CITE:  509 U.S. 579
CMON:  June 1993
PLAIN: Daubert
DEFND: Merrell Dow Pharmacueticals
COURT: United States Supreme Court
DATE:  June 28, 1993

HISTORY:

On Writ of Certiorari to the United States Court of Appeals for the Ninth
Circuit.  951 F.2d 1128, vacated and remanded. 

Petitioners, two minor children and their parents, alleged in their suit
against respondent that the children's serious birth defects had been
caused by the mothers' prenatal ingestion of Bendectin, a prescription drug
marketed by respondent. The District Court granted respondent summary judgment
based on a well-credentialed expert's affidavit concluding, upon reviewing the
extensive published scientific literature on the subject, that maternal use of
Bendectin has not been shown to be a risk factor for human birth defects.
Although petitioners had responded with the testimony of eight other
well-credentialed experts, who based their conclusion that Bendectin can cause
birth defects on animal studies, chemical structure analyses, and the
unpublished "reanalysis" of previously published human statistical studies,
the court determined that this evidence did not meet the applicable "general
acceptance" standard for the admission of expert testimony. The Court of
Appeals agreed and affirmed, citing Frye v. United States, 54 App. D.C. 46,
47, 293 F. 1013, 1014, for the rule that expert opinion based on a scientific
technique is inadmissible unless the technique is "generally accepted" as
reliable in the relevant scientific community. 
 
Held: The Federal Rules of Evidence, not Frye, provide the standard for
admitting expert scientific testimony in a federal trial. Pp. 585-597. 
 
(a) Frye's "general acceptance" test was superseded by the Rules' adoption.
The Rules occupy the field, United States v. Abel, 469 U.S. 45, 49, 83 L. Ed.
2d 450, 105 S. Ct. 465, and, although the common law of evidence may serve as
an aid to their application, id., at 51-52, respondent's assertion that they
some-how assimilated Frye is unconvincing. Nothing in the Rules as a whole or
in the text and drafting history of Rule 702, which specifically governs
expert testimony, gives any indication that "general acceptance" is a
necessary precondition to the admissibility of scientific evidence. Moreover,
such a rigid standard would be at odds with the Rules' liberal thrust and
their general approach of relaxing the traditional barriers to "opinion"
testimony. Pp. 585-589. 
 
(b) The Rules -- especially Rule 702 -- place appropriate limits on the
admissibility of purportedly scientific evidence by assigning to the trial
judge the task of ensuring that an expert's testimony both rests on a reliable
foundation and is relevant to the task at hand. The reliability standard is
established by Rule 702's requirement that an expert's testimony pertain to
"scientific . . . knowledge", since the adjective "scientific" implies a
grounding in science's methods and procedures, while the word "knowledge"
connotes a body of known facts or of ideas inferred from such facts or
accepted as true on good grounds. The Rule's requirement that the testimony
"assist the trier of fact to understand the evidence or to determine a fact in
issue" goes primarily to relevance by demanding a valid scientific connection
to the pertinent inquiry as a precondition to admissibility. Pp. 589-592. 
 
(c) Faced with a proffer of expert scientific testimony under Rule 702, the
trial judge, pursuant to Rule 104(a), must make a preliminary assessment of
whether the testimony's underlying reasoning or methodology is scientifically
valid and properly can be applied to the facts at issue. Many considerations
will bear on the inquiry, including whether the theory or technique in
question can be (and has been) tested, whether it has been subjected to peer
review and publication, its known or potential error rate and the existence
and maintenance of standards controlling its operation, and whether it has
attracted widespread acceptance within a relevant scientific community. The
inquiry is a flexible one, and its focus must be solely on principles and
methodology, not on the conclusions that they generate. Throughout, the
judge should also be mindful of other applicable Rules. Pp. 592-595. 
 
(d) Cross-examination, presentation of contrary evidence, and careful
instruction on the burden of proof, rather than wholesale exclusion under an
uncompromising "general acceptance" standard, is the appropriate means by
which evidence based on valid principles may be challenged. That even limited
screening by the trial judge, on occasion, will prevent the jury from hearing
of authentic scientific breakthroughs is simply a consequence of the fact
that the Rules are not designed to seek cosmic understanding but, rather,
to resolve legal disputes. Pp. 595-597. 

SUMMARY:
  To be scientific, a theory has to be tested, has to be subject to
  peer review and publication, have a known or potential error rate and
  the existence and maintenance of standards controlling its operation,
  and has to attract widespread acceptance within a relevant scientific
  community.

JUDGE: MR. JUSTICE BLACKMUN
BLACKMUN, J., delivered the opinion for a unanimous Court with respect
to Parts I and II-A, and the opinion of the Court with respect to Parts II-B,
II-C, III, and IV, in which WHITE, O'CONNOR, SCALIA, KENNEDY, SOUTER, and
THOMAS, JJ., joined.

REHNQUIST, C. J., filed an opinion concurring in part and dissenting in
part, in which STEVENS, J., joined, post, p. 598. 

DECISION:

In this case we are called upon to determine the standard for admitting expert
scientific testimony in a federal trial. 

