Amicus Curiae Brief — Union Carbide Corporation, et al., Petitioners v. Lee Ann Sommerville

No. 25-919

IN THE

Supreme Court of the United States

____________

UNION CARBIDE CORPORATION; COVESTRO LLC,

v.

Petitioners,

LEE ANN SOMMERVILLE, individually and on behalf

of all others similarly situated,

Respondent.

____________

On Petition for a Writ of Certiorari

to the United States Court of

Appeals for the Fourth Circuit

____________

BRIEF OF WASHINGTON LEGAL FOUNDATION AS

AMICUS CURIAE SUPPORTING PETITIONER

____________

LEE MICKUS

EVANS FEARS SHUTTERT

MCNULTY MICKUS

1805 Shea Center Drive

Littleton, CO 80129

(303) 656-2199

lmickus@efsmmlaw.com

February 17, 2026

CORY L. ANDREWS

Counsel of Record

WASHINGTON LEGAL

FOUNDATION

2009 Mass. Ave., NW

Washington, DC 20036

(202) 588-0302

candrews@wlf.org

QUESTION PRESENTED

Whether, under Federal Rule of Evidence 702,

challenges to the factual basis of an expert witness’s

testimony always go to the weight of the evidence

rather than to admissibility, as the First and Fourth

Circuits hold, or whether such challenges go to weight

only if a court first finds it more likely than not that an

expert has a sufficient basis to support the testimony,

as the Fifth, Sixth, Eighth, Ninth, and Federal Circuits

hold.

iii

TABLE OF CONTENTS

Page

QUESTION PRESENTED ......................................... ii

TABLE OF CONTENTS ............................................ iii

TABLE OF AUTHORITIES ...................................... iv

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

INTRODUCTION & SUMMARY OF ARGUMENT .. 2

REASONS FOR GRANTING THE PETITION ......... 4

I.

REVIEW IS NEEDED TO ENSURE THAT

SCIENTIFIC RELIABILITY REMAINS A

QUESTION FOR THE DISTRICT COURT, NOT

THE JURY ........................................................... 4

II.

THE DECISION BELOW RESTS ON A

DEFUNCT

PRE-DAUBERT

VIEW

OF

ADMISSIBILITY ................................................... 8

III.

ONLY THIS COURT CAN PRESERVE THE

VITAL GOAL OF UNIFORMITY BEHIND THE

FEDERAL RULES OF EVIDENCE ......................... 15

CONCLUSION .......................................................... 18

iv

TABLE OF AUTHORITIES

Cases:

Page(s)

Allison v. McGhan Med. Corp.,

184 F.3d 1300 (11th Cir. 1999) ..................................7

Bresler v. Wilmington Tr. Co.,

855 F.3d 178 (4th Cir. 2017) ............ 3, 8, 9, 11–14, 17

Daubert v. Merrell Dow Pharms., Inc.,

509 U.S. 579 (1993) ............................................ 1, 4, 9

Gen. Elec. Co. v. Joiner,

522 U.S. 136 (1997) ................................................ 1, 4

Harris v. FedEx Corp. Servs., Inc.,

92 F.4th 286 (5th Cir. 2024) .................................... 13

Hurst v. United States,

882 F.2d 306 (8th Cir. 1989) .............................. 10, 11

Kumho Tire Co. v. Carmichael,

526 U.S. 137 (1999) ................................................ 1, 7

Loudermill v. Dow Chem. Co.,

863 F.2d 566 (8th Cir. 1988) .............................. 10–12

Michael’s Fabrics, LLC v. Donegal Mut. Ins. Co.,

2025 WL 2624280 (D. Md. Sept. 11, 2025) .............. 17

Nairne v. Landry,

151 F.4th 666 (5th Cir. 2025)................................... 13

Polk v. Ford Motor Co.,

529 F.2d 259 (8th Cir. 1976) .............................. 10, 11

R&J Components Corp. v. Centimark Corp.,

2025 WL 3732157 (D.S.C. Nov. 18, 2025) ............... 17

Sommerville v. Union Carbide Corp.,

149 F.4th 408 (4th Cir. 2025)................................... 17

v

TABLE OF AUTHORITIES

(continued)

Page(s)

