JurisdictionUnited States


As noted earlier, there are two boundaries that need to be defined in determining the proper subject matter for expert testimony — first a test for instances where expertise is not needed, a topic discussed in the previous section. A second standard is required to determine when expert testimony is too unreliable or uncertain. But first some background.

The reliability of evidence derived from a scientific theory or principle depends upon three factors: (1) the validity of the underlying theory, (2) the validity of the technique applying that theory, and (3) the proper application of the technique on a particular occasion. In short, neither an invalid technique nor a valid technique improperly applied will produce reliable results.38

The first two factors — the validity of the underlying theory and the validity of the technique — are distinct issues. One could accept, for example, the validity of the premise underlying DNA profiling — the uniqueness of DNA (except for identical twins) — but still question whether a particular DNA technique can identify that uniqueness. Similarly, the underlying psychological and physiological principles of polygraph testing could be acknowledged without endorsing the proposition that a polygraph examiner can detect deception by means of the polygraph technique.

Methods of proof. The validity of a scientific principle and the validity of the technique applying that principle may be established through (1) judicial notice, (2) legislative recognition, (3) stipulation, or (4) the presentation of evidence, typically expert testimony.

Judicial notice. First, if the validity of a theory or technique has been sufficiently established, a court may take judicial notice of the technique's validity, thereby relieving the offering party of the burden of introducing expert testimony on this issue.39 The principles underlying intoxication tests, fingerprint comparisons, and firearms identifications have all been judicially recognized in this fashion.40

Statutory recognition. Second, the validity of a scientific technique may be recognized legislatively. At an earlier time, most statutory provisions were limited to motor vehicle codes and paternity cases. Radar, intoxication tests, and blood tests are still often subject to legislative regulation. These techniques are typically subject to judicial notice as well. Like judicial notice, legislative "notice" or recognition relieves the proponent of the burden of introducing evidence on the validity issue. Frequently, these statutes specify additional requirements for admissibility, such as periodic testing as a prerequisite for introducing breathalyzer results in evidence.41

More recent enactments have extended legislative recognition to more controversial techniques — for example, polygraph,42 hypnosis,43 rape trauma syndrome,44 and battered wife syndrome45 evidence. Many of these techniques would not be subject to judicial notice because their validity is disputable. The most important of the newer statutes deal with DNA evidence.46

Stipulation. The third way to establish reliability is by stipulation. The most notable example is polygraph evidence. As one court noted: "The primary effect of the stipulation is that it operates as a waiver of objection or challenge to the validity of the basic theory of polygraph testing and eliminates the necessity of or the opportunity for the parties to establish a foundation in each case to satisfy the trial court of the basic theory and validity of polygraphs."47 Nevertheless, most jurisdictions reject the admissibility of polygraph results by stipulation.48

Evidentiary foundation. Finally, the validity of a particular scientific technique may be established by introducing evidence, including, but not limited to, expert testimony. At least four different approaches can be gleaned from the cases: (1) the Frye "general acceptance" test, (2) a relevancy approach, (3) the Supreme Court's reliability approach, as set forth in Daubert v. Merrell Dow Pharm., Inc.,49 and (4) other reliability tests.

[A] Frye "General Acceptance" Test

For most of the last century, Frye v. United States50 was the leading case on the admissibility of novel scientific evidence.51 In rejecting the results of a precursor of the modern polygraph, the D.C. Circuit set forth what has come to be known as the "general acceptance" test:

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.52

The court went on to hold that the polygraph had "not yet gained such standing and scientific recognition among physiological and psychological authorities."53 The general-acceptance standard was subsequently used to decide the admissibility of a plethora of techniques, including voiceprints, neutron activation analysis, gunshot residue tests, bite mark comparisons, psycholinguistics, truth serum, hypnosis, blood analysis, hair comparison, intoxication testing, and DNA profiling.54

The general-acceptance test is justified on reliability grounds: "The requirement of general acceptance in the scientific community assures that those most qualified to assess the general validity of a scientific method will have the determinative voice."55 Nevertheless, courts and scholars criticized the Frye test on several grounds. First, there are problems associated with applying the standard. For example, DNA analysis covers several fields (molecular biology and population genetics). In which is general acceptance required? Both? In addition, is acceptance by scientists rather than technicians required? If polygraph examiners are defined as the relevant field under Frye, polygraph results would be admissible because there is "general acceptance" in the community of polygraph examiners. Second, there is also a concern that reliable evidence would be excluded under Frye while courts awaited general acceptance in the scientific community. In short, the general-acceptance standard is too conservative. Third, "[p]erhaps the most important flaw in the Frye test is that by focusing attention on the general acceptance issue, the test obscures critical problems in the use of a particular technique."56 Each technique is different, and a "one-size-fits-all" general-acceptance test may overlook crucial issues.

Although Frye is no longer the majority rule, it is still followed in approximately a dozen jurisdictions, including California, Florida, Illinois, Maryland, Minnesota, New Jersey, New York, Pennsylvania, and Washington.57 Because these are populous states and most crimes are prosecuted in state courts, Frye remains an important test.

[B] Relevancy Approach

In his 1954 text, Professor McCormick argued that a special test, such as general acceptance, was not necessary and that the traditional evidentiary rules on relevancy and expert testimony should be applied to scientific facts.58 In effect, qualifying the expert presumptively qualifies the technique used by that expert. Because most trial judges do not possess scientific backgrounds, the judge "will generally be forced to accept the probative value of the evidence as what a qualified expert testifies it to be."59 This lax standard suffers from several drawbacks: "The major flaw in the relevancy analysis is its failure to recognize the distinctive problems of scientific evidence. The judge frequently is forced to defer to an expert, thereby permitting admissibility based on the views of a single individual in some cases."60 The Supreme Court implicitly rejected it in Daubert, which requires the trial judge to determine reliability in addition to relevancy.61 Few states follow this rule today.

[C] Daubert Reliability Test

After the Federal Rules were adopted in 1975, the federal courts divided over the continued validity of the general-acceptance standard.62 Some courts assumed Frye had survived the enactment of the Federal Rules, while other courts rejected this view. In Daubert v. Merrell Dow Pharm., Inc.,63 the Supreme Court discarded the Frye test as a matter of statutory interpretation.64 Daubert involved the admissibility of expert testimony concerning whether Bendectin, an anti-nausea drug, caused birth defects.65

The Court held that scientific evidence must satisfy a reliability test. Such a standard, in the Court's view, is derived from Rule 702 (since amended), which uses the terms "scientific" and "knowledge."66 Under this analysis, the trial court must make an admissibility determination pursuant to Rule 104(a). This task "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."67 This task has come to be known as the trial judge's "gatekeeper" role.

One of the more problematic passages in the opinion was the following admonition: "The focus, of course, must be solely on principles and methodology, not on the conclusions that they generate."68 This statement implies a clear dichotomy between methods and conclusions, an assumption drawn into question by the Court's later decisions.

The Supreme Court followed Daubert with General Electric Co. v. Joiner69 and Kumho Tire Co. v. Carmichael,70 to make up what is now known as the Daubert trilogy. In 2000, Rule 702 was amended to codify Daubert and Kumho.

[1] Daubert Factors

In performing the Daubert "gatekeeping function," the trial court may consider a number of factors. The Supreme Court specified five...

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