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Scientific Evidence


The first American crime laboratories were established about 1930. The principal techniques used in these laboratories were fingerprinting, handwriting comparisons, toolmark and firearms ("ballistics") identifications, drug analysis, blood tests, and trace analysis (hair, fiber, and glass). However, by the late 1960s the nature of scientific evidence had changed dramatically; new techniques had been developed, and courts faced decisions about the admissibility of testimony based upon a much wider array of scientific techniques: sound spectrography ("voiceprint"), neutron activation, atomic absorption, electrophoretic blood testing, scanning electron microscopy, mass spectrometry, gas chromatography, and bite mark comparisons. Even fingerprint identification had moved into the high-tech age with laser technology for visualizing latent prints and powerful computers for searching databases including millions of sets of prints.

Contributing factors

Funding. Several factors may have contributed to this increased use of scientific evidence. At one time there was substantial funding for "forensic" science researchthe application of science in a legal setting. The creation of the Law Enforcement Assistance Administration (LEAA) in 1968 undoubtedly played a significant role. In the 1970s the LEAA underwrote a number of research projects designed to encourage the forensic application of scientific knowledge, and the development of some techniques can be traced directly to this research. Voiceprint analysis is the most prominent example. Other funded projects dealt with blood analysis, blood flight characteristics (blood spatter evidence), trace metal detection, and the polygraph. Later, the F.B.I. spent considerable resources refining the forensic application of DNA, and the Bureau of Alcohol, Tobacco, and Firearms joined the F.B.I. in establishing computerized firearms comparison systems.

Supreme Court decisions. Some commentators attribute the expanded use of scientific evidence to another factor, namely, U.S. Supreme Court decisions of the 1960s, in which the Warren Court severely restricted the acquisition of evidence for criminal cases by traditional crime-solving techniques such as interrogations and lineups. There is some suggestion in the Supreme Court's own cases that the Court deliberately encouraged greater reliance on scientific techniques. For example, in one case the Court wrote: "Modern community living requires modern scientific methods of crime detection lest the public go unprotected" (Breithaupt v. Abram, 352 U.S. 432, 439 (1957)). In Escobedo the Court observed: "We have learned the lesson of history, ancient and modern, that a system of criminal law enforcement which comes to depend on the 'confession' will, in the long run, be less reliable and more subject to abuses than a system which depends on extrinsic evidence independently secured through skillful investigation" (Escobedo v. Illinois, 378 U.S. 478, 48889 (1964)).

Interestingly, at the same time the Court was erecting constitutional barriers to the use of confessions and lineups, it was removing Fourth and Fifth Amendment obstacles to the use of scientific evidence. The most important case was Schmerber v. California, 384 U.S. 757 (1966). The Court, in an opinion by Justice Brennan, held that the privilege against compulsory self-incrimination applies only to testimonial evidence and not to physical evidence. Thus, the police could extract blood from Schmerber for blood-alcohol analysis without violating the Fifth Amendment privilege. This ruling had broader significance; the ruling also meant that handwriting exemplars, fingerprints, voice exemplars, and, later, biological samples for DNA testing could be compelled from a suspect without running afoul of the self-incrimination clause. Justice Brennan likewise authored Warden v. Hayden, 387 U.S. 294 (1967), in which the Supreme Court overruled its prior cases prohibiting the seizure of "mere evidence." Under the "mere evidence" rule the police could seize only contraband, instrumentalities of a crime, or fruits of a crime. Most scientific evidence, such as blood, hair, gunshot residues, and blood-stained clothing, would have been "mere evidence" and hence immune from seizure.

Technology. It is unclear that either of these reasonsresearch funding or Supreme Court decisionsfully explains the increased use of scientific evidence. The answer may be more basic. It is simply expectable that a society so dependent on science and technology should turn to such knowledge as a method of proof. With computer technology running modern businesses, magnetic resonance imaging (MRI) aiding medicine, and Nintendo and other electronic devices captivating children, no one should be surprised to see DNA evidence in the courtroom.

Novel scientific evidence

In any event, what is clear is that reliance on scientific proof has increased. One survey concluded: "About one quarter of the citizens who had served on juries which were presented with scientific evidence believed that had such evidence been absent, they would have changed their verdictsfrom guilty to not guilty" (Peterson et al., p. 1748).