S. Cent. Petroleum, Inc. v. Long Brothers Oil Co.,

974 F.2d 1015 (8th Cir. 1992) .................................. 10

Sprafka v. Med. Device Bus. Servs., Inc.,

139 F.4th 656 (8th Cir. 2025)................................... 12

Structural Polymer Grp. v. Zoltek Corp.,

543 F.3d 987 (8th Cir. 2008) ................................ 9, 10

Viterbo v. Dow Chem. Co.,

826 F.2d 420 (5th Cir. 1987) .............................. 11, 13

Weisgram v. Marley Co.,

528 U.S. 440 (2000) ................................................ 4, 6

Williams v. BP Exploration & Production, Inc.,

143 F.4th 593 (5th Cir. 2025)................................... 13

Statutes:

28 U.S.C. § 2072(a) ........................................................9

28 U.S.C. § 2072(b) .............................................. 3, 9, 14

Rules:

Fed. R. Civ. P. 26(a)(2)(B) ........................................... 11

Fed. R. Civ. P. 43(a) ....................................................15

Fed. R. Evid. 104(a) ........................................... 5, 13, 14

Fed. R. Evid. 702 ................................... 1–5, 7–9, 11–16

Fed. R. Evid. 705 ......................................................... 11

vi

TABLE OF AUTHORITIES

(continued)

Page(s)

Miscellaneous Sources:

David E. Bernstein, The Misbegotten Judicial

Resistance to the Daubert Revolution, 89 Notre

Dame L. Rev. 27 (2013) ............................................ 16

Jules Epstein, Cross-Examination: Seemingly

Ubiquitous, Purportedly Omnipotent, and “At

Risk”, 14 Widener L. Rev. 427 (2009) .................... 5, 6

Thomas F. Green, Jr., Drafting Uniform Federal

Rules of Evidence, 52 Cornell L.Q. 177 (1967) ........ 15

Lee Mickus, Amended Rule 702 in 2025: Circuit

Courts Embrace the Changed Standard, WLF

Legal Backgrounder (Oct. 7, 2025),

https://perma.cc/Z5SY-JPSH .....................................1

Kimberly S. Moore, Exploring the Inconsistencies

of Scrutinizing Expert Testimony Under the

Federal Rules of Evidence, 22 Tex. Tech L. Rev.

885 (1991) ................................................................. 16

Christopher B. Mueller, Daubert Asks the Right

Questions: Now Appellate Courts Should Help

Find the Right Answers, 33 Seton Hall L. Rev.

987 (2003) ...................................................................7

N.J. Schweitzer & Michael J. Saks, The Gatekeeper

Effect: The Impact of Judges’ Admissibility

Decisions on the Persuasiveness of Expert

Testimony, 15 Psychol. Pub. Pol'y & L. 1 (2009) .......7

vii

TABLE OF AUTHORITIES

(continued)

Page(s)

Victor E. Schwartz & Cary Silverman, The

Draining of Daubert and the Recidivism of Junk

Science in Federal and State Courts, 35 Hofstra

L. Rev. 217 (2006).................................................... 7,8

Hon. Patrick J. Schiltz, Report of the Advisory Comm.

on Evid. Rules (May 15, 2022),

https://perma.cc/QN6N-KEHA ..................................5

Hon. Thomas D. Schroeder, Toward a More Apparent

Approach to Considering the Admission of Expert

Testimony, 95 Notre Dame L. Rev. 2039

(2020) .................................................................. 13, 14

S. Rep. No. 93-1277 (1974) .......................................... 16

Cass R. Sunstein, Conformity: The Power of Social

Influences (2019) ...................................................... 14

19 Charles Alan Wright, Arthur R. Miller & Edward

H. Cooper, Federal Practice and Procedure § 4512

(2d ed. 1996) ............................................................. 15

1

INTEREST OF AMICUS CURIAE *

Washington Legal Foundation is a nonprofit,

public-interest law firm and policy center with

supporters nationwide. WLF promotes free enterprise,

individual rights, limited government, and the rule of

law. It often appears as an amicus curiae to urge

exclusion of scientifically unreliable expert evidence.

An early critic of “junk science,” WLF filed an amicus

brief in each of this Court’s Daubert trilogy cases. See

Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999);

Gen. Elec. Co. v. Joiner, 522 U.S. 136 (1997); Daubert

v. Merrell Dow Pharms., Inc., 509 U.S. 579 (1993).

WLF’s Legal Studies Division, the foundation’s

publishing arm, distributes articles by outside experts

on the proper reliability threshold for expert testimony.

See, e.g., Lee Mickus, Amended Rule 702 in 2025:

Circuit Courts Embrace the Changed Standard, WLF

Legal Backgrounder (Oct. 7, 2025), https://perma.cc/

Z5SY-JPSH.