                                       I 

Petitioners Jason Daubert and Eric Schuller are minor children born with
serious birth defects. They and their parents sued respondent in California
state court, alleging that the birth defects had been caused by the mothers'
ingestion of Bendectin, a prescription antinausea drug marketed by respondent.
Respondent removed the suits to federal court on diversity grounds. 

After extensive discovery, respondent moved for summary judgment, contending
that Bendectin does not cause birth defects in humans and that petitioners
would be unable to come forward with any admissible evidence that it does. In
support of its motion, respondent submitted an affidavit of Steven H. Lamm,
physician and epidemiologist, who is a well-credentialed expert on the risks
from exposure to various chemical substances. {1} Doctor Lamm stated that he
had reviewed all the literature on Bendectin and human birth defects -- more
than 30 published studies involving over 130,000 patients. No study had found
Bendectin to be a human teratogen (i.e., a substance capable of causing
malformations in fetuses). On the basis of this review, Doctor Lamm concluded
that maternal use of Bendectin during the first trimester of pregnancy
has not been shown to be a risk factor for human birth defects. 
 
Petitioners did not (and do not) contest this characterization of the
published record regarding Bendectin. Instead, they responded to respondent's
motion with the testimony of eight experts of their own, each of whom also
possessed impressive credentials. {2}   These experts had concluded that
Bendectin can cause birth defects. Their conclusions were based upon
"in vitro" (test tube) and "in vivo" (live) animal studies that found a
link between Bendectin and malformations; pharmacological studies of the
chemical structure of Bendectin that purported to show similarities between
the structure of the drug and that of other substances known to cause birth
defects; and the "reanalysis" of previously published epidemiological
(human statistical) studies. 

The District Court granted respondent's motion for summary judgment. The court
stated that scientific evidence is admissible only if the principle upon which
it is based is "'sufficiently established to have general acceptance in the
field to which it belongs.'" 727 F. Supp. 570, 572 (SD Cal. 1989), quoting
United States v. Kilgus, 571 F.2d 508, 510 (CA9 1978). The court concluded
that petitioners' evidence did not meet this standard. Given the vast body of
epidemiological data concerning Bendectin, the court held, expert opinion
which is not based on epidemiological evidence is not admissible to
establish causation. 727 F. Supp. at 575. Thus, the animal-cell studies,
live-animal studies, and chemical-structure analyses on which petitioners
had relied could not raise by themselves a reasonably disputable jury issue
regarding causation. Ibid. Petitioners' epidemiological analyses, based as
they were on recalculations of data in previously published studies that had
found no causal link between the drug and birth defects, were ruled to be
inadmissible because they had not been published or subjected to peer review.
Ibid. 

The United States Court of Appeals for the Ninth Circuit affirmed. 951 F.2d
1128 (1991). Citing Frye v. United States, 54 App. D.C. 46, 47, 293 F. 1013,
1014 (1923), the court stated that expert opinion based on a scientific
technique is inadmissible unless the technique is "generally accepted" as
reliable in the relevant scientific community. 951 F.2d at 1129-1130. The
court declared that expert opinion based on a methodology that diverges
"significantly from the procedures accepted by recognized authorities in
the field . . . cannot be shown to be 'generally accepted as a reliable
technique.'" Id., at 1130, quoting United States v. Solomon, 753 F.2d 1522,
1526 (CA9 1985). 

The court emphasized that other Courts of Appeals considering the risks of
Bendectin had refused to admit reanalyses of epidemiological studies that had
been neither published nor subjected to peer review. 951 F.2d at 1130-1131.
Those courts had found unpublished reanalyses "particularly problematic in
light of the massive weight of the original published studies supporting
[respondent's] position, all of which had undergone full scrutiny from the
scientific community." Id., at 1130. Contending that reanalysis is generally
accepted by the scientific community only when it is subjected to verification
and scrutiny by others in the field, the Court of Appeals rejected
petitioners' reanalyses as "unpublished, not subjected to the normal peer
review process and generated solely for use in litigation." Id., at 1131.
The court concluded that petitioners' evidence provided an insufficient
foundation to allow admission of expert testimony that Bendectin caused their
injuries and, accordingly, that petitioners could not satisfy their burden
of proving causation at trial. 

We granted certiorari, 506 U.S. 914 (1992), in light of sharp divisions
among the courts regarding the proper standard for the admission of expert
testimony. Compare, e.g., United States v. Shorter, 257 U.S. App. D.C. 358,
363-364, 809 F.2d 54, 59-60 (applying the "general acceptance" standard),
cert. denied, 484 U.S. 817, 98 L. Ed. 2d 35, 108 S. Ct. 71 (1987), with DeLuca
v. Merrell Dow Pharmaceuticals, Inc., 911 F.2d 941, 955 (CA3 1990) (rejecting
the "general acceptance" standard). 

