KEEPING THE GATE
The evolving role of the judiciary
in admitting scientific evidence
by Joseph T. Walsh
The 1993 decision of the United States Supreme Court in Daubert v.
Merrill Dow Pharm., Inc.,1 was viewed as a watershed event.
In abolishing the 70 year old Frye test for evaluating scientific
evidence, the Court established a new, and somewhat controversial, standard.
Under Frye, trial courts were simply required to determine whether
the expert's methodology was generally accepted in the relevant scientific
Daubert introduced a more elaborate, open-ended approach
keyed to validating the reliability and relevance of the methods employed
by the expert. The new standard placed increased responsibility on the
trial judge who was assigned a gatekeeper's role.
Daubert's teaching has since been refined and expanded both at
the Supreme Court level and among federal and state courts who have sought
to implement it. This article addresses post-Daubert developments
and discusses the questions it, and its progeny, leave unanswered.
Despite its longevity, the Frye rule of general acceptance in
the relevant scientific community was not without its critics. Although
adopted in pure or modified form in most jurisdictions, federal and state,
the Frye standard posed a significant ambiguity: what is the relevant
scientific community and who defines it? The Frye approach came
under greater scrutiny over time. Courts wrestled with its application
in technical areas lacking clear scientific underpinnings (such as psychological
syndromes and voice printing). Perhaps the greatest barrier to Frye's
continued viability, however, arose with the emergence of the Federal Rules
of Evidence which, in turn, became the model for evidentiary standards
in many state courts.
Rule 702 of the Federal Rules of Evidence, with its emphasis on the
reliability of the expert, appears almost at cross-purposes to Frye's
focus on the subject matter of the expert's opinion. Similarly, Rule 703,
which permits an expert to use data not necessarily admissible in evidence
in formulating an opinion provided such data is "of a type" reasonably
relied upon by experts "in the particular field," seems to suggest a Frye-like
test without the general acceptance requirement.
Courts seeking to reconcile Frye's general acceptance test with
the more specific criteria imparted by Rules 702 and 703 struggled to provide
a consistent practical guide for practitioners. To the extent that Frye
was viewed as unduly conservative, courts sought to relax its application
to avoid the exclusion of evidence, particularly in criminal cases. Also,
as more scientific studies and methodology were brought to bear in toxic
and pharmaceutical based tort actions, courts struggled to permit the use
of innovative science to establish causation.
In the period immediately proceeding Daubert, some courts, lacking
consistent doctrinal standards, opted to treat close questions of admissibility
of scientific evidence as matters of "weight" to be resolved by the trier
of fact, typically a jury. Some trial judges applied the highly subjective
probative value/prejudice balance of Rule 403 to resolve contests over
the admissibility of scientific evidence. This relaxed approach placed
a premium on the securing of a favorable expert witness and led to the
much-criticized emergence of the hired gun expert.2
Regardless of one's view of the continued efficacy of Frye, the
controversy engendered by the use of confrontational experts opining on
unusual, and sometimes novel, issues of scientific evidence created a demand
for clarification. Not only was there division among the federal circuits,
but varying admissibility standards promulgated by state courts led to
claims of forum shopping. Thus, the time was ripe for an authoritative
The sweep of Daubert
There is continued debate about the sweep of Daubert and its
impact on the existing law of evidence. Two points, however, are clear.
both dispels any doubt that Federal Rule of Evidence 702 supersedes Frye
and significantly enlarges the role of the trial judge as the arbiter of
the admissibility of scientific evidence.
