SUPREME COURT OF THE UNITED STATES
WILLIAM DAUBERT, ET UX., ETC., ET AL., PETITIONERS V. MERRELL DOW PHARMACEUTICALS, INC.
509 U.S. 579 (1993)
JUSTICE BLACKMUN delivered the opinion of the Court.
]In this case we are called upon to determine the standard for admitting expert scientific testimony in a federal trial.
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. 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. 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
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
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).
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.
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. 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. 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
"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.
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. 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." 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.
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, 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.
Faced with a proffer of expert scientific testimony, then, the trial
judge must determine at the outset, pursuant to Rule 104(a), 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. 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, 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.
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
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.
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.