                                       II 

                                       A 

In the 70 years since its formulation in the Frye case, the "general
acceptance" test has been the dominant standard for determining the
admissibility of novel scientific evidence at trial. See E. Green & C. Nesson,
Problems, Cases, and Materials on Evidence 649 (1983). Although under
increasing attack of late, the rule continues to be followed by a majority
of courts, including the Ninth Circuit. {3}
 
The Frye test has its origin in a short and citation-free 1923 decision
concerning the admissibility of evidence derived from a systolic blood
pressure deception test, a crude precursor to the polygraph machine. In what
has become a famous (perhaps infamous) passage, the then Court of Appeals for
the District of Columbia described the device and its operation and
declared: 
   
    "Just when a scientific principle or discovery crosses the line
    between the experimental and demonstrable stages is difficult to
    define.  Somewhere in this twilight zone the evidential force of
    the principle must be recognized, and while courts will go a long
    way in admitting expert testimony deduced from a well-recognized
    scientific principle or discovery, the thing from which the deduction
    is made must be sufficiently established to have gained general
    acceptance in the particular field in which it belongs." 54 App. D.C.
    at 47, 293 F. at 1014 (emphasis added).
 
Because the deception test had "not yet gained such standing and scientific
recognition among physiological and psychological authorities as would justify
the courts in admitting expert testimony deduced from the discovery,
development, and experiments thus far made", evidence of its results
was ruled inadmissible. Ibid. 
 
The merits of the Frye test have been much debated, and scholarship on its
proper scope and application is legion. {4}  Petitioners' primary attack,
however, is not on the content but on the continuing authority of the rule.
They contend that the Frye test was superseded by the adoption of the
Federal Rules of Evidence. {5}  We agree. 
 
We interpret the legislatively enacted Federal Rules of Evidence as we would
any statute. Beech Aircraft Corp. v. Rainey, 488 U.S. 153, 163, 102 L. Ed. 2d
445, 109 S. Ct. 439 (1988). Rule 402 provides the baseline:

    "All relevant evidence is admissible, except as otherwise provided by
    the Constitution of the United States, by Act of Congress, by these
    rules, or by other rules prescribed by the Supreme Court pursuant to
    statutory authority. Evidence which is not relevant is not admissible."
 
"Relevant evidence" is defined as that which has "any tendency to make the
existence of any fact that is of consequence to the determination of the
action more probable or less probable than it would be without the evidence."
Rule 401. The Rules' basic standard of relevance thus is a liberal one. 

Frye, of course, predated the Rules by half a century. In United States v.
Abel, 469 U.S. 45, 83 L. Ed. 2d 450, 105 S. Ct. 465 (1984), we considered the
pertinence of background common law in interpreting the Rules of Evidence. We
noted that the Rules occupy the field, id., at 49, but, quoting Professor
Cleary, the Reporter, explained that the common law nevertheless could
serve as an aid to their application:
   
    "'In principle, under the Federal Rules no common law of evidence
    remains.  "All relevant evidence is admissible, except as otherwise
    provided . . . ."  In reality, of course, the body of common law
    knowledge continues to exist, though in the somewhat altered form
    of a source of guidance in the exercise of delegated powers.'"
    Id., at 51-52.
 
We found the common-law precept at issue in the Abel case entirely consistent
with Rule 402's general requirement of admissibility, and considered it
unlikely that the drafters had intended to change the rule. Id., at 50-51.
In Bourjaily v. United States, 483 U.S. 171, 97 L. Ed. 2d 144, 107 S.Ct.
2775 (1987), on the other hand, the Court was unable to find a particular
common-law doctrine in the Rules, and so held it superseded. 
 
Here there is a specific Rule that speaks to the contested issue. Rule 702,
governing expert testimony, provides:

  "If scientific, technical, or other specialized knowledge will assist
  the trier of fact to understand the evidence or to determine a fact in
  issue, a witness qualified as an expert by knowledge, skill, experience,
  training, or education, may testify thereto in the form of an opinion
  or otherwise."
 
Nothing in the text of this Rule establishes "general acceptance" as an
absolute prerequisite to admissibility. Nor does respondent present any clear
indication that Rule 702 or the Rules as a whole were intended to incorporate
a "general acceptance" standard. The drafting history makes no mention of
Frye, and a rigid "general acceptance" requirement would be at odds with the
"liberal thrust" of the Federal Rules and their "general approach of relaxing
the traditional barriers to 'opinion' testimony." Beech Aircraft Corp. v.
Rainey, 488 U.S. at 169 (citing Rules 701 to 705). See also Weinstein, Rule
702 of the Federal Rules of Evidence is Sound; It Should Not Be Amended,
138 F.R.D. 631 (1991) ("The Rules were designed to depend primarily upon
lawyer-adversaries and sensible triers of fact to evaluate conflicts").
Given the Rules' permissive backdrop and their inclusion of a specific rule on
expert testimony that does not mention "general acceptance," the assertion
that the Rules somehow assimilated Frye is unconvincing. Frye made "general
acceptance" the exclusive test for admitting expert scientific testimony. That
austere standard, absent from, and incompatible with, the Federal Rules of
Evidence, should not be applied in federal trials. {6}
 
That the Frye test was displaced by the Rules of Evidence does not mean, 
however, that the Rules themselves place no limits on the admissibility
of purportedly scientific evidence. {7} Nor is the trial judge disabled
from screening such evidence. To the contrary, under the Rules the trial
judge must ensure that any and all scientific testimony or evidence
admitted is not only relevant, but reliable. 
 