The quality of decision-making in the federal

courts turns increasingly on the willingness of federal

judges to take seriously their responsibilities as

gatekeepers to stop unsound expert evidence from ever

reaching the jury. The decision below, if allowed to

stand, would severely erode this vital gatekeeping

function. Unless this Court intervenes, WLF fears that

the Fourth Circuit’s rule barring trial judges from

scrutinizing the “factual underpinnings” of expert

evidence will hollow out Federal Rule of Evidence 702,

* No party’s counsel authored any part of this brief. No one,

other than Washington Legal Foundation and its counsel, helped

pay for the brief’s preparation or submission. Counsel for WLF

timely notified all parties’ counsel of WLF’s intent to file this brief.

2

inviting injustices well beyond the facts and parties of

this case.

INTRODUCTION &

SUMMARY OF ARGUMENT

The Petition raises a profound question about

the integrity of our federal justice system—whether

trial judges must faithfully enforce Federal Rule of

Evidence 702 as a bulwark against unreliable expert

testimony, or whether they must sidestep that duty

and pass the buck to juries. Joining the First Circuit,

the Fourth Circuit has chosen the latter approach by

holding that challenges to the factual foundation of an

expert’s opinion go only to weight, not admissibility.

That ruling revives a lax, pre-Daubert-era view that

Rule 702 has abolished. As the Petition ably shows, this

view stands in direct conflict with the rigorous

approaches of the Fifth, Sixth, Eighth, Ninth, and

Federal Circuits. See Pet. 12–21.

This circuit split goes to the heart of fair trials,

where expert evidence often sways outcomes in

complex cases like this toxic-tort class action. By

defying the plain text of Rule 702—as clarified by the

2023 amendment—the decision below undermines

judicial gatekeeping, invites inconsistent verdicts, and

erodes public confidence in the courts. Three

compelling reasons support review.

First, Rule 702 demands that judges, not juries,

decide whether expert testimony is grounded in

sufficient facts or data before that testimony may reach

the jury. As Judge Diaz emphasized in his dissent, “A

modeling expert’s methodology doesn’t end with

selecting a modeling system. The expert must also

3

develop the inputs and assumptions used to create the

model.” Pet. App. 35a–36a. The Fourth Circuit’s

insistence that factual-input deficiencies are mere jury

questions abolishes this threshold inquiry, inviting

verdicts built on speculation rather than sound science.

Because jurors lack the tools to dissect and resolve

scientific flaws in evidence, this approach to

admissibility not only ignores the Daubert trilogy’s

mandate for vigilant gatekeeping but also the Advisory

Committee’s explicit warning that it is “an incorrect

application” of Rule 702. Fed. R. Evid. 702, Advisory

Comm. note to 2023 amendment

Second, the Fourth Circuit’s ruling hinges on its

prior precedent in Bresler v. Wilmington Tr. Co., 855

F.3d 178, 195 (4th Cir. 2017). But Bresler is a bygone

artifact of superseded caselaw, spawned by a daisy

chain of pre-Daubert decisions that even their

originating circuits—the Fifth and Eighth—have now

abandoned in favor of the amended Rule’s clear

standard. Still clinging to this “weight not

admissibility” relic, the decision below elevates stale

precedent over textualism and greenlights the very

junk science that Rule 702 excludes, flouting the Rule’s

evolution and purpose. See 28 U.S.C. § 2072(b) (“All

laws in conflict with such rules shall be of no further

force or effect after such rules have taken effect.”).

Third, the Federal Rules of Evidence were

enacted to foster national uniformity, ensuring that

evidentiary standards do not vary from circuit to

circuit. Yet this widening split now ensnares the

district courts in nine States, encouraging forumshopping and unequal justice under the law. In an era

of nationwide litigation, such fragmentation threatens

4

the coherence of federal law and the evenhanded

administration of justice that our system demands.

The interests of fairness, predictability, and the

rule of law were all injured in this case. WLF joins with

Petitioner in urging the Court to grant certiorari.

REASONS FOR GRANTING THE PETITION

I.

REVIEW IS NEEDED TO ENSURE THAT

SCIENTIFIC RELIABILITY REMAINS A QUESTION

FOR THE DISTRICT COURT, NOT THE JURY.