Voiceprints. During the last quarter of the twentieth century, each decade seemedto produce its own "novel" technique for identification. In the 1970s the admissibility of voice identification by spectrographic examination of speech samples, often referred to as "voiceprint" evidence, divided the courts. The technique was designed to identify a person's voice, often an important fact in telephone threats, extortion, and kidnaping cases. Even the name "voiceprint," which was initially used to describe this technique, sparked controversy because it suggested an unwarranted comparison with fingerprint identification. Some speech scientists concluded that the differences between these techniques "seem to exceed the similarities" (Bolt et al., p. 599). Although numerous early cases had admitted voiceprint evidence, a National Academy of Sciences report undermined the scientific basis for the technique: "Estimates of error rates now available pertain to only a few of the many combinations of conditions encountered in real-life situations. These estimates do not constitute a generally adequate basis for a judicial or legislative body to use in making judgments concerning the reliability and acceptability of aural-visual voice identification in forensic applications" (National Research Council, 1979, p. 60). This report, however, has not rendered voiceprint evidence inadmissible in all jurisdictions (State v. Coon, 974 P.2d 386 (Alaska 1999)).

Hypnotically-refreshed testimony. In the early 1980s hypnotically enhanced testimony became the focal point of the admissibility battles. The courts had rejected hypnotic evidence when first offered at trial (People v. Ebanks, 49 P. 1049, 1053 (Cal. 1897)). The issue remained dormant until the 1970s when "a dramatic rise in the use of hypnotism as an aid in criminal investigations" occurred (People v. Lucas, 435 N.Y.S.2d 461, 462 (Sup. Ct. 1980)). After traumatic crimes, victims often experience difficulties recalling the details. Police decided to resort to hypnosis to help victims retrieve the memories. Indeed, it was "hailed as a major breakthrough in police investigation" (Orne et al., p. 171).

Several factors may have accounted for this trend. First, the leading case permitting hypnotically refreshed testimony was decided in 1968. (Harding v. State, 246 A.2d 302 (Md. App. 1968), cert. denied, 395 U.S. 949 (1969)). Second, by the 1960s professional organizations, notably the American Medical Association and the American Psychiatric Association, had recognized the validity of hypnosis for therapeutic purposes, such as psychotherapy, treatment of psychosomatic illnesses, and amnesia. Third, hypnotic induction is easily learned. "A police officer can become a reasonably skilled hypnotist in a few hours of practice, with or without formal instruction" (Diamond, p. 314). Fourth, a number of books and articles on hypnosis had advocated its use, often claiming that hypnosis yielded valuable leads in many investigations: "In 77% of cases, important information was elicited that had not been available by routine interrogation" (Reiser and Nielson, p. 76).

The early judicial decisions that considered the admissibility of hypnotically refreshed testimony took the position that hypnosis affects the credibility of the testimony but not its admissibility. Under this view, the use of hypnosis to stimulate recall is treated no differently than other methods of refreshing recollection. Accordingly, cross-examination, the presentation of expert testimony on the dangers of hypnosis, and cautionary instructions are thought adequate to both protect the defendant against unreliable evidence and enable a jury to evaluate the credibility of previously hypnotized witnesses. In retrospect, there seems little dispute that this approach rested on dubious scientific grounds, and by the early 1980s courts began to more closely scrutinize this type of testimony. In rejecting hypnotically enhanced testimony in State v. Mack, 292 N.W.2d 764, 768 (Minn. 1980), the Minnesota Supreme Court recognized that for hypnosis to be "therapeutically useful, it need not produce historically accurate memory." In therapy, the mental health expert is often more concerned about what the patient believes than in what actually occurred.

Two years later, the California Supreme Court identified several dangers of hypnotically enhanced testimony (People v. Shirley, 641 P.2d 775, cert. denied, 459 U.S. 860 (1982)). These dangers include hypersuggestibility and hypercompliance. In addition, there is an inability to distinguish accurate from inaccurate recall; neither the subject nor the hypnotist can differentiate between true memories and pseudomemories. Moreover, "a witness who is uncertain of his recollections before being hypnotized will become convinced by that process that the story he told under hypnosis is true and correct in every respect" (p. 803). There is, however, no correlation between confidence and accuracy in this context. The jury may find the testimony convincing precisely because the witness displays (unjustified) confidence in his or her demeanor on the stand. Finally, the danger of confabulationfilling in details from the imagination in order to make a memory more coherent and completeis present. After much litigation, the majority of courts concluded that the dangers outweighed any possible benefit and adopted a per se rule of exclusion, albeit with some exceptions. Other courts left the admissibility decision to the trial judge, under what became known as the "totality of the circumstances" approach.