In place of Frye's general reliability standard the Court fashioned
a more elaborate inquiry for the trial judge to determine the scientific
validity of proffered evidence. The Court pointed to four, non-exclusive,
factors: (1) testability; (2) subjection to peer-review; (3) known or potential
rate of error; and (4) widespread acceptance (a factor closely akin to
the discarded Frye standard). An overarching and separate requirement
for scientific evidence, even if found reliable, is that it be relevant,
i.e. that it bear upon and materially advance the resolution of a fact
With respect to the role of the trial judge, Daubert is quite
explicit. Judges must be more active in screening expert scientific testimony,
hence the notion of judges as gatekeepers. Are most judges adequate to
the task? In the view of certain commentators, judges, despite "their general
enthusiasm and diligence tend to be highly resistant to the sort of learning
demands."3 In partial response to this concern, the Federal
Judicial Center, in 1994, published a guide for federal judges. This publication,
Manual on Scientific Evidence, itself has become a source of controversy
with the organizations representing plaintiffs voicing criticism of the
manual as defense-oriented and a misinterpretation of Daubert's
The Supreme Court has attempted to flesh out the Daubert framework
in subsequent decisions by expanding the doctrinal basis underlying Daubert's
reliability/relevancy approach. In late 1997, the Court decided Gen.
Elec. Co. v. Joiner4 which involved a claim by a worker
for alleged illness stemming from toxic exposure. The trial judge in Joiner
denied admissibility to the plaintiff's expert's conclusion of causation
because it was based, in part, on extrapolation from animal studies. The
Eleventh Circuit Court of Appeals reversed, noting that the Federal Rules
of Evidence demonstrate a preference for admissibility and, thus, require
"a stringent standard of review."5
In reversing the court of appeals, the Supreme Court aligned itself
strongly on the side of the trial judge by mandating that evidentiary rulings
made at trial be reviewed under an abuse of discretion standard. Moreover,
the Court explicitly extended the reach of this lenient standard to the
trial court's acceptance, or rejection, of both the conclusions and the
methodology of the expert. Additionally, the Court stated that the trial
judge's discretion is not controlled by "the ipsi dixit" of the expert.
The judge's role
In a more recent decision, the Supreme Court elaborated further on
the scope of the trial judge's role as the gatekeeper of the evidence.
In March 1999 the Court issued its opinion in Kumho Tire Co. Ltd. v.
Carmichael,6 a products liability action brought for recovery
of injuries resulting from a tire blow out. The plaintiffs' proof of causation
rested exclusively on the testimony of its expert who opined on the tire's
defectiveness. The trial court, applying its view of Daubert, excluded
the expert's testimony because it found insufficient indications of reliability,
i.e., a sufficient scientific basis for his opinion. The court of appeals
reversed, ruling that the subject of the expert's opinion, tire technology,
fell outside Daubert's scope and its rigorous standards for the
admissibility of scientific evidence.
The Supreme Court in Kumho Tire again championed the trial judge's discretionary
gatekeeping role. The Court emphasized that Rule 702 makes no distinction
between types of specialized knowledge, whether "technical" or "scientific."
addressed only scientific knowledge because that was what was at issue
in that case. But Rule 702, as interpreted in
Daubert, imposes a
reliability finding as a prerequisite for all expert testimony in areas
beyond the knowledge and experience of lay jurors.
The Court also noted that Daubert's list of criteria intended
to guide the trial judge's discretion was not exclusive. In discharging
its duty to determine reliability and relevancy, the trial court is extended
considerable latitude, not only in the acceptance or rejection of the expert's
opinion, but also in the evaluation of the factors leading to that conclusion.
Under an abuse of discretion standard, the trial judge could not be faulted
for his rejection of the tire expert's opinion and, in the view of the
Supreme Court, the court of appeals erred in second guessing that determination.
The Supreme Court's decision in Kumho Tire represents a continuation of
Joiner's expansive view of the trial judge's role in applying Daubert's
Daubert-implementing rulings provide at least three
clear interpretive guidelines under Rule 702. First, the range of subjects
to which an expert's opinion is directed, and which is subject to judicial
scrutiny, is not limited to the purely scientific so long as the methodology
is scientific-based. Second, the trial court may extend its gatekeeping
role to the expert's conclusions, not simply the expert's methodology.
Finally, the trial judge's discretionary authority of acceptance or rejection
is subject to reversal only if arbitrary and lacking any record support.