In holding that the district court abused its

discretion by excluding Dr. Sahu’s speculative airmodeling opinions, the Fourth Circuit departed from

this Court’s clear directive, now firmly embedded in

Rule 702, that all expert testimony must meet

“exacting standards of reliability.” Weisgram v. Marley

Co., 528 U.S. 440, 455 (2000). Under this Court’s

Daubert trilogy, district courts play a critical

“gatekeeping” role in shielding jurors from unreliable

expert evidence. It is thus incumbent on every trial

judge to “ensure that any and all [expert] testimony or

evidence admitted is not only relevant, but reliable.”

Daubert, 509 U.S. at 589.

That is why Rule 702 explicitly assigns the task

of ensuring the reliability of expert testimony to

district judges alone. Because an expert’s “conclusions

and methodology are not entirely distinct from one

another,” Joiner, 522 U.S. at 146, “an analysis of the

sufficiency of the expert’s basis cannot be divorced from

the ultimate reliability of the expert’s opinion.” Fed. R.

Evid. 702, Advisory Comm. note to 2000 amendment.

5

Under Rule 702, it’s no longer enough to invoke

the cross-examination of expert testimony while

leaving any dispute about scientific reliability to the

weight a jury decides to give that testimony. Yes,

challenges to the factual basis underlying an expert’s

conclusion may ultimately go to weight, but only if a

court first finds it more likely than not that the expert

has a sufficient basis to support the testimony.

In 2023, the Judicial Conference amended the

Rule to clarify this very point. The Advisory Committee

stressed that many courts were still wrongly treating

defects in the factual basis and methodology of an

expert’s opinion as questions of weight, not

admissibility, contrary to Rules 702 and 104(a). See

Fed. R. Evid. 702, Advisory Comm. note to 2023

amendment (“These rulings are an incorrect

application of Rules 702 and 104(a).”); see also see Hon.

Patrick J. Schiltz, Report of the Advisory Committee,

at 6 (May 15, 2022), https://perma.cc/QN6N-KEHA

(explaining that “treating these questions as ones of

weight rather than admissibility . . . is contrary to the

Supreme Court’s holdings”). The Fourth Circuit’s

insistence here that the district court should have

punted expert reliability to the jury—and, in fact, erred

by not doing so—defies this command.

Rule 702’s mandate that insufficient facts and

data behind an expert’s conclusion go not to weight but

to admissibility also comports with experience and

common sense. For while cross-examination has its

benefits, it is no panacea; it cannot readily distinguish

validly derived expert opinions from junk science. And

it can never take the court’s place in determining the

reliability of an expert’s opinion in the first instance.

As Professor Jules Epstein has explained:

6

This treatment of cross-examination as the

palliative of choice has its flaws, not merely

in its expectation that cross-examination

without other resources can fairly respond to

an expert witness. The mythic status of

cross-examination in this regard actually

impedes accurate fact-finding because

leading questions are not always an

appropriate or sufficient tool for truth

finding. Courts have not acknowledged these

limitations.

Jules

Epstein,

Cross-Examination:

Seemingly

Ubiquitous, Purportedly Omnipotent, and “At Risk,” 14

Widener L. Rev. 427, 437 (2009) (internal citations

omitted). That is why simply subjecting an expert

witness to a rigorous cross-examination does nothing

to ensure the reliability of that expert’s testimony;

reliability must always be assessed before the

testimony is admitted to the jury.

Here the Fourth Circuit overrode the district

court’s decision to exclude patently unreliable expert

testimony, insisting that a jury can somehow muddle

through the relevant science with the aid of competing

expert evidence and cross-examination. But dismissing

key, objective flaws in expert evidence as bearing only

on the weight of that evidence inevitably leaves jurors

with the rarefied task of resolving the basic reliability

of the expert’s scientific findings. Jurors cannot be

expected—and should not be permitted—to make those

sorts of reliability determinations. Above all,

unreliable evidence “contributes nothing to a ‘legally

sufficient evidentiary basis’” for a jury verdict.

Weisgram, 528 U.S. at 454 (internal citation omitted).

As for how much “weight” a jury should give unreliable

7

expert evidence, the only acceptable answer is none. It

should never reach the jury in the first place.

Legal scholars have long noted that “crossexamination does little to affect jury appraisals of

expert testimony.” Christopher B. Mueller, Daubert

Asks the Right Questions: Now Appellate Courts

Should Help Find the Right Answers, 33 Seton Hall L.

Rev. 987, 993 (2003). Indeed, multiple studies have

revealed jurors’ commonly held assumption that,

because the trial judge admitted the expert evidence, it

must have at least passed some test of reliability. See,

e.g., N.J. Schweitzer & Michael J. Saks, The Gatekeeper

Effect: The Impact of Judges’ Admissibility Decisions

on the Persuasiveness of Expert Testimony, 15 Psychol.