The primary locus of this obligation is Rule 702, which clearly contemplates
some degree of regulation of the subjects and theories about which an expert
may testify. "If scientific, technical, or other specialized knowledge will
assist the trier of fact to understand the evidence or to determine a fact in
issue" an expert "may testify thereto." (Emphasis added.) The subject of an
expert's testimony must be "scientific . . . knowledge." {8} The adjective
"scientific" implies a grounding in the methods and procedures of science.
Similarly, the word "knowledge" connotes more than subjective belief or
unsupported speculation. The term "applies to any body of known facts or
to any body of ideas inferred from such facts or accepted as truths on
good grounds." Webster's Third New International Dictionary 1252 (1986). Of
course, it would be unreasonable to conclude that the subject of scientific
testimony must be "known" to a certainty; arguably, there are no certainties
in science. See, e.g., Brief for Nicolaas Bloembergen et al. as Amici Curiae 9
("Indeed, scientists do not assert that they know what is immutably 'true' --
they are committed to searching for new, temporary, theories to explain, as
best they can, phenomena"); Brief for American Association for the Advancement
of Science et al. as Amici Curiae 7-8 ("Science is not an encyclopedic body of
knowledge about the universe. Instead, it represents a process for proposing
and refining theoretical explanations about the world that are subject to
further testing and refinement" (emphasis in original)). But, in order to
qualify as "scientific knowledge," an inference or assertion must be derived
by the scientific method. Proposed testimony must be supported by appropriate
validation -- i.e., "good grounds," based on what is known. In short, the
requirement that an expert's testimony pertain to "scientific knowledge"
establishes a standard of evidentiary reliability. {9}
 
Rule 702 further requires that the evidence or testimony "assist the trier
of fact to understand the evidence or to determine a fact in issue". This
condition goes primarily to relevance. "Expert testimony which does not relate
to any issue in the case is not relevant and, ergo, non-helpful." 3 Weinstein
& Berger P702[02], p. 702-18. See also United States v. Downing, 753 F.2d
1224, 1242 (CA3 1985) ("An additional consideration under Rule 702 -- and
another aspect of relevancy -- is whether expert testimony proffered in
the case is sufficiently tied to the facts of the case that it will aid the
jury in resolving a factual dispute"). The consideration has been aptly
described by Judge Becker as one of "fit." Ibid. "Fit" is not always obvious, 
and scientific validity for one purpose is not necessarily scientific
validity for other, unrelated purposes. See Starrs, Frye v. United States
Restructured and Revitalized: A Proposal to Amend Federal Evidence Rule 702,
26 Jurimetrics J. 249, 258 (1986). The study of the phases of the moon, for
example, may provide valid scientific "knowledge" about whether a certain
night was dark, and if darkness is a fact in issue, the knowledge will assist
the trier of fact. However (absent creditable grounds supporting such a link),
evidence that the moon was full on a certain night will not assist the trier
of fact in determining whether an individual was unusually likely to have
behaved irrationally on that night. Rule 702's "helpfulness" standard
requires a valid scientific connection to the pertinent inquiry as a
precondition to admissibility. 
 
That these requirements are embodied in Rule 702 is not surprising. Unlike an
ordinary witness, see Rule 701, an expert is permitted wide latitude to offer
opinions, including those that are not based on firsthand knowledge or
observation. See Rules 702 and 703. Presumably, this relaxation of the usual
requirement of firsthand knowledge -- a rule which represents "a 'most
pervasive manifestation' of the common law insistence upon 'the most reliable
sources of information,'" Advisory Committee's Notes on Fed. Rule Evid. 602,
28 U.S.C. App., p. 755 (citation omitted) -- is premised on an assumption
that the expert's opinion will have a reliable basis in the knowledge
and experience of his discipline. 

                                       C 
 
Faced with a proffer of expert scientific testimony, then, the trial judge
must determine at the outset, pursuant to Rule 104(a), {10} whether the expert
is proposing to testify to (1) scientific knowledge that (2) will assist the
trier of fact to understand or determine a fact in issue. {11} This entails
a preliminary assessment of whether the reasoning or methodology underlying
the testimony is scientifically valid and of whether that reasoning or
methodology properly can be applied to the facts in issue. We are confident
that federal judges possess the capacity to undertake this review. Many
factors will bear on the inquiry, and we do not presume to set out a
definitive checklist or test. But some general observations are appropriate. 
 
Ordinarily, a key question to be answered in determining whether a theory
or technique is scientific knowledge that will assist the trier of fact
will be whether it can be (and has been) tested. "Scientific methodology today
is based on generating hypotheses and testing them to see if they can be
falsified; indeed, this methodology is what distinguishes science from other
fields of human inquiry." Green 645. See also C. Hempel, Philosophy of Natural
Science 49 (1966) ("The statements constituting a scientific explanation
must be capable of empirical test"); K. Popper, Conjectures and Refutations:
The Growth of Scientific Knowledge 37 (5th ed. 1989) ("The criterion of the
scientific status of a theory is its falsifiability, or refutability, or
testability") (emphasis deleted). 

