F.3d 173, 178-79 (3d Cir. 2005); United States

v. Lopez-Lopez, 282 F.3d 1, 13-14 (1st Cir. 2002); David

L. Faigman et al., Mod. Sci. Evidence § 1:27 (20232024 Ed.) (“[C]ourts continue to permit many prosecution experts with hardly a glance at the methods underlying their testimony. Perhaps the best example is

the testimony of police officers testifying as expert witnesses.”).

That trend is troubling for many reasons—including that there is reason to doubt whether experiencebased officer testimony actually relies on “specialized

knowledge” that assists the trier of fact. See United

States v. Campos, 217 F.3d 707, 719 (9th Cir. 2000)

(Pregerson, J., concurring in part, dissenting in part)

(“Agent Darvas testified that his ‘expert’ opinion on

drug courier’s knowledge was based as much, if not

more, on a common sense review of the facts than on

his ‘specialized knowledge’ as a law enforcement officer.”); Hon. Jack B. Weinstein, Science, and the Challenges of Expert Testimony in the Courtroom, 77 Or. L.

Rev. 1005, 1008 (1998) (“Much of the so-called expert

testimony, such as that of police officers who opine

that criminals keep revolvers in glove compartments,

or that the mafia is a gang, seems useless. This

19

information really does not help the jury, but rather

amounts to preliminary summation.”).

As Kumho Tire teaches, “training and experience”

is not a talisman for reliability under Daubert; qualifications are not a substitute for reliability. But that

is the precise error too many courts are committing—

often because they have no idea how to assess the reliability of an officer expert’s asserted experience.

Under a proper application of the Rules, an officer’s

“training and experience” would be deemed an insufficient measure of reliability in its own right to permit

officer expert testimony. See, e.g., United States v. Valencia-Lopez, 971 F.3d 891, 898-99 (9th Cir. 2020) (noting that “the record contains no evidence as to why []

experience, by itself, equals reliability for his testimony” and that in the context of experience-based expert testimony “not subject to routine testing, error

rate, or peer review type analysis,” the reliability determination “becomes more, not less, important”); cf.

United States v. Mrabet, No. 23 Cr. 69, 2023 WL

8179685, at *2 (S.D.N.Y. Nov. 27, 2023) (Rakoff, J.)

(rejecting proposed expert testimony based only on an

officer’s “training, education, and experience” because

“an expert opinion requires some explanation as to

how the expert came to his conclusion and what methodologies or evidence substantiate that conclusion”)

(cleaned up).

Far too often, though, courts look the other way,

treating qualification as inherent evidence of reliability for officer experts. The result is that such testimony is often admitted with inadequate gatekeeping,

and thus in contravention of an important guardrail.

In such cases, proper application of Rule 704(b)

20

becomes even more important as a safeguard for defendants, since other protections have fallen by the

wayside.

2. Often, defendants can neither crossexamine nor rebut experience-based

“officer expert” testimony

That conclusion is amplified by yet another practical difficulty in many officer-expert cases: the unique

challenges that defendants face in responding to it.

Cross-examination takes on greater practical importance when a witness is offering purportedly “expert” testimony. “Unlike an ordinary witness … an expert is permitted wide latitude to offer opinions, including those that are not based on firsthand

knowledge or observation.” Daubert, 509 U.S. at 592.

This latitude risks admission of “powerful and quite

misleading” testimony. Id. at 595. And in criminal

cases—unlike in civil cases like Daubert—expert testimony can contribute to wrongful convictions. Crossexamination thus serves as a bulwark against dubious

testimony.

But when law enforcement officers testify as experts based on their experiences alone, that otherwise

formidable bulwark crumbles. Cf. United States v.

Dukagjini, 326 F.3d 45, 53 (2d Cir. 2003) (“[E]xpert

testimony by a fact witness or case agent can inhibit

cross-examination, thereby impairing the trial’s truthseeking function,” in part, because “he is providing an

opinion that, unlike a factual matter, is not easily contradicted.”). In practice, “the Daubert factors (peer review, publication, potential error rate, etc.) simply are

not applicable” to officer experts, and there is an

21

“absence of any empirical research-based confirmation” to draw on. See Holguin, 51 F.4th at 867 (Berzon,

J., concurring in part, dissenting in part) (citation

omitted).