Pub. Pol’y & L. 1, 7 (2009). This is precisely why trial

judges have a duty to avoid “dumping a barrage of

questionable” evidence on a jury likely to be “awestruck

by the expert’s mystique.” Allison v. McGhan Med.

Corp., 184 F.3d 1300, 1310 (11th Cir. 1999).

“The basic calipers that jurors use to evaluate

testimony—their own life experience—are of little

value when jurors evaluate whether an expert is telling

the truth.” Victor E. Schwartz & Cary Silverman, The

Draining of Daubert and the Recidivism of Junk

Science in Federal and State Courts, 35 Hofstra L. Rev.

217, 220 (2006). This Court has long insisted that any

questions about the “factual basis, data, principles, [or]

methods” of expert testimony—or “their application”—

require the trial judge to determine whether that

testimony is reliable before sending it to the jury.

Kumho Tire Co., 526 U.S. at 149 (emphasis added); see

Fed. R. Evid. 702(b), (d).

8

“When a court looks to the data underlying

expert opinion but neglects to evaluate its relation to

the expert’s conclusion . . . ostensibly legitimate data

may serve as a Trojan horse that facilitates the

surreptitious advance of junk science and spurious,

unreliable opinions.” Schwartz & Silverman, The

Draining of Daubert, supra, at 237–38. Again, the only

way to ensure that a jury does not give too much weight

to unreliable evidence is to exclude it. Yet in the Fourth

Circuit, as well as the First, doing so is deemed an

abuse of discretion. That is untenable.

II.

THE DECISION BELOW RESTS ON A DEFUNCT

PRE-DAUBERT VIEW OF ADMISSIBILITY.

Among other things, a proponent of expert

testimony must show that it is more likely than not

that “the testimony is based on sufficient facts or data.”

Fed. R. Evid. 702(b). The Fourth Circuit’s misguided

notion that objections to the factual basis of an expert’s

opinion go only to the weight and credibility of that

opinion, not to its admissibility, cannot be reconciled

with Rule 702. No surprise, then, that close

examination of the authorities cited by the panel

majority reveals that the Fourth Circuit’s rule of

decision derives not from an attentive analysis of Rule

702’s words, but from a hapless game of follow-theleader stretching back half a century.

In holding that the district court abused its

discretion by scrutinizing the factual basis for Dr.

Sahu’s opinions, the Fourth Circuit relies on its 2017

decision in Bresler v. Wilmington Trust Co.:

This was an abuse of discretion because

“questions

regarding

the

factual

9

underpinnings of the [expert witness’]

opinion affect the weight and credibility of

the

witness’

assessment,

not

its

admissibility.” See Bresler v. Wilmington Tr.

Co., 855 F.3d 178, 195 (4th Cir.

2017) (cleaned up).

Pet. App. 21a. Twice again for good measure, the

opinion quotes this same language from Bresler. Pet.

App. 22a, 27a.

Yet in relying on Bresler to reverse the district

court, the Fourth Circuit commits two critical errors.

First, the panel majority mistakenly assumes that

caselaw, rather than Rule 702, supplies the proper

standard for admissibility. See 28 U.S.C. § 2072(a)

(authorizing the U.S. Supreme Court to prescribe

“rules of evidence for cases in the United States district

courts . . . and courts of appeals”); 28 U.S.C. § 2072(b)

(“All laws in conflict with such rules shall be of no

further force or effect after such rules have taken

effect.”); Daubert, 509 U.S. at 588–589 (Rule 702 is the

authority “governing expert testimony”).

Second,

Bresler’s

sweeping

“factual

underpinnings” rule does not spring from the text of

Rule 702, nor even caselaw construing it. Rather, it

smuggles in a pre-Daubert view of expert admissibility

that some judges have reflexively recycled, for decades,

without analysis or scrutiny. In fact, the Bresler rule’s

lineage traces back to Eighth and Fifth Circuit

opinions from the 1970s and 1980s.