Many of these issues resurfaced in the 1990s with "repressed memory" cases. Perhaps the most publicized case was Franklin v. Duncan, 884 F. Supp. 1435 (N. Cal.), aff'd, 70 F.3d 75, 78 (9th Cir. 1995), where the defendant was convicted of a 1969 murder based on the testimony of his daughter. She did not recall the event until twenty years later following hypnosis. The conviction was overturned on constitutional, not evidentiary, grounds.

Social science research. During the 1980s, expert testimony based on social science research found its way into American courtrooms. The first inroad was evidence of battered wife syndrome (BWS). While being a battered woman by itself is no defense to homicide, the syndrome may assist a jury in understanding two elements of a self-defense claim: (1) the defendant's subjective fear of serious injury or death, and (2) the reasonableness of that belief. For example, the evidence may explain why a battered woman has not left her mate. According to the New Jersey Supreme Court, "[o]nly by understanding these unique pressures that force battered women to remain with their mates, despite their longstanding and reasonable fear of severe bodily harm and the isolation that being a battered woman creates, can a battered woman's state of mind be accurately and fairly understood" (State v. Kelly, 478 A.2d 364, 372 (N.J. 1984)).

Another illustration is rape trauma syndrome (RTS), a phrase coined by Burgess and Holmstrom to describe the behavioral, somatic, and psychological reactions of rape and attempted rape victims. Other studies elaborated on the initial research, sometimes confirming the earlier studies and occasionally providing additional insights. "Subsequent research, which is much more rigorous, conceptualizes rape trauma in terms of specific symptoms rather than more general stages of recovery" (Frazier and Borgida, p. 299).

While BWS was primarily used by defense attorneys, RTS became an important tool for prosecutors. By the early 1980s prosecutors had begun to use this research in rape trials. RTS evidence was offered at trial for two different purposes: (1) to prove lack of consent by the alleged victim, and (2) to explain postincident conduct by a victim, such as delayed reporting of the incident, that a jury might perceive as inconsistent with the claim of rape and therefore impeaching. The courts divided over the first use but generally accepted the second use. In 1982 the Kansas Supreme Court in State v. Marks, 647 P.2d 1292, 1299 (Kan. 1982), became the first state supreme court to uphold the admission of RTS evidence. The court concluded: "An examination of the literature clearly demonstrates that the so-called 'rape trauma syndrome' is generally accepted to be a common reaction to sexual assault." Many other courts followed this precedent.

In contrast, courts rejecting RTS as proof of lack of consent disputed the scientific validity of the syndrome when offered for the first purpose. For example, in People v. Bledsoe, 681 P.2d 291 (Cal. 1984), the California Supreme Court noted that "rape trauma syndrome was not devised to determine the 'truth' or 'accuracy' of a particular past eventi.e., whether, in fact, a rape in the legal sense occurredbut rather was developed by professional rape counselors as a therapeutic tool, to help identify, predict and treat emotional problems experienced by the counselors' clients or patients" (p. 300). Thus, according to the court, although generally accepted by the scientific community for a therapeutic purpose, expert testimony on RTS was not generally accepted as a fact-finding technique "to prove that a rape, in fact, occurred" (p. 301).

Although the California Supreme Court rejected RTS evidence offered to prove lack of consent, it approved prosecution use of RTS evidence where the defendant attacked the victim's credibility and suggested to the jury that the victim's conduct after the incident was inconsistent with the claim of rape. In this situation, the court wrote, "expert testimony on rape trauma syndrome may play a particularly useful role by disabusing the jury of some widely held misconceptions about rape and rape victims, so that it may evaluate the evidence free of popular myths" (p. 298). Most courts accepted this position, admitting expert testimony to account for a victim's (1) passive resistance during a rape, (2) delay in reporting the crime, (3) failure to attempt to escape, and (4) calm demeanor after an attack. RTS evidence has also been introduced to explain that "in the context of a trust relationship, such as a doctor-patient relationship, some victims may return to the trusted relationship for further contact with the perpetrator of the assault" (Commonwealth v. Mamay, 553 N.E.2d 945, 951 (Mass. 1990)).