Daubert and its progeny have significantly affected the standards
for admissibility of scientific evidence under Rule 702. But these decisions
were issued by the Supreme Court under its supervisory power and are, thus,
binding only on federal courts. However, Daubert's emergence has
led to repeated challenges to Frye-based standards historically
followed in state courts. The result has been mixed. Since the overwhelming
majority of states have adopted a counterpart of Rule 702, Daubert's
interpretation of the rule, while not entitled to deference, has caused
most state courts to reexamine the decisional underpinnings of the norms
for admissibility of scientific evidence. Other states, notably Florida,
have rejected the lure of
Daubert and have elected to stay with
the Frye test.7 Some states have adopted
as consistent with Rule 702 while others have found Daubert "instructive,"
but not necessarily required, of trial courts.
The controversy over the merits of Daubert continues in academia
with some critics questioning whether trial judges possess a sufficient
level of scientific sophistication to assume the gatekeeping role in determining
complex scientific issues. Protagonists on both sides of the tort reform
debate also dispute whether Daubert places too much power in the
hands of the trial judge, whose rulings to exclude expert opinions, particularly
in products liability cases, may deprive a plaintiff of redress at the
hands of a jury. Even in the federal system, where
application is not open to question, appellate courts, in particular, continue
to struggle with "the enormous power [of a trial court] to foreclose submission
of a party's case to a jury on the basis of a threshold determination of
nonreliability of opinion evidence."8
Despite the increase in rulings at both the federal and state level
that seek to map the standards and define the limits for admitting scientific
evidence proffered through experts, the problem is far from being resolved.
There are two factors that hinder the effort to formulate a consistent
framework for testing the admissibility of scientific evidence. The first
is the evolving nature of the scientific knowledge as it is brought to
the courtroom; the second is the highly subjective judgment brought to
bear under a gatekeeper construct. Each of these factors deserves elaboration.
For at least the past 20 years, expert testimony has become increasingly
more complex as it is directed to a variety of technical and scientific
issues. Presentations such as epidemiological studies presented in toxic
exposure cases and product liability cases are often based on cutting-edge
science. The emergence of DNA evidence as a forensic tool for identification
purposes and as a prediction of physical and emotional abnormality is a
good example of how knowledge outstrips the ability of courts to accommodate
its implications. DNA matching evidence, once viewed as controversial,
is now readily accepted for identification purposes. The scientific basis
for this evidence is now so well established that its admissibility is
sanctioned by statute in many jurisdictions with only the projection of
a random match left to expert opinion. The current state of the law seems
to sanction the general scientific basis for DNA identification by permitting
only the challenge to individual results.
DNA evidence as a prediction of genetic influence on physical and emotional
abnormality is just now in the early stages of cautious development. Yet,
it is now generally expected that within five years the Human Genome Project,
sponsored by the Department of Energy and the National Institute of Health,
will have succeeded in completely mapping and sequencing the human genome,
a sequence of three billion characters. The evolving understanding of the
underlying causes of thousands of genetic diseases, including many forms
of cancer, will pose significant new responsibilities for health care providers
and pharmaceutical companies. With litigation likely, however, whenever
new legal relationships are created, advances in genetic science will bring
to the courtroom an array of expert witnesses opining on the emerging science
of genetics. The opinions they will give (and the counter views which will
inevitably arise) will occur on the developing edge of science. Will testability,
general acceptance, and peer review continue to be appropriate criteria
for determining the admissibility of such testimony?
Daubert's elaborate constraints may prove too rigid to accommodate
the novelty of these innovations when offered in the courtroom in cutting-edge
cases. It seems reasonable to insist that the current doctrinal framework
spawned by Daubert be flexible enough to accommodate "novel" evidence.