That means officer experts can (and do) offer opinions based on self-validating experiences without any

reliable norms, standards, or otherwise objective scientific measures to effectively undermine their lived

experienced. See id. (observing that “cross-examination during such testimony can provide only limited

means for testing”). As a result, the defendant simply

cannot conduct a meaningful examination to attack

the testimony of the officer expert. Instead, defendants may opt to forgo examination altogether rather

than risk bolstering the officer expert’s testimony. Cf.

Dukagjini, 326 F.3d at 53-54 (“Challenges to the expert are often risky because they can backfire and end

up bolstering the credibility of the witness,” so “a defendant may have to make the strategic choice of declining to cross-examine the witness at all.”).

These problems are exacerbated when the officer

expert testifies about mens rea. With even less means

to challenge that expert’s testimony, the defendant’s

ability to cross-examine goes from difficult to impracticable. Examination is futile where the officer expert—testifying based on personal training and experience—can respond as if cross-examination were a

mere Thanksgiving dinner dispute: “Well, that has

been my experience.” This case illustrates the point.

On direct examination, Agent Flood described his

28 years of service with Homeland Security Investigation, 500-plus investigations, and 50-plus times in

court. See Pet. App. C at 10a-13a. The government

22

asked, “based on [his] training and experience,”

whether “large quantities of drugs [are] entrusted to

drivers that are unaware of those drugs.” Id. at 15a.

Agent Flood responded, “No,” then added, “in most circumstances, the driver knows they are hired.” Id.

Ms. Diaz, on cross-examination, tried to challenge

that testimony, referencing Homeland Security announcements that identified schemes where drug trafficking organizations used unknowing couriers. Id. at

23a. But Agent Flood reframed. Although he noted

some familiarity with such schemes (“I—I know of

three schemes”), he opined that fact patterns fitting

those schemes did not “necessarily mean that they are

unknowing couriers” because, in his personal experience, Agent Flood “had investigations involving each

[scheme] where actually the person stated they were

hired to bring drugs across the border.” Id.

Thus, in this case, the personal experiences of the

officer expert, Agent Flood—and his investigations—

permitted him to neutralize any challenge to his generalized testimony about what occurs in “most circumstances.” And if Agent Flood did have personal experience with schemes involving unknowing couriers, such

that it would undermine any generalizations about

classwide mens rea that the government wants to

make, the government knows what to do: find another

agent with different experiences. Instead of Agent

Flood, maybe call Agent Johnson, who would testify

that, based on his experience, the use of an unknowing

courier “has not happened.” Jury Trial Tr. at 10-11,

United States v. Venegas-Reynoso, No. 10 Cr. 1257 (D.

Ariz. filed Aug. 15, 2011), ECF No. 74. Or perhaps call

DEA Agent Hella, who has “[n]ever been able to

23

corroborate the story of a blind mule.” United States v.

Medina-Copete, 757 F.3d 1092, 1106 n.4 (10th Cir.

2014).

Prosecutors in the Ninth Circuit and other jurisdictions with similar interpretations of Rule 704(b) need

only find the right agent, with the right experience, to

testify “generally” about the mens rea of a class of individuals that conveniently mirror the defendant at

trial. See, e.g., Nat’l Ass’n of Fed. Defenders Amicus

Br. at 22-24. Defendants like Ms. Diaz simply cannot

meaningfully prepare for and examine an agent based

on such personalized experience. See Holguin, 51

F.4th at 868 (Berzon, J., concurring in part, dissenting

in part) (noting that “objections and cross-examination during such testimony can provide only limited

means for testing” because “[s]uch inquiries require

the defendant … to respond to the witness’s testimony

on the fly without prior knowledge of the assumed connection between the expert’s background and the specific testimony offered”). Instead, those defendants are

left in the untenable position of arguing their innocence divorced from reality in a vacuum of experiences

unique to Agents Flood, Johnson, Hella, or some other

curated agent to testify based solely on their experience. As a result, they lose access to vigorous crossexamination, “the greatest legal engine ever invented

for the discovery of truth.” California v. Green, 399

U.S. 149, 158 (1970) (cleaned up).

Compounding the inability to conduct a meaningful cross-examination is the lack of “contrary evidence” to present to the jury. See Daubert, 509 U.S. at

596 (describing “presentation of contrary evidence” as

a traditional means to attack expert evidence). Defendants like Ms. Diaz are, for all practical purposes,

24

unable to call an opposing expert to rebut the officer

expert who testifies based on his experience, untethered to objective principles or methodology.