DNA profiling. We turn now to the late 1980s and 1990s. DNA (deoxyribonucleic acid) is a chemical messenger of genetic information, a code that gives both common and individual characteristics to people. Except for identical twins, no two individuals share the same DNA pattern. DNA is found in every body cell with a nucleus. Red blood cells lack a nucleus. However, blood may still be used as evidence because white cells and other components of blood have DNA. With few exceptions, DNA does not vary from cell to cell. Each cell contains the entire genetic code, although each cell reads only the part of the code that it needs to perform its job. Thus, blood obtained from a suspect can be compared with semen or hair cells from a crime scene. In the World Trade Center bombing prosecution, an F.B.I. expert matched saliva on an envelope sent by the terrorists to the New York Times with the DNA of one of the defendants.

In effect, the United States imported DNA typing from the United Kingdom. In 1985 Dr. Alec Jeffreys of the University of Leicester, England, recognized the utility of DNA profiling in criminal cases. Its first use in American courts came the following year. The initial appellate case, Andrews v. State, 533 So. 2d 841 (Fla. App. 1988), rev. denied, 542 So. 2d 1332 (Fla. 1989), was reported in 1988. By January 1990, forensic DNA evidence had been admitted in at least 185 cases by 38 states and the U.S. military. At the close of the twentieth century, DNA evidence, in one form or the other, was admissible in every state and federal circuit. These developments are remarkable. No other scientific technique had gained such widespread acceptance so quickly. No other technique had been as complex or evolved so rapidly. DNA profiling raised issues at the cutting edge of modern science. New DNA technologies were introduced even as cases litigating the older procedures worked their way through the court system; there have already been three generations of testsRestriction Fragment Length Polymorphism, Polymerase Chain Reaction, and the current state of the art, Short-Tandem Repeat (STR) analysis.

Finally, no other technique has been as potentially valuable. One court called DNA evidence the "single greatest advance in the 'search for truth'. . . since the advent of cross-examination" (People v. Wesley, 533 N.Y.S.2d 643, 644 (Co. Ct. 1988), aff'd, 633 N.E.2d 451 (N.Y. 1994)). Even its critics acknowledged that "[a]ppropriately carried out and correctly interpreted, DNA typing is possibly the most powerful innovation in forensics since the development of fingerprinting in the last part of the 19th Century" (Lewontin and Hartl, p. 1746).

Yet, much of the initial euphoria that accompanied the introduction of DNA was dispelled by People v. Castro, 545 N.Y.S.2d 985, 996 (Sup. Ct. 1989), the first reported case to successfully challenge DNA evidence. In Castro the court accepted the general validity of the DNA technique but ruled that the test results in Castro were inadmissible. The court found fault with the specific manner in which the analysts applied the technique and conducted the test. Later, the F.B.I.'s top DNA expert conceded these deficiencies: "The initial outcry over DNA typing standards concerned laboratory problems: poorly defined rules for declaring a match; experiments without controls; contaminated probes and samples; and sloppy interpretation of autoradiograms. Although there is no evidence that these technical failings resulted in any wrongful convictions, the lack of standards seemed to be a recipe for trouble" (Lander and Budowle, p. 735).

After Castro was decided, the National Academy of Science's National Research Council convened a committee to study the forensic use of DNA. That committee's 1992 report recommended stringent laboratory procedureswritten laboratory protocols, objective and quantitative procedures for identifying patterns, clearly defined procedures for declaring a match, and methods for identifying potential artifacts (National Research Council, 1992, pp. 52 55). The report bemoaned the fact that there was no mandatory proficiency testing. The NAS report added: "No laboratory should let its results with a new DNA typing method be used in court, unless it has undergone such proficiency testing via blind trials" (p. 55). The publication of the National Academy of Science report did not resolve all the issues. Indeed, it sparked a heated controversy, which an experienced prosecutor would later describe as follows: "[S]cientists fighting for principle displayed an intensity, even a savagery, unmatched by the most aggressive lawyers" (Levy, p. 106). The report recommended statistical techniques designed to ensure that any random match probabilities quoted in court were relatively conservative. Critics charged that these techniques were nothing more than policy judgments masquerading as science. The criticisms of its first report were so forceful that the NAS commissioned a second report, published in 1996. The 1992 "report did not eliminate all controversy. Indeed, in propounding what the committee regarded as a moderate position [for dealing with the statistical issues] the report itself became the target of criticism from scientists and lawyers on both sides of the debate on DNA evidence in the courts" (National Research Council, 1996, p. 1).