Another pertinent consideration is whether the theory or technique has been
subjected to peer review and publication. Publication (which is but one
element of peer review) is not a sine qua non of admissibility; it does not
necessarily correlate with reliability, see S. Jasanoff, The Fifth Branch:
Science Advisors as Policymakers 61-76 (1990), and in some instances
well-grounded but innovative theories will not have been published, see
Horrobin, The Philosophical Basis of Peer Review and the Suppression of
Innovation, 263 JAMA 1438 (1990). Some propositions, moreover, are too
particular, too new, or of too limited interest to be published. But
submission to the scrutiny of the scientific community is a component of
"good science", in part because it increases the likelihood that substantive
flaws in methodology will be detected. See J. Ziman, Reliable Knowledge: An
Exploration of the Grounds for Belief in Science 130-133 (1978); Relman &
Angell, How Good Is Peer Review?, 321 New Eng. J. Med. 827 (1989). The fact
of publication (or lack thereof) in a peer reviewed journal thus will
be a relevant, though not dispositive, consideration in assessing the
scientific validity of a particular technique or methodology on which an
opinion is premised. 

Additionally, in the case of a particular scientific technique, the court
ordinarily should consider the known or potential rate of error, see, e.g.,
United States v. Smith, 869 F.2d 348, 353-354 (CA7 1989) (surveying studies of
the error rate of spectrographic voice identification technique), and the
existence and maintenance of standards controlling the technique's operation,
see United States v. Williams, 583 F.2d 1194, 1198 (CA2 1978) (noting
professional organization's standard governing spectrographic analysis),
cert. denied, 439 U.S. 1117, 59 L. Ed. 2d 77, 99 S. Ct. 1025 (1979). 

Finally, "general acceptance" can yet have a bearing on the inquiry. A
"reliability assessment does not require, although it does permit, explicit
identification of a relevant scientific community and an express determination
of a particular degree of acceptance within that community." United States v.
Downing, 753 F.2d at 1238. See also 3 Weinstein & Berger P702[03], pp. 702-41
to 702-42. Widespread acceptance can be an important factor in ruling
particular evidence admissible, and "a known technique which has been able to
attract only minimal support within the community," Downing, 753 F.2d at 1238,
may properly be viewed with skepticism. 

The inquiry envisioned by Rule 702 is, we emphasize, a flexible one. {12}
Its overarching subject is the scientific validity -- and thus the
evidentiary relevance and reliability -- of the principles that underlie a
proposed submission. The focus, of course, must be solely on principles and
methodology, not on the conclusions that they generate. 
 
Throughout, a judge assessing a proffer of expert scientific testimony under
Rule 702 should also be mindful of other applicable rules. Rule 703 provides
that expert opinions based on otherwise inadmissible hearsay are to be
admitted only if the facts or data are "of a type reasonably relied upon by
experts in the particular field in forming opinions or inferences upon the
subject".  Rule 706 allows the court at its discretion to procure the
assistance of an expert of its own choosing. Finally, Rule 403 permits the
exclusion of relevant evidence "if its probative value is substantially
outweighed by the danger of unfair prejudice, confusion of the issues, or
misleading the jury . . . ." Judge Weinstein has explained: "Expert evidence
can be both powerful and quite misleading because of the difficulty in
evaluating it. Because of this risk, the judge in weighing possible prejudice
against probative force under Rule 403 of the present rules exercises more
control over experts than over lay witnesses." Weinstein, 138 F.R.D. at 632. 

                                      III 
 
We conclude by briefly addressing what appear to be two underlying concerns of
the parties and amici in this case. Respondent expresses apprehension that
abandonment of "general acceptance" as the exclusive requirement for admission
will result in a "free-for-all" in which befuddled juries are confounded
by absurd and irrational pseudoscientific assertions.  In this regard
respondent seems to us to be overly pessimistic about the capabilities of
the jury and of the adversary system generally. Vigorous cross-examination,
presentation of contrary evidence, and careful instruction on the burden of
proof are the traditional and appropriate means of attacking shaky but
admissible evidence. See Rock v. Arkansas, 483 U.S. 44, 61, 97 L. Ed. 2d 37,
107 S. Ct. 2704 (1987). Additionally, in the event the trial court concludes
that the scintilla of evidence presented supporting a position is insufficient
to allow a reasonable juror to conclude that the position more likely than not
is true, the court remains free to direct a judgment, Fed. Rule Civ. Proc.
50(a), and likewise to grant summary judgment, Fed. Rule Civ. Proc. 56. Cf.,
e.g., Turpin v. Merrell Dow Pharmaceuticals, Inc., 959 F.2d 1349 (CA6)
(holding that scientific evidence that provided foundation for expert
testimony, viewed in the light most favorable to plaintiffs, was not
sufficient to allow a jury to find it more probable than not that defendant 
caused plaintiff's injury), cert. denied, 506 U.S. 826, 121 L. Ed. 2d 47,
113 S. Ct. 84 (1992); Brock v. Merrell Dow Pharmaceuticals, Inc., 874 F.2d
307 (CA5 1989) (reversing judgment entered on jury verdict for plaintiffs
because evidence regarding causation was insufficient), modified, 884 F.2d 166
(CA5 1989), cert. denied, 494 U.S. 1046 (1990); Green 680-681. These
conventional devices, rather than wholesale exclusion under an uncompromising
"general acceptance" test, are the appropriate safeguards where the basis of
scientific testimony meets the standards of Rule 702. 