Yet, at the same time, the prospect of "new" scientific learning presents
the risk that practitioners of junk science will seek to enter the courtroom
to take advantage of the lack of a formalized body of knowledge. The real
challenge for gatekeeper judges in the future will be to balance these
The second, and equally challenging, consequence of the Daubert
standards is the highly subjective duty imposed upon the trial judge. As
the gatekeeper, the trial judge is expected to screen expert testimony
to insure that the jury does not consider it unless it is relevant and
reliable. Daubert's underlying rationale is a sound one: lay jurors
should not be exposed to unfiltered scientific or technical testimony that
may adversely influence their findings of fact. But this rationale is built
on two underlying assumptions: (1) that the trial judge is more knowledgeable
in assessing complex scientific testimony than is the average lay juror
and (2) that each judge brings to the specific task of gatekeeping a general
attitude or philosophy concerning the level of scrutiny appropriate for
scientific gatekeepers. Experience, however, has demonstrated that judges
are not fungible. Intelligence aside, judges vary considerably in how they
view their role in the courtroom; active or passive, dominating or deferential
to counsel, prone to independent inquiry or content to let the lawyers
try the case.
The presence of a significant subjective factor in the gatekeeper calculus
poses a real risk of differing results depending on the idiosyncracies
or predisposition of the trial judge. Thus, scientific evidence which would
gain admissibility in one courtroom might be rejected in another. To make
matters worse, an aggrieved litigant seeking to appeal a lower court ruling
on scientific evidence will be required to overcome the highest standard
of review—abuse of discretion. Moreover, it may plausibly be argued that,
unlike the reliability prong of Daubert which is fact-intensive,
the determination of relevancy is more akin to an issue of law and, thus,
not requiring the same level of deference.
Traditionally, the right of review has provided the best guarantee for
the correction of errors at the trial level. The higher the threshold of
review, however, the more difficult it will be for the appellant to demonstrate
error. Since abuse of discretion sets the bar at the highest mark, a trial
judge making a Daubert-based ruling excluding vital expert testimony
is fairly well insulated against reversal. At least one circuit court of
appeals, acknowledging this problem has called for greater scrutiny, or
a "hard look" regarding a trial court's rulings that set too high a standard
for the admissibility of scientific testimony under Daubert.9
An approach that provides greater scrutiny to rulings of exclusion, however,
raises questions about the fairness of examining rulings admitting evidence
under a more lenient standard.
The broad discretion given to trial judges performing the gatekeeper
role in passing upon the admissibility of scientific evidence apparently
was of some concern to at least three members of the Supreme Court who
concurred in Kumho Tire. Justice Antonin Scalia, joined by Justices
Sandra Day O'Connor and Clarence Thomas, noted that Daubert's grant
of discretion is not open-ended. While Daubert's factors "are not
holy writ," the failure to apply "one or another of them" in a particular
case may be unreasonable and thus an abuse of discretion.
In Daubert and its progeny, the United States Supreme Court accomplished
the task of repudiating the Frye rule and replacing it with a standard
vesting significant discretion in the trial judge. The new standards, however,
have not won acceptance in all state jurisdictions and pose significant
problems in application. Courts following Daubert's lead will be
required to deal with a fundamental shifting of the responsibility for
dealing with suspect scientific evidence. The contest for admissibility
will be less and less a competition between opposing experts and more and
more the independent responsibility of the gatekeeping judge. It remains
to be seen whether this expanded duty assigned to the trial judge will
disturb the traditional role of the fact finder as determiners of the weight
of testimony. Therein lies the challenge facing litigators and judges as
DNA science evolves.
Joseph T. Walsh is a justice of the Delaware Supreme Court.
1. 509 U.S. 579 (1993).
2. See Huber, Galileo's Revenge: Junk Science in the
Courtroom (1991). For a reply to Huber, see Chesebro, Galileo's Retort:
Peter Huber's Junk Scholarship, 42 Am. U.L. Rev. 1637 (1993).
3. Conley and Peterson, The Science of Gatekeeping, 74
N. C. L. Rev. 1183, 1205 (1996).
4. 522 U.S. 136 (1997).
5. Joiner v. Gen. Elec. Co., 78 F.3d 524, 529 (11th Cir.
6. 119 S.Ct. 1167 (1999).
7. Flanagan v. State, 625 So.2d 827, 829 (Fla. 1993).
8. In re Paoli R.R. Yard PCB Litig., 35 F.3d 717, 733
(3rd Cir. 1994) cert. denied 513 U.S. 1190 (1995).