This episode illustrates the difficulties courts face when trying to understand complicated scientific procedures and their suitability for forensic use. In 2000, DNA evidence is routinely admitted at trial. The importance of DNA profiling was underscored by a Department of Justice report that discussed the exoneration of twenty-eight convicts through the use of DNA technologysome of whom had been sentenced to death (Connors et al.). By mid-1999, more than seventy convicts had obtained postconviction relief based on exculpatory DNA test results.

Frye v. United States

When the courts began confronting new and more sophisticated scientific evidence in the late 1960s and early 1970s, they needed a legal test for determining admissibility. Three different approaches emerged. One treats the validity of the underlying principle and the validity of the technique as aspects of relevancy. A second approach, ultimately adopted by the U.S. Supreme Court, is known as the reliability test. (The relevancy and reliability approaches are discussed below.) A third approach, which requires the proponent of a novel technique to establish its general acceptance in the scientific community, is based on Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), a federal case decided by the District of Columbia Circuit in 1923. In Frye the D.C. Circuit considered the admissibility of testimony based on the systolic blood pressure test, a precursor of the modern polygraph. The court announced that a novel scientific technique "must be sufficiently established to have gained general acceptance in the particular field in which it belongs" (p. 1014). The court found that the systolic test had "not yet gained such standing and scientific recognition among physiological and psychological authorities" (ibid.). Thus, under the Frye standard, it is not enough that a qualified individual expert, or even several experts, testify that a particular technique is valid. Frye imposes a special burden: the technique must be "generally" accepted by the relevant scientific community.

The Frye court's brief two-page opinion offered no explanation for adopting the "general acceptance" test. As a federal decision, it did not apply to state courts; for that matter, it was not binding on any of the other federal circuit courts. For the next five decades, Frye remained largely dormant; it was rarely cited, and then, mostly in polygraph cases. As late as 1972, a federal district court correctly observed that "[t]here is notably an absence of any discussion of the 'general acceptance' standard in federal decisions" (United States v. Zeiger, 350 F. Supp. 685, 687 n. 6 (D.D.C.), rev'd, 475 F.2d 1280 (D.C. Cir. 1972)).

This state of affairs was understandable. For most of the period between the rendition of Frye and the 1970s, the scientific techniques confronting the courts did not raise significant Frye issuesthe admissibility of novel scientific evidence. A 1966 amendment to Federal Criminal Rule 16, which governs the pretrial disclosure of scientific reports in criminal litigation, provides some insight into the types of expert testimony used during this period. The accompanying committee note mentioned reports of "fingerprint and handwriting comparisons." Ballistics, blood tests, paint, fibers, and autopsies could be added to this list. None of these techniques presented Frye issues; they were traditional techniques, long accepted by the courts.

Over the years, the general acceptance standard gradually became the overwhelming majority rule in both federal and state courts until 1993 when the U.S. Supreme Court rejected Frye in Daubert v. Merrell Dow Pharmaceuticals, Inc. , 509 U.S. 579 (1993). By 1993, Frye had been applied to voiceprints, neutron activation, gunshot residue tests, fingerprints, bite mark comparisons, psycholinguistics, truth serum, scanning electron microscopic analysis, hypnosis, blood analysis, hair analysis, intoxication testing, instrumental analysis, and numerous other forensic techniques. Later, the Frye test would be applied to DNA evidence as well as to social science techniques such as the battered woman syndrome, rape trauma syndrome, child abuse accommodation syndrome, profile evidence, and psychiatric testimony.

Rationale. The stated justification for the general acceptance standard is that it tends to indirectly ensure the reliability of scientific evidence. The D.C. Circuit, the progenitor of the Frye test, later stated in rejecting voiceprint evidence that 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" (United States v. Addison, 498 F.2d 741, 74344 (D.C. Cir. 1974)).