Petitioners and, to a greater extent, their amici exhibit a different concern.
They suggest that recognition of a screening role for the judge that allows
for the exclusion of "invalid" evidence will sanction a stifling and
repressive scientific orthodoxy and will be inimical to the search for truth.
See, e.g., Brief for Ronald Bayer et al. as Amici Curiae. It is true that open
debate is an essential part of both legal and scientific analyses. Yet there
are important differences between the quest for truth in the courtroom and
the quest for truth in the laboratory. Scientific conclusions are subject
to perpetual revision. Law, on the other hand, must resolve disputes finally
and quickly. The scientific project is advanced by broad and wide-ranging
consideration of a multitude of hypotheses, for those that are incorrect will
eventually be shown to be so, and that in itself is an advance. Conjectures
that are probably wrong are of little use, however, in the project of reaching
a quick, final, and binding legal judgment -- often of great consequence --
about a particular set of events in the past. We recognize that, in practice,
a gatekeeping role for the judge, no matter how flexible, inevitably on
occasion will prevent the jury from learning of authentic insights and
innovations. That, nevertheless, is the balance that is struck by Rules of
Evidence designed not for the exhaustive search for cosmic understanding
but for the particularized resolution of legal disputes.

To summarize: "General acceptance" is not a necessary precondition to the
admissibility of scientific evidence under the Federal Rules of Evidence, but
the Rules of Evidence -- especially Rule 702 -- do assign to the trial judge
the task of ensuring that an expert's testimony both rests on a reliable
foundation and is relevant to the task at hand. Pertinent evidence based
on scientifically valid principles will satisfy those demands. 
 
The inquiries of the District Court and the Court of Appeals focused almost
exclusively on "general acceptance," as gauged by publication and the
decisions of other courts. Accordingly, the judgment of the Court of Appeals
is vacated, and the case is remanded for further proceedings consistent
with this opinion. 

It is so ordered. 

DISSENT:

CHIEF JUSTICE REHNQUIST, with whom JUSTICE STEVENS joins, concurring in
part and dissenting in part. 

The petition for certiorari in this case presents two questions: first,
whether the rule of Frye v. United States, 54 App. D.C. 46, 293 F. 1013
(1923), remains good law after the enactment of the Federal Rules of Evidence;
and second, if Frye remains valid, whether it requires expert scientific
testimony to have been subjected to a peer review process in order to be
admissible. The Court concludes, correctly in my view, that the Frye rule did
not survive the enactment of the Federal Rules of Evidence, and I therefore
join Parts I and II-A of its opinion. The second question presented in the
petition for certiorari necessarily is mooted by this holding, but the Court
nonetheless proceeds to construe Rules 702 and 703 very much in the abstract,
and then offers some "general observations." Ante, at 593. 

"General observations" by this Court customarily carry great weight with lower
federal courts, but the ones offered here suffer from the flaw common to most
such observations -- they are not applied to deciding whether particular
testimony was or was not admissible, and therefore they tend to be not only
general, but vague and abstract. This is particularly unfortunate in a case
such as this, where the ultimate legal question depends on an appreciation of
one or more bodies of knowledge not judicially noticeable, and subject to
different interpretations in the briefs of the parties and their amici.
Twenty-two amicus briefs have been filed in the case, and indeed the Court's
opinion contains no fewer than 37 citations to amicus briefs and other
secondary sources. 

The various briefs filed in this case are markedly different from typical
briefs, in that large parts of them do not deal with decided cases or
statutory language -- the sort of material we customarily interpret. Instead,
they deal with definitions of scientific knowledge, scientific method,
scientific validity, and peer review -- in short, matters far afield from the
expertise of judges. This is not to say that such materials are not useful or
even necessary in deciding how Rule 702 should be applied; but it is to say
that the unusual subject matter should cause us to proceed with great caution
in deciding more than we have to, because our reach can so easily exceed our
grasp. 

But even if it were desirable to make "general observations" not necessary
to decide the questions presented, I cannot subscribe to some of the
observations made by the Court. In Part II-B, the Court concludes that
reliability and relevancy are the touchstones of the admissibility of expert
testimony. Ante, at 590-592. Federal Rule of Evidence 402 provides, as the
Court points out, that "evidence which is not relevant is not admissible".
But there is no similar reference in the Rule to "reliability." The Court
constructs its argument by parsing the language "if scientific, technical,
or other specialized knowledge will assist the trier of fact to understand
the evidence or to determine a fact in issue, . . . an expert . . . may
testify thereto . . . ." Fed. Rule Evid. 702. It stresses that the subject
of the expert's testimony must be "scientific . . . knowledge," and points
out that "scientific" "implies a grounding in the methods and procedures of
science" and that the word "knowledge" "connotes more than subjective belief
or unsupported speculation." Ante, at 590. From this it concludes that
"scientific knowledge" must be "derived by the scientific method." Ibid.
Proposed testimony, we are told, must be supported by "appropriate
validation." Ibid. Indeed, in footnote 9, the Court decides that "in a case
involving scientific evidence, evidentiary reliability will be based
upon scientific validity." Ante, at 591, n. 9 (emphasis in original). 