Consider, again, this case. The government proffered Agent Flood as an expert to testify based on his

almost 30 years of experience and his 500 investigations. The source of his expert testimony is his personal experience on the beat, investigating crimes.

There is no rebuttal expert available to swear, under

oath, that Agent Flood in fact had a different experience over those 30 years. Because there is no methodology, there is no expert on the other side applying a

different methodology and reaching a different result.

Moreover, even in the unlikely event that the defendant succeeds in finding a suitable rebuttal expert,

she will be fighting the perception that juries tend to

view non-officer experts as “hired guns.” See, e.g.,

Scott E. Sundby, The Jury as Critic: An Empirical

Look at How Capital Juries Perceive Expert and Lay

Testimony, 83 Va. L. Rev. 1109, 1125-30 (1997);

Mnookin, supra, at 769-71; Seth Stoughton, Evidentiary Rulings As Police Reform, 69 U. Miami L. Rev.

429, 449-50 (2015). And the court may not even be

willing to admit her rebuttal expert, as empirical evidence demonstrates that while courts are happy to

hear from expert officers, they are reluctant to allow

defendants to introduce their own expert testimony.

See Moreno, supra, at 3 (“A recent study of federal appellate criminal cases found that more than 95% of

prosecutors’ experts are admitted at trial, while fewer

than 8% of defense experts are allowed to testify.”);

Wes R. Porter, Repeating, Yet Evading Review: Admitting Reliable Expert Testimony in Criminal Cases Still

25

Depends Upon Who Is Asking, 36 Rutgers L. Rec. 48,

49 (2009) (“One of the unfortunate truths in criminal

litigation is that trial courts frequently admit testimony from the government’s experts and exclude the

defendant’s proposed expert testimony.”).

Rather than the “battle of the experts” traditionally waged in civil matters, criminal defendants frequently are subjected to the tyranny of the single curated police officer expert opining on the mens rea of

defendants as a class based only on their own views.

B.

Officer experts often present an aura of

trustworthiness but lack neutrality

The prejudice of admitting officer expert testimony

about classwide mens rea is compounded because officer experts are testifying with the “aura of special reliability and trustworthiness surrounding expert testimony.” See United States v. Young, 745 F.2d 733, 766

(2d Cir. 1984) (Newman, J., concurring) (cleaned up);

see Ake v. Oklahoma, 470 U.S. 68, 81 n.7 (1985) (noting

that “testimony emanating from the depth and scope

of specialized knowledge is very impressive to a jury”).

So, when the prosecution calls a law enforcement

officer to testify as an expert “about participation in

prior and similar cases, the possibility that the jury

will give undue weight to the expert’s testimony is

greatly increased.” United States v. Alvarez, 837 F.2d

1024, 1030 (11th Cir. 1988). This expert-imprimatur

phenomenon is especially true for law enforcement officers, who juries perceive as particularly trustworthy.

See Stoughton, supra, at 449-50 (jurors may “perceive

that police officers, unlike many other expert witnesses, are not ‘hired guns,’ and so their testimony

26

may be viewed as more credible than that of a paid,

professional witness”); Stephen Garvey et al., Juror

First Votes in Criminal Trials, 1 J. Empirical Legal

Stud. 371, 396 (2004) (in a 2004 analysis of juror decision-making in criminal trials, finding that “[m]ost of

the jurors believed the police testimony they heard”).

This aura surrounding qualification should further

caution against the uncritical admission of experiencebased officer expert testimony. This is especially true

because these officer experts are not like the traditional reticent expert appearing at the Old Bailey. See

Mnookin, supra, at 769-71. These agents are not neutral experts studying the issue dispassionately. Their

job is investigating crime. Whatever training and experience they bring to the courtroom has been acquired through a lens focused on finding criminality.

In short, these agents are biased, and that bias “might

lead the witness to slant, unconsciously or otherwise,

his testimony in favor of or against a party.” United

States v. Abel, 469 U.S. 45, 51 (1984); see Moreno, supra, at 30 (discussing the hazards of reasoning from

experience, which include the risk that the witness

will rely on intuition or will reach conclusions that are

“personal, idiosyncratic, and subjective”); Gross et al.,

supra, at 156-58 (discussing the risk of bias in law enforcement testimony based on experience).

C.

The Near Automatic Admission of Officer Expert Testimony Leads to Constitutional Concerns

The practical problems we have just described also

give rise to weighty constitutional concerns—which,

as explained, militate against an unduly broad view of

permissible testimony under Rule 704(b).