Application of Frye test. The Frye rule was not without its detractors. One criticism focused on the difficulties involved in applying the test. The general acceptance test requires a two-step analysis: first, identifying the field in which the underlying principle falls, and second, determining whether that principle has been generally accepted by members of the identified field. Neither step is free of difficulties.

The first step can be problematic. Many scientific techniques do not fall within the domain of a single academic discipline or professional field. Consequently, selecting the proper field may prove troublesome. For example, "voiceprint" analysis requires a knowledge of anatomy, physiology, physics, psychology, and linguistics. Similarly, DNA involves several disciplinesmolecular biology, genetics, environmental biology, physical anthropology, evolutionary biology, population genetics, and statistics. The selection of the field, moreover, will often affect whether a novel technique satisfies the generalacceptance test. If polygraph examiners are selected as the relevant field, polygraph results would be admissible.

Once the relevant field has been identified, the second step requires determining whether the technique has been "generally accepted" by members of that field. The percentage of those in the field who must accept the technique has never been clearly specified. For instance, one court has defined general acceptance as "widespread; prevalent; extensive though not universal" (United States v. Zeiger, 350 F. Supp. 685, 688 (D.D.C.), rev'd, 475 F.2d 1280 (D.C. Cir. 1972)). Another court conceded that "a degree of scientific divergence of view is inevitable," without elaborating on how much divergence would be fatal to admissibility (Commonwealth v. Lykus, 327 N.E.2d 671, 678 n.6 (Mass. 1975)).

An additional issue arises in multiple-step procedures such as DNA profiling, which consists of a molecular biology component (declaring a match) and a population genetics component (statistical calculations). Are both aspects subject to the general-acceptance requirement? A related issue concerns subsequent developments or variants of a technique. For example, the general acceptance of stationary radar should not automatically lead to the admissibility of moving radar, in which the patrol car as well as the suspect car is in motion. Similarly, general acceptance of RFLP(DNA) does not mean that PCR(DNA), or even a particular type of PCR such as DQalpha, is necessarily admissible.

Moreover, some jurisdictions following Frye do not apply the general-acceptance test to all types of "scientific" evidence. For example, a California appellate court refused to apply the Frye test to bitemark comparisons (People v. Marx, 126 Cal. Rpt. 350, 35556 (App. 1975)). The court reasoned that bite mark evidence did not require blind acceptance by the jury. The basis on which the expert reached his conclusionsmodels, photographs, and X raysare shown to the trier of fact, and the trier could therefore independently second-guess the expert's conclusions. Similarly, the Arizona Supreme Court has ruled that the "Frye analysis is not applicable to footprints [comparisons]" (State v. Murray, 906 P.2d 542, 562 (Ariz. 1995), cert. denied, 518 U.S. 1011 (1996)). In short, many jurisdictions that profess adherence to Frye exempt some techniques from scrutiny under Frye.

Criticisms. In addition to the attacks concerning the difficulties of applying the Frye test discussed above, the test has been criticized on other grounds. Another criticism of the general-acceptance test is that it exacts too high a cost and often bars the admission of reliable evidence. Critics of Frye assert that the delay to permit the technique to win general acceptance "precludes too much relevant evidence for purposes of the fact determining process" (United States v. Sample, 378 F. Supp. 43, 53 (E.D. Pa. 1974)). In contrast, courts favoring the general acceptance test recognize its conservative nature but believe that this aspect does not exact an "unwarranted cost" (United States v. Addison, 498 F.2d 741, 743 (D.C. Cir. 1974)). The California Supreme Court has stated that the "primary advantage. . .of the Frye test lies in its essentially conservative nature" (People v. Kelly, 549 P.2d 1240, 1245 (Cal. 1976)). The criticism that the Frye test is too conservative begs a question. The question is not whether the Frye test is conservative (which it is), but whether other standards would better accomplish the objective of preventing the admission of unreliable scientific evidence.