Questions arise simply from reading this part of the Court's opinion, and
countless more questions will surely arise when hundreds of district judges
try to apply its teaching to particular offers of expert testimony. Does all
of this dicta apply to an expert seeking to testify on the basis of "technical
or other specialized knowledge" -- the other types of expert knowledge to
which Rule 702 applies -- or are the "general observations" limited only to
"scientific knowledge"? What is the difference between scientific knowledge
and technical knowledge; does Rule 702 actually contemplate that the phrase
"scientific, technical, or other specialized knowledge" be broken down into
numerous subspecies of expertise, or did its authors simply pick general
descriptive language covering the sort of expert testimony which courts have
customarily received? The Court speaks of its confidence that federal judges
can make a "preliminary assessment of whether the reasoning or methodology
underlying the testimony is scientifically valid and of whether that reasoning
or methodology properly can be applied to the facts in issue." Ante, at
592-593. The Court then states that a "key question" to be answered in
deciding whether something is "scientific knowledge" "will be whether it can
be (and has been) tested." Ante, at 593. Following this sentence are three
quotations from treatises, which not only speak of empirical testing, but one
of which states that the "'criterion of the scientific status of a theory is
its falsifiability, or refutability, or testability.'" Ibid. 

I defer to no one in my confidence in federal judges; but I am at a loss to
know what is meant when it is said that the scientific status of a theory
depends on its "falsifiability," and I suspect some of them will be, too. 

I do not doubt that Rule 702 confides to the judge some gatekeeping
responsibility in deciding questions of the admissibility of proffered expert
testimony. But I do not think it imposes on them either the obligation or
the authority to become amateur scientists in order to perform that role. I
think the Court would be far better advised in this case to decide only the
questions presented, and to leave the further development of this important
area of the law to future cases. 

FOOTNOTES:

{1} Doctor Lamm received his master's and doctor of medicine degrees from
the University of Southern California. He has served as a consultant in
birth-defect epidemiology for the National Center for Health Statistics and
has published numerous articles on the magnitude of risk from exposure to
various chemical and biological substances. App. 34-44.

{2} For example, Shanna Helen Swan, who received a master's degree in
biostatistics from Columbia University and a doctorate in statistics from
the University of California at Berkeley, is chief of the section of the
California Department of Health and Services that determines causes of birth
defects and has served as a consultant to the World Health Organization, the
Food and Drug Administration, and the National Institutes of Health. Id., at
113-114, 131-132. Stuart A. Newman, who received his bachelor's degree in
chemistry from Columbia University and his master's and doctorate in chemistry
from the University of Chicago, is a professor at New York Medical College and
has spent over a decade studying the effect of chemicals on limb development.
Id., at 54-56. The credentials of the others are similarly impressive. See
id., at 61-66, 73-80, 148-153, 187-192, and Attachments 12, 20, 21, 26, 31,
and 32 to Petitioners' Opposition to Summary Judgment in No. 84-2013-G(I)
(SD Cal.). 

{3} For a catalog of the many cases on either side of this controversy,
see P. Giannelli & E. Imwinkelried, Scientific Evidence, Sections 1-5,
pp. 10-14 (1986 and Supp. 1991). 

{4} See, e.g., Green, Expert Witnesses and Sufficiency of Evidence in Toxic
Substances Litigation: The Legacy of Agent Orange and Bendectin Litigation, 86
Nw. U. L. Rev. 643 (1992) (hereinafter Green); Becker & Orenstein, The Federal
Rules of Evidence After Sixteen Years -- The Effect of "Plain Meaning"
Jurisprudence, the Need for an Advisory Committee on the Rules of Evidence,
and Suggestions for Selective Revision of the Rules, 60 Geo. Wash. L. Rev.
857, 876-885 (1992); Hanson, James Alphonzo Frye is Sixty-Five Years Old;
Should He Retire?, 16 West. St. U. L. Rev. 357 (1989); Black, A Unified Theory
of Scientific Evidence, 56 Ford. L. Rev. 595 (1988); Imwinkelried, The "Bases"
of Expert Testimony: The Syllogistic Structure of Scientific Testimony, 67 N.
C. L. Rev. 1 (1988); Proposals for a Model Rule on the Admissibility of
Scientific Evidence, 26 Jurimetrics J. 235 (1986); Giannelli, The
Admissibility of Novel Scientific Evidence: Frye v. United States, a
Half-Century Later, 80 Colum. L. Rev. 1197 (1980); The Supreme Court,
1986 Term, 101 Harv. L. Rev. 7, 119, 125-127 (1987). 

Indeed, the debates over Frye are such a well-established part of the academic
landscape that a distinct term -- "Frye-ologist" -- has been advanced to
describe those who take part. See Behringer, Introduction, Proposals for a
Model Rule on the Admissibility of Scientific Evidence, 26 Jurimetrics J. 237,
239 (1986), quoting Lacey, Scientific Evidence, 24 Jurimetrics J. 254, 264
(1984). 