27

1. Right to Confrontation

The Confrontation Clause entitles a defendant not

only to the right of physical confrontation but also the

right of cross-examination. It “commands, not that evidence be reliable, but that reliability be assessed … by testing in the crucible of cross-examination.” Crawford v. Washington, 541 U.S. 36, 61 (2004).

But officer experts often rely on conversations with

non-testifying witnesses, cooperators, codefendants,

or other police officers. See Mejia, 545 F.3d at 188-89.

For instance, in United States v. Campos, where an

alleged drug-courier denied knowledge, an agent testified about the mens rea of such defendants as a class

and relied, in part, on his “number of occasions to talk

to individuals in organizations that smuggle marijuana across the border,” who told him that “nobody in

their right mind would entrust that amount of marijuana to somebody that doesn’t know what they are doing.” 217 F.3d 707, 714-15 (9th Cir. 2000) (Pregerson,

J., concurring in part, dissenting in part).

Similarly, in United States v. Mejia, the officer expert grounded his testimony on “between fifteen and

fifty custodial interrogations of Long Island MS–13

members.” 545 F.3d at 199 (holding that the testimony

violated the Confrontation Clause).

Even in this very case, Agent Flood testified based

on “various investigation techniques,” including

speaking with cooperating defendants, cooperating

sources, and other agents. Pet. App. 11a-12a.

Where officer experts are given an undue berth under Rule 704(b), they often smuggle in out-of-court

statements to the jury that raise serious Sixth

28

Amendment concerns. Several courts have raised this

concern. See United States v. Maher, 454 F.3d 13, 19

(1st Cir. 2006) (“Post-Crawford, the admission of nontestifying informants’ out-of-court testimonial statements, through the testimony of police officers, is a recurring issue in the courts of appeals.”); Mejia, 545

F.3d at 198-99. So have some scholars. See, e.g., Ross

Andrew Oliver, Testimonial Hearsay As the Basis for

Expert Opinion: The Intersection of the Confrontation

Clause and Federal Rule of Evidence 703 After Crawford v. Washington, 55 Hastings L.J. 1539, 1555-58

(2004). Nonetheless, this kind of testimony regularly

gets to the jury, often without scrutiny. A proper interpretation of Rule 704(b) would mitigate that issue.

2. Right to a Fair Trial

“[C]ourts must be alert to factors that … undermine the fairness of the factfinding process.” Estelle v.

Williams, 425 U.S. 501, 503 (1976). In criminal cases,

the factfinding process is “assigned solely to juries.”

Carella, 491 U.S. at 265 (per curiam). But officer expert testimony about classwide mens rea can far too

readily usurp that function. Cf. Sandstrom v. Montana, 442 U.S. 510, 520 (1979) (“[T]he decision on the

issue of intent must be left to the trier of fact alone.”).

As the Second Circuit has observed, “[a]n increasingly thinning line separates the legitimate use of an

officer expert … from the illegitimate and impermissible substitution of expert opinion for factual evidence.”

Mejia, 545 F.3d at 190. “In such instances, it is a little

too convenient that the Government has found an individual who is expert on precisely those facts that the

Government must prove to secure a guilty verdict.” Id.

at 191. Yet despite that warning, prosecutors in the

29

Ninth Circuit—and in jurisdictions with a similarly

loose reading of Rule 704(b)—have found that “convenien[ce]” to be an effective means to secure guilty

pleas and verdicts in drug courier cases. See Nat’l

Ass’n of Fed. Defenders Amicus Br. at 17-21 (explaining how prosecutors use officer experts).

In far too many cases, in far too many jurisdictions,

the “officer expert” appears not as an expert assisting

the jury, but as “a chronicler of the recent past whose

pronouncements on elements of the charged offense

serve as shortcuts to proving guilt.” Mejia, 545 F.3d at

190; Gallini, supra, at 408 (“The government’s … wide

latitude to tacitly comment on a drug defendant’s

mens rea relieves it from proving defendant’s mens rea

beyond a reasonable doubt.”). As a result, “[t]he officer

expert transforms into the hub of the case, displacing

the jury by connecting and combining all other testimony and physical evidence into a coherent, discernible, internally consistent picture of the defendant’s

guilt.” Mejia, 545 F.3d at 190-91; see also Gallini, supra, at 409 (“When a member of law enforcement takes

the stand as an ‘expert’ and thereafter testifies that

defendant’s activities are consistent with drug trafficking, the witness has unconstitutionally usurped

the jury’s role.”). By displacing the jury’s role in finding mens rea, officer experts imperil due process values.