Still another criticism of Frye is that it rests upon an invalid assumption, namely, that jurors are overwhelmed by scientific evidence. For example, in excluding voiceprint evidence under the Frye test, the D.C. Circuit asserted that scientific evidence may "assume a posture of mystic infallibility in the eyes of a jury of laymen" (United States v. Addison, 498 F.2d 741, 744 (D.C. Cir. 1974)). Commentators, however, have argued that the available empirical research on jury reaction to different types of scientific evidence does not support the D.C. Circuit's assertion (Rogers and Ewing; Imwinkelried, 1983).

Finally, "[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" (Giannelli, 1980, p. 1226). There is a great diversity in the underlying bases of expert testimony, varying from the complexity of DNA to the social sciencebased research of rape trauma syndrome. The Frye "one-test-fits-all" approach is too blunt an instrument.

Relevancy test

One alternative approach to Frye is to treat scientific evidence in the same way as other evidence, weighing its probative value against countervailing dangers and considerations. Charles McCormick, a noted evidence scholar, advocated this position. In his 1954 text, he wrote that "Any relevant conclusions which are supported by a qualified expert witness should be received unless there are other reasons for exclusion. Particularly, its probative value may be overborne by the familiar dangers of prejudicing or misleading the jury, unfair surprise and undue consumption of time" (pp. 363364).

In practice, however, the relevancy test affords inadequate assurance of the reliability of scientific evidence; this test often means that qualifying the expert automatically renders testimony about the technique admissible. Even if the expert is qualified, the expert may be relying on a bogus theory or technique. "The major flaw in the relevancy analysis . . . is its failure to recognize the distinctive problems of scientific evidence. . . . [T]he judge frequently is forced to defer to an expert, thereby permitting admissibility based on the views of a single individual in some cases" (Giannelli, 1980, p. 1250).

Barefoot v. Estelle. Barefoot v. Estelle, 463 U.S. 880 (1983), a capital murder case decided by the Supreme Court in 1983, illustrates the weakness of the relevancy approach even though the case was decided on constitutional, rather than evidentiary, grounds. In the penalty phase, the prosecution offered psychiatric testimony concerning Barefoot's future dangerousness, a qualifying factor under the Texas death penalty statute. One psychiatrist, Dr. James Grigson, without ever examining Barefoot, testified that there was a "'one hundred percent and absolute' chance that Barefoot would commit future acts of criminal violence" (463 U.S. at 919). Barefoot challenged the admission of this evidence on constitutional grounds due to its unreliability.

In an amicus ("friend of the court") brief, the American Psychiatric Association (APA) stated that the "large body of research in this area indicates that, even under the best of conditions, psychiatric predictions of long-term future dangerousness are wrong in at least two out of every three cases" (p. 9). In a later passage, the brief noted that the "unreliability of psychiatric predictions of long term future dangerousness is by now an established fact within the profession" (p. 12). A substantial body of research supported the APA position.

Nevertheless, the Court rejected Barefoot's argument. According to the Court, "[n]either petitioner nor the [APA] suggests that psychiatrists are always wrong with respect to future dangerousness, only most of the time" (p. 901). In one passage the Court virtually adopts McCormick's relevancy approach. As a result, the Barefoot Court admitted evidence "at the brink of quackery" (Dix, p. 172). Justice Blackmun, who later authored the Daubert opinion, dissented. He noted that "[i]n the present state of psychiatric knowledge, this is too much for me. One may accept this in a routine lawsuit for money damages, but when a person's life is at stake . . . a requirement of greater reliability should prevail. In a capital case, the specious testimony of a psychiatrist, colored in the eyes of an impressionable jury by the inevitable untouchability of a medical specialist's words, equates with death itself" (p. 916). Even a proponent of capital punishment believed that the execution of Thomas Barefoot on 24 October 1984 was based on "junk science." One could favor the death penalty and "yet still recoil at the thought that a junk science fringe of psychiatry . . . could decide who will be sent to the gallows" (Huber, p. 220).