Bresler stands on a single quote from the Eighth

Circuit’s opinion in Structural Polymer Grp. v. Zoltek

Corp., 543 F.3d 987, 997 (8th Cir. 2008). See Bressler,

10

855 F.3d at 195 (“[Q]uestions regarding the factual

underpinnings of the [expert witness’] opinion affect

the weight and credibility’ of the witness’ assessment,

‘not its admissibility.’”) (quoting Structural Polymer,

543 F.3d at 997).

“As a rule,” Structural Polymer maintains,

“questions regarding the factual underpinnings of the

expert’s opinion affect the weight and credibility of her

testimony, not its admissibility.” 543 F.3d at 997. But

in support of this supposed “rule,” Structural Polymer

cites and paraphrases a pre-Daubert case, South

Central Petroleum, Inc. v. Long Brothers Oil Co., 974

F.2d 1015, 1019 (8th Cir. 1992) (“The expert’s opinion

should be excluded only if it is ‘so fundamentally

unsupported that it cannot help the factfinder.’”). Id.

South Central Petroleum, in turn, draws from

Hurst v. United States, 882 F.2d 306, 311 (8th Cir.

1989). Recycling the discredited pre-Daubert view that

“[a] trial court should exclude an expert opinion only if

it is so fundamentally unsupported that it cannot help

the factfinder,” Hurst insists that “[a]ny weaknesses in

the factual underpinnings of (the expert’s) opinion go

to the weight and credibility of his testimony, not to its

admissibility.” 882 F.2d at 311.

Yet Hurst borrows this notion from Loudermill

v. Dow Chem. Co., 863 F.2d 566, 570 (8th Cir. 1988)

(“Any weaknesses in the factual underpinnings of

Wuebben’s opinion go to the weight and credibility of

his testimony, not to its admissibility.”), and Polk v.

Ford Motor Co., 529 F.2d 259, 271 (8th Cir. 1976) (“The

weakness in the underpinnings of such opinions may

be developed upon cross-examination and such

11

weakness goes to the weight and credibility of the

testimony.”).

Loudermill repeats the “general rule” that the

“factual basis of an expert opinion goes to the

credibility of the testimony, not the admissibility, and

it is up to the opposing party to examine the factual

basis for the opinion in cross-examination.” 863 F.2d at

570. But Loudermill derives this supposed rule from

the Fifth Circuit’s opinion in Viterbo v. Dow Chem. Co.,

826 F.2d 420, 422 (5th Cir. 1987). Id.

Decided thirteen years before Rule 702’s postDaubert-trilogy overhaul in 2000, Viterbo supplies this

foundational statement: “As a general rule, questions

relating to the bases and sources of an expert’s opinion

affect the weight to be assigned that opinion rather

than its admissibility and should be left for the jury’s

consideration.” 826 F.2d at 422.

And Polk, also cited by Hurst, includes language

that closely tracks the Fourth Circuit’s Bresler rule.

529 F.2d at 271. But that assertion is based on an

unusually lax interpretation of then-Rule 705:

Indeed, the newly adopted Federal Rules of

Evidence make it clear that an expert may

give his conclusions without prior disclosure

of the underlying facts. Fed. R. Evid. 705.

The weakness in the underpinnings of such

opinions may be developed upon crossexamination and such weakness goes to the

weight and credibility of the testimony.

Id. Polk turns 50 this year, and its anachronistic

reliance on Rule 705 no longer holds water, given

12

Federal Rule of Civil Procedure 26(a)’s robust

disclosure requirements since 1993. See Fed. R. Civ. P.

26(a)(2)(B) (mandating disclosure of “the facts or data

considered by the witness in forming” an expert

opinion).

Although Bresler’s wrong turn originated in the

Eighth and Fifth Circuits, both those courts have,

following adoption of the 2023 amendment to Rule 702,

disclaimed any suggestion that the “factual

underpinnings” of an expert’s opinions go to weight and

not admissibility. In Sprafka v. Medical Device Bus.

Svcs., 139 F.4th 656 (8th Cir. 2025), the Eighth Circuit

notes that the 2023 amendment was “necessary

because many courts had incorrectly held ‘that the

critical questions of the sufficiency of an expert’s basis,

and the application of the expert’s methodology, are

questions of weight and not admissibility.’” Id. at 660

& n.3 (citing Fed. R. Evid. 702 Advisory Comm. note to

2023 amendment).

Of course, this concession pulls out the rug from

under Loudermill’s “general rule” that an expert’s

factual basis is not a proper gatekeeping consideration.

But the court in Sprafka goes beyond simply

recognizing the Advisory Committee’s purpose—it

declares that expert opinions “lack reliability” and

must be excluded if the court finds that they lack an

adequate factual basis. Id. at 660.

The 2023 amendment also changed the Fifth

Circuit’s understanding of the gatekeeping standard.