And that is just one aspect of the issue. Viewed

from another perspective, officer expert testimony on

classwide mens rea can come awfully close to group

propensity arguments—which are widely recognized

to threaten defendants’ right to a fair trial. See Kadish, supra, at 776 (noting how officers often “testify …

that the defendant[] … acted like a drug courier,

30

therefore, he is a drug courier”). As a result of this testimony, the burden of proof is essentially reversed: the

defendant must establish she is an outlier, and not

within the overwhelmingly predominant cohort described by the expert witness.

Simply put, classwide mens rea testimony amounts

to improper “profile” evidence that asks the jury “to

infer the defendant’s guilt from the past acts of third

persons.” Wright & Miller, supra, § 5234. “Such evidence presents the defendant with an impossible dilemma” because “evidence of crimes committed by a

third person who is not on trial saddles a defendant

with the burden of proving the innocence of another.”

Kadish, supra, at 785 (cleaned up). Testimony about

the mens rea of defendants as a class therefore represents a perverse form of impermissible propensity evidence, and it erodes “a basic premise of our criminal

justice system: Our law punishes people for what they

do, not who they are.” Buck v. Davis, 580 U.S. 100, 123

(2017).

* * *

Rule 704(b) is unambiguous and should be read as

Petitioner explains. That follows from considerations

of text and structure. But the practical and constitutional concerns that stem from the reading of Rule

704(b) advanced by Respondent offer yet another reason to adopt Petitioner’s straightforward reading.

III.

ALLOWING OFFICER EXPERTS TO TESTIFY ABOUT CLASSWIDE MENS REA

WOULD POSE ADDITIONAL RISKS

This case arises in the context of drug trafficking

prosecutions. But the decision in this case will extend

31

beyond drug-related cases. Blessing the Ninth Circuit’s atextual interpretation of Rule 704(b) would add

a new, dubious maneuver to every prosecutor’s playbook. Expert testimony from law enforcement officers

will conveniently mirror the defendant’s actions, but

framed in terms like “generally” or “usually.”

Looking ahead, it would be all too easy for law enforcement agencies to develop “expertise” to infer the

knowledge or intent for various crimes based on generalized profiles. See Mejia, 545 F.3d at 190 (“Any effective law enforcement agency will necessarily develop expertise on the criminal organizations it investigates ….”); see also Pet’r Br. at 21-22. We could expect law enforcement agencies to cultivate the perfect

expert across a range of offenses. One for securities

fraud, another for income tax fraud, a third for accounting fraud, and still others for money laundering,

terrorism, gangs, and so on. These concerns would also

extend beyond offense-related “expert” testimony to,

for example, the mens rea of the defendant who joins

an unpopular political group, see, e.g., United States v.

Lightfoot, 228 F.2d 861, 865-67 (7th Cir. 1956) (holding that defendant’s familiarity with excerpts of the

“writings of Marx, Engels, Lenin and Stalin” were

“properly admitted as bearing upon [defendant’s]

knowledge and intent”), rev’d, 355 U.S. 2 (1957); or the

defendant who practices a certain religion, see, e.g.,

United States v. Hayat, 710 F.3d 875, 900-01 (9th Cir.

2013) (relying on drug-courier precedent to affirm expert testimony that the “kind of person” who would

carry a certain Islamic supplication was “[a] person

who is engaged in jihad”).

32

The way to prohibit these nefarious ends is to forbid their beginning. And they begin with a misreading

of Rule 704(b). The Court should therefore adopt the

interpretation of that rule advocated by Petitioner.

CONCLUSION

For the reasons set out above and in Petitioner’s

brief, the judgment below should be reversed.

Respectfully submitted,

SEAN HECKER

DAVID PATTON

SEAN KELLY

KAPLAN HECKER & FINK LLP

350 Fifth Avenue

63rd Floor

New York, NY 10118

CARMEN IGUINA GONZÁLEZ

Counsel of Record

JOSHUA MATZ

KAPLAN HECKER & FINK LLP

1050 K Street NW|Suite 1040

Washington, DC 20001

(212) 763-0883

ciguinagonzalez@kaplanhecker.com

JOSHUA L. DRATEL

LAW OFFICES OF

DRATEL & LEWIS

29 Broadway, Suite 1412

New York, NY 10006

Counsel for Amicus Curiae