AuthorImwinkelried, Edward J.
PositionMedical condition overview

In the past few decades, the American criminal justice system has had to come to terms with the reality that wrongful convictions are not exceedingly rare occurrences. By 2017, post-conviction DNA testing had established the innocence of over 340 wrongfully convicted accused. (1) The National Registry of Exonerations lists almost 2,000 wrongful convictions. (2) It is particularly disturbing that in many cases, flawed or at least overstated expert testimony appears to have contributed to these wrongful convictions. (3) Inaccurate expert testimony is a "recurrent theme 1:1" in the wrongful conviction studies. (4) In one study of 156 accused later exonerated by post-conviction DNA testing, during at least 60 percent of the trials, "forensic analysts called by the prosecution provided invalid testimony.... [including] the misuse of empirical population data and... conclusions regarding the probative value of evidence that were unsupported by empirical data." (5) Researchers have faulted erroneous opinions by forensic experts in a wide range of fields, "including blood serology, hair evidence, soil comparison, early DNA tests, bitemark analysis, dog sniffs, spectrographic voice identification, shoeprints, and fiber comparisons." (6)

To be sure, in some cases the miscarriage of justice was virtually unavoidable. In most scientific fields, research is ongoing. At one point in time--the time of an accused's original trial--the available empirical data might have seemed to support the validity of a particular scientific theory or technique. On that assumption, at the trial it is understandable that an honest expert would be willing to testify based on the technique and that a trial judge would admit the testimony. However, as the research evolved, later discovered data might undermine confidence in the technique. No matter how careful early researchers are, it is always possible that the outcomes in their studies will be flawed because, by happenstance, the researchers drew unrepresentative samples from the relevant universe. Unless the researchers conduct a complete census of the universe, any given "random" sample can turn out to be atypical.

Microscopic hair analysis is a case in point. For decades, trace evidence experts testified that based on a detailed comparative microscopic analysis of hair strands found at a crime scene and obtained from an accused, an accused was the likely source of the crime scene strands. (7) DNA experts later developed techniques for analyzing mitochondrial DNA ("mtDNA") in hair. (8) In an F.B.I. study of eighty hair comparisons in which the trace analysts had reported a microscopic match or association, mtDNA analysis demonstrated that nine samples (12.5 percent) came from different persons. (9) In these situations, given the earlier state of the empirical research the prior conviction may have been defensible; but the question that arises later is whether the accused is entitled to post-conviction relief. Some courts have awarded new trials under the existing statutes governing post-conviction relief. (10) In other jurisdictions such as California and Texas, the legislatures have recently amended their statutes to expressly provide relief when later scientific research invalidates a technique or theory that played a major role in securing an accused's conviction. (11)

In other cases, though, the wrongful conviction is avoidable. In these cases, at the very time that the forensic expert took the stand to testify about a technique or theory, there was empirical data demonstrating the invalidity of the technique or theory and the data was reasonably accessible to the accused's defense attorney. On occasion, when the facts are extreme enough to clearly demonstrate the defense attorney's negligence, courts can grant relief based on ineffective assistance of counsel in violation of the Sixth Amendment. (12) However, the optimal solution is to devise and enforce an admissibility standard that separates the wheat from the chaff and prevents the introduction of the junk science.

Part I of this short article critiques two admissibility standards that the courts have sometimes applied in the past. Part II proposes an alternative admissibility standard, arguing that the proposed standard reflects both sound scientific methodology and a sensible synthesis of the governing statutes and cases. Part III discusses several applications of the proposed standard. That discussion is intended to provide a sense of the utility and limitations of the proposed standard.


    1. Expert Testimony Should Be Admissible So Long as the Expert's Opinion Is Relevant.

      One possible approach is to admit any relevant opinion. (13) Under this approach, the trial judge admits the opinion so long as the expert vouches that in his or her opinion, the technique or theory is valid. (14)

      While this approach would radically simplify the trial judge's admissibility decision, the approach is at odds with both scientific methodology and the controlling statutes and cases. (15) As the Supreme Court itself acknowledged in its seminal 1993 Daubert decision, there are three steps in classic Newtonian scientific methodology: first, the scientist proposes an hypothesis about a phenomenon; then the scientist subjects the hypothesis to empirical testing in the form of controlled laboratory experimentation or systematic field observation; and finally, the scientist critically evaluates the test results to determine whether they falsify or validate the hypothesis. (16) Rather than embodying the scientific method, the relevancy approach represents the antithesis of scientific methodology; the approach allows the expert to opine on the basis of unscientific "subjective belief or unsupported speculation." (17) No matter how vociferously the expert proclaims his or her personal belief in the truth of the hypothesis, at most that proclamation amounts to the proposal of a hypothesis. If the hypothesis is plausible, it may be worth an empirical investigation. However, without more the proposal is no substitute for either the subsequent empirical testing or the still later evaluation of the results of the test. Plausible or not, the hypothesis has not yet been scientifically proven.

      In a 1997 decision, General Electric v. Joiner, (18) the Supreme Court explicitly stated that a trial judge may not admit expert testimony resting solely on the expert's ipse dixit claim that the underlying technique or theory is valid. (19) To drive the point home, the Court reiterated the point in 1999 in Kumho Tire Company v. Carmichael. (20) The point was certainly not lost on the Federal Rules of Evidence Advisory Committee. In 2000, the Committee drafted an amendment to Federal Rule of Evidence 702 in part to codify the teachings of the Daubert-Joiner-Kumho line of authority. (21) As is customary, the Committee prepared an official Advisory Committee Note to accompany the amendment. (22) Citing the three cases, the Committee declared that the trial judge may not "simply tak[e] the expert's word for it.'" (23)

    2. Expert Testimony Should Be Admissible So Long as It Is Based on a Technique or Theory that is Generally Accepted Within the Pertinent Scientific Field(s).

      In the first approach, the personal opinion of the expert is dispositive: if his or her opinion is relevant and based on a technique or theory that the expert personally vouches for, the opinion is admissible. (24) The second approach is fundamentally different. Rather than accepting the personal opinion of the individual expert, this approach demands a showing that the technique or theory has gained widespread, general acceptance among the scientists in the relevant fields. (25) This is the famous Frye (26) 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. (27) At one time the federal courts and the clear majority of states subscribed to this test. (28)

      To its credit, the Frye test provides greater assurance of the reliability of the expert testimony than the relevancy test. (29) It no longer suffices that a single expert vouches for the technique or theory. (30) Rather, the proponent of the testimony has to demonstrate that the technique or theory has garnered widespread acceptance by the specialists in the field. (31) If the technique or theory had achieved that status, there is an inference that at least some experts in the field have reviewed the underlying empirical research and found it to be satisfactory. (32) However, in the final analysis, the existence of the technique's extensive popularity is merely circumstantial evidence that the technique or theory passes muster under scientific methodology. (33) Without more, general acceptance does not establish that there has been empirical testing or that the test outcomes validate the hypothesis. (34) General acceptance represents little more than the collective ipse dixit of a large number of experts in the field. (35) Moreover, in numerous cases such as microscopic hair analysis, (36) the compositional analysis of bullet lead ("CABL") technique for identifying the source of bullets, (37) and the paraffin test for gunshot residue, (38) techniques that once enjoyed great popularity have been discredited by later scientific research. (39) In short, compliance with the general acceptance test furnishes inadequate assurance that the technique or theory is scientifically sound. The test is a crude proxy for a direct...

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