Now “[t]here is no question that Federal Rule of

Evidence 702 governs the admissibility of expert

testimony,” and under that rule “expert testimony may

not be admitted unless the proponent demonstrates to

13

the court that it is more likely than not that the

proffered

testimony

meets

the

admissibility

requirements set forth in the rule.” Nairne v. Landry,

151 F.4th 666, 697, 698 n.20 (5th Cir. 2025).

The Fifth Circuit has also heeded the Advisory

Committee’s caution that prior decisions suggesting

that “critical questions of the sufficiency of an expert’s

basis, and the application of the expert’s methodology,

are questions of weight and not admissibility” are “an

incorrect application of Rules 702 and 104(a).” Id. at

697–98 (quoting Fed. R. Evid. 702 Advisory Comm.

note to 2023 amendment). Breaking with the Viterbo

line, the Fifth Circuit has clarified in multiple decisions

that an expert’s opinions must be excluded under Rule

702(b) if it is not based on sufficient facts or data.

Harris v. FedEx Corp. Svcs., Inc., 92 F.4th 286, 303 (5th

Cir. 2024); Williams v. BP Exploration & Production,

Inc., 143 F.4th 593, 601 (5th Cir. 2025), Nairne, 151

F.4th at 698. Indeed, the trial court “abdicate[s] its role

as gatekeeper” if it allows an expert “to testify without

a proper foundation” under Rule 702(b). Harris, 92

F.4th at 303–04. Yet again, in the First and Fourth

Circuits, this view constitutes an abuse of discretion.

Judge Thomas D. Schroeder, who chaired the

Rule 702 Subcommittee that drafted the 2023

amendment, has openly criticized the Fourth Circuit’s

Bresler rule for causing great mischief. See Hon.

Thomas D. Schroeder, Toward a More Apparent

Approach to Considering the Admission of Expert

Testimony, 95 Notre Dame L. Rev. 2039 (2020). In his

law review article, Judge Schroeder catalogues a

number of “illustrative cases” that the Advisory

Committee identified “as evidence that courts are

14

abdicating their gatekeeper role.” Id. at 2043–59.

Bresler is one of them. Id. at 249–50.

“Without explanation,” Judge Schroeder

explains, Bresler unaccountably “concluded that the

defendants’ challenge amounted to a ‘disagreement’

with the values the expert chose for certain variables

in his opinion and consequently ‘affect[ed] the weight

and credibility’ of [the expert’s] assessment, not its

admissibility.’” 95 Notre Dame L. Rev. at 2049–50

(quoting Bresler, 855 F.3d at 195–96). “As a general

rule, the Fourth Circuit’s statement effectively vitiated

the application of Rule 104(a) to Rule 702(b).” Id.

(emphasis added).

The Fourth Circuit’s “factual underpinnings”

rule has all the hallmarks of what Cass Sunstein calls

a “precedential cascade,” in which some courts “come

into line” with a few decisionmakers’ initial (mistaken)

judgments. Cass Sunstein, Conformity: The Power of

Social Influences 42 (2019). As the number of

authorities repeating the wrong rule accumulates,

courts stop looking at the matter with fresh eyes. They

feel “the great weight” of prior courts’ repeated

assertions of the supposed rule, “and perhaps

insufficiently appreciat[e] the extent to which that

weight is a product of an early and somewhat

idiosyncratic judgment.” Id. “This can happen a lot,”

actually, and “it makes for bad law.” Id.

In sum, these throwback cases conflict with Rule

702’s renewed rigor and are no longer good law. See 28

U.S.C. § 2072(b). This Court should intervene and

bring admissibility in the First and Fourth Circuits

into the 21st century.

15

III.

ONLY THIS COURT CAN PRESERVE THE VITAL

GOAL OF UNIFORMITY BEHIND THE FEDERAL

RULES OF EVIDENCE.

The proponent of an expert witness must first

show by a preponderance of the evidence that the

expert’s testimony answers the “critical questions of

the sufficiency of an expert’s basis, and the application

of the expert’s methodology.” Fed. R. Evid.

702, Advisory Comm. note to 2023 amendment. Rule

702 cannot mean one thing in the First and Fourth

Circuits and the opposite in their sister circuits. It was

precisely this kind of disparity on evidentiary

questions among federal courts that was the catalyst

for adopting federal rules of evidence in the first place.