{5} Like the question of Frye's merit, the dispute over its survival has
divided courts and commentators. Compare, e.g., United States v. Williams,
583 F.2d 1194 (CA2 1978) (Frye is superseded by the Rules of Evidence), cert.
denied, 439 U.S. 1117, 59 L. Ed. 2d 77, 99 S. Ct. 1025 (1979), with
Christophersen v. Allied-Signal Corp., 939 F.2d 1106, 1111, 1115-1116 (CA5
1991) (en banc) (Frye and the Rules coexist), cert. denied, 503 U.S. 912, 117
L. Ed. 2d 506, 112 S. Ct. 1280 (1992), 3 J. Weinstein & M. Berger, Weinstein's
Evidence P702[03], pp. 702-36 to 702-37 (1988) (hereinafter Weinstein &
Berger) (Frye is dead), and M. Graham, Handbook of Federal Evidence Sectopm
703.2 (3d ed. 1991) (Frye lives). See generally P. Giannelli & E.
Imwinkelried, Scientific Evidence  1-5, at 28-29 (citing authorities). 

{6} Because we hold that Frye has been superseded and base the discussion
that follows on the content of the congressionally enacted Federal Rules of
Evidence, we do not address petitioners' argument that application of the Frye
rule in this diversity case, as the application of a judgemade rule affecting
substantive rights, would violate the doctrine of Erie R. Co. v. Tompkins,
304 U.S. 64, 82 L. Ed. 1188, 58 S. Ct. 817 (1938). 

{7} THE CHIEF JUSTICE "do[es] not doubt that Rule 702 confides to the judge
some gatekeeping responsibility," post, at 600, but would neither say how it
does so nor explain what that role entails. We believe the better course
is to note the nature and source of the duty. 

{8} Rule 702 also applies to "technical, or other specialized knowledge." Our
discussion is limited to the scientific context because that is the nature of
the expertise offered here. 

{9} We note that scientists typically distinguish between "validity" (does
the principle support what it purports to show?) and "reliability" (does
application of the principle produce consistent results?). See Black, 56 Ford.
L. Rev., at 599. Although "the difference between accuracy, validity, and
reliability may be such that each is distinct from the other by no more than
a hen's kick," Starrs, Frye v. United States Restructured and Revitalized:
A Proposal to Amend Federal Evidence Rule 702, 26 Jurimetrics J. 249, 256
(1986), our reference here is to evidentiary reliability -- that is,
trustworthiness. Cf., e.g., Advisory Committee's Notes on Fed. Rule Evid. 602,
28 U.S.C. App., p. 755 ("'The rule requiring that a witness who testifies
to a fact which can be perceived by the senses must have had an opportunity
to observe, and must have actually observed the fact' is a 'most pervasive
manifestation' of the common law insistence upon 'the most reliable sources
of information'" (citation omitted)); Advisory Committee's Notes on Art. VIII
of Rules of Evidence, 28 U.S.C. App., p. 770 (hearsay exceptions will be
recognized only "under circumstances supposed to furnish guarantees of
trustworthiness"). In a case involving scientific evidence, evidentiary
reliability will be based upon scientific validity. 
 
{10} Rule 104(a) provides: 
 
"Preliminary questions concerning the qualification of a person to be a
witness, the existence of a privilege, or the admissibility of evidence shall
be determined by the court, subject to the provisions of subdivision (b)
[pertaining to conditional admissions]. In making its determination it is not
bound by the rules of evidence except those with respect to privileges." These
matters should be established by a preponderance of proof. See Bourjaily v.
United States, 483 U.S. 171, 175-176, 97 L. Ed. 2d 144, 107 S. Ct. 2775
(1987). 
 
{11} Although the Frye decision itself focused exclusively on "novel"
scientific techniques, we do not read the requirements of Rule 702 to
apply specially or exclusively to unconventional evidence. Of course,
well-established propositions are less likely to be challenged than those that
are novel, and they are more handily defended. Indeed, theories that are so
firmly established as to have attained the status of scientific law, such as
the laws of thermodynamics, properly are subject to judicial notice under
Federal Rule of Evidence 201. 

{12} A number of authorities have presented variations on the reliability
approach, each with its own slightly different set of factors. See, e.g.,
Downing, 753 F.2d at 1238-1239 (on which our discussion draws in part); 3
Weinstein & Berger P702[03], pp. 702-41 to 702-42 (on which the Downing court
in turn partially relied); McCormick, Scientific Evidence: Defining a New
Approach to Admissibility, 67 Iowa L. Rev. 879, 911-912 (1982); and Symposium
on Science and the Rules of Evidence, 99 F.R.D. 187, 231 (1983) (statement
by Margaret Berger). To the extent that they focus on the reliability of
evidence as ensured by the scientific validity of its underlying principles,
all these versions may well have merit, although we express no opinion
regarding any of their particular details. 

{13} This is not to say that judicial interpretation, as opposed to
adjudicative factfinding, does not share basic characteristics of the
scientific endeavor: "The work of a judge is in one sense enduring and in
another ephemeral. . . . In the endless process of testing and retesting,
there is a constant rejection of the dross and a constant retention of
whatever is pure and sound and fine." B. Cardozo, The Nature of the
Judicial Process 178-179 (1921).