See generally Thomas F. Green, Jr., Drafting Uniform

Federal Rules of Evidence, 52 Cornell L.Q. 177, (1967)

(explaining that the new rules will be “uniform” in

“that the precepts will be the same throughout the

United States and will not vary from state to state or

circuit to circuit to the extent that they do in some

instances today”).

Before enactment of the Federal Rules of

Evidence, Federal Rule of Civil Procedure 43(a)

governed evidentiary matters in federal courts. Under

that rule, federal courts typically had to apply the

forum state’s evidentiary law to questions about the

admissibility of evidence. 19 Charles Alan Wright,

Arthur R. Miller & Edward H. Cooper, Federal Practice

and Procedure § 4512 (2d ed. 1996) (“[T]he norm was to

follow forum state law on evidentiary matters, not

because Erie demanded it, but simply because the Civil

Rule so directed.”).

16

That disjointed approach to admissibility

created “a real need for a comprehensive code of

evidence intended to govern the admissibility of proof

in all trials before the Federal courts because of the

lack of uniformity and clarity in the present law of

evidence on the Federal level.” S. Rep. No. 93-1277

(1974). The “culmination of 13 years of study by

distinguished judges, Members of Congress, lawyers,

and others interested in and affected by the

administration of justice in the Federal courts,” the

Federal Rules of Evidence sought to eliminate the

widespread disparity among federal courts on the

admissibility of evidence. Id.

Indeed, the Federal Rules of Evidence were

bottomed entirely on the need for uniformity and

predictability in the federal courts. Kimberly S. Moore,

Exploring the Inconsistencies of Scrutinizing Expert

Testimony Under the Federal Rules of Evidence, 22 Tex.

Tech. L. Rev. 885, 885 (1991) (“[T]he purpose of codified

rules of evidence is to ensure consistency, uniformity,

and fairness throughout the judicial system.”). Rule

702 “not only codifies revolutionary changes in the

substantive law” but also imposes “substantial new

demands on judges by requiring a far more managerial

role for judges than they are used to assuming in the

American adversarial system.” David E. Bernstein,

The Misbegotten Judicial Resistance to the Daubert

Revolution, 89 Notre Dame L. Rev. 27, 66 (2013).

Despite the renewed clarity of the Rule’s recent

2023 amendment, the First and Fourth Circuits

remain hopelessly lost on the issue of whether the

court, and not a jury, should decide if an expert’s

testimony satisfies the Rule’s stringent requirements.

Because this disparity in the rule of law arises from a

17

fundamental disagreement over the proper application

of the Federal Rules of Evidence, only this Court’s

intervention can provide a unified, coherent fix.

The Fourth Circuit’s deeply misguided rule is

already having harmful downstream effects. A

defendant’s “questions regarding the factual

underpinnings of the [expert witness’] opinion,” the

District of Maryland recently expounded, “‘affect the

weight and credibility’ of the witness’ assessment, ‘not

its admissibility.’” Michael’s Fabrics, LLC v. Donegal

Mut. Ins. Co., 2025 WL 2624280, at *4 (D. Md. Sep. 11,

2025) (quoting Bresler, 855 F.3d at 195, and citing

Sommerville v. Union Carbide Corp, 149 F.4th 408, 427

n.7 (4th Cir. 2025)). Bound by the misguided decision

below, district courts in the Fourth Circuit are now

obliged to shirk their gatekeeping duties or else risk

reversal for abuse of discretion. See, e.g., R&J

Components Corp v. Centimark Corp., 2025 WL

3732157, at *6 (D.S.C. Nov. 18, 2025) (admitting

testimony of expert who “assigned value to items that

his own investigation concluded did not show any

evidence of damage or contamination”) (citing and

quoting Sommerville, 149 F.4th at 423).

It is impossible to overstate the detrimental

effect the decision below—and imminent decisions in

the nine federal districts bound by it—will have on the

very uniformity the Federal Rules of Evidence are

meant to accomplish. More than 30 years after its

seminal holding in Daubert, the Court should end this

alarming trend by interceding now—before the Fourth

Circuit’s recalcitrance can do any further damage.

18

CONCLUSION

This Court should grant the writ.

Respectfully submitted,

LEE MICKUS

EVANS FEARS SHUTTERT

MCNULTY MICKUS

1805 Shea Center Drive

Littleton, CO 80129

(303) 656-2199

lmickus@efsmmlaw.com

February 17, 2026

CORY L. ANDREWS

Counsel of Record

WASHINGTON LEGAL

FOUNDATION

2009 Mass. Ave., NW

Washington, DC 20036

(202) 588-0302

candrews@wlf.org