Reliability Test

A third admissibility test eventually emerged from the Frye debates; the new test rejected the Frye standard but demanded proof of reliability. The Supreme Court adopted this approach in 1993, ruling that the Federal Rules of Evidence had displaced the Frye test. The case, Daubert v. Merrell Dow Pharmaceuticals, Inc. , involved the admissibility of expert epidemiological testimony concerning whether Bendectin, an anti-nausea drug, causes birth defects. In place of Frye, the Court substituted a reliability test. The test, in the Court's view, derived from a federal statute, Evidence Rule 702, which uses the terms "scientific" and "knowledge." The Court embraced a classically Newtonian understanding of scientific methodologythe process of formulating a hypothesis and then engaging in experimentation or observation to falsify or validate the hypothesis. "[I]n 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" (p. 590). In performing this "gatekeeping role," the trial court may consider a number of factors. First, the court should determine whether the scientific theory or technique has been tested. Citing scientific authorities, the Court recognized that a hallmark of science is empirical testing. Second, whether a theory or technique had been subjected to peer review and publication is "a relevant, though not dispositive, consideration in assessing. . . scientific validity." The peer review and publication process increase the likelihood that flaws will be revealed. They thus serve as circumstantial evidence that the hypothesis has been validated by sound scientific methodology. Third, a technique's "known or potential rate of error" is a pertinent factor. Fourth, the "existence and maintenance of standards controlling the technique's operation" is another indicia of trustworthiness. Finally, "general acceptance" remains an important consideration. Although the Court rejected "general acceptance" as the sole criterion for admissibility as in the Frye test, it recognized its relevance in assessing the reliability of scientific evidence. Again, this factor can be important circumstantial evidence of the soundness of the research underlying the expert's hypothesis.

Although the Daubert decision was a civil case, it applied to federal criminal trials as well. Its ancestry can be traced to civil litigation concerning toxic torts (asbestos, Agent Orange, and silicone breast implants) in the 1980s. Frye had rarely been applied to civil cases until the 1980s. At this point, the Frye test, which had previously been championed by the criminal defense bar, was adopted by big business in their battle against "junk science." A plank against junk science appeared in the Republican platform in 1988; and President George Bush, by executive order, imposed a modified Frye test on Justice Department lawyers in civil cases. Inconsistently, the same Justice Department was advocating a lower standard, the complete rejection of Frye, in criminal DNA cases (Giannelli, 1993).

Application in the states. Daubert 's effect on state jurisdictions depends on several factors. As previously stated, Daubert rests on an interpretation of the Federal Rules of Evidence, a federal statute. As a statutory rather than a constitutional case, Daubert is not binding on the states, which are therefore free to continue to follow Frye. The state courts have that freedom even in the forty jurisdictions with evidence codes modeled after the Federal Rules, some of which still adhere to the Frye test. Although numerous jurisdictions have rejected Frye in favor of the Daubert approach, other courts have retained the Frye test. For example, the Arizona Supreme Court declined to follow Daubert, noting that it was "not bound by the United States Supreme Court's non-constitutional construction of the Federal Rules of Evidence when we construe the Arizona Rules of Evidence" (State v. Bible, 858 P.2d 1152, 1183 (Ariz. 1993), cert. denied, 511 U.S. 1046 (1994)). Indeed, Frye still has adherents in some of the most populous statesCalifornia, New York, Pennsylvania, Michigan, and Florida. Courts retaining Frye have found Daubert wanting. The Washington Supreme Court declared: "While Frye may be difficult to apply in some contexts, this is a result of the complexity of the particular science at issue, the extent to which the scientific community has made its view known, and the extent of any dispute in the scientific community. . . . Questions of admissibility of complex, controversial scientific techniques or methods, like those involving DNA evidence, are going to be difficult under either standard" (State v. Copeland, 922 P.2d 1304, 1314 (Wash. 1996)).

Post- Daubert issues. Although the Daubert decision resolved a number of significant issues, like most landmark cases, it left many questions unanswered. One question raised by the Daubert opinion is whether the Supreme Court intended its reliability test to be more permissive than the Frye test. There is some language in the opinion that points in this direction, and a number of courts have embraced this position. The polygraph cases are a good example. In United States v. Posado, 57 F.3d 428, 429 (5th Cir. 1995), the Fifth Circuit stated that "the rationale underlying this circuit's per se rule against admitting polygraph evidence did not survive Daubert." The court did not hold that polygraph evidence was admissible but only that admissibility should be left to the discretion of the trial judge.

There is, however, another viewthat Daubert sets forth a different, but not necessarily less stringent, standard. Some language in Daubert supports this position, particularly the Court's emphasis on the "gatekeeper" function of the trial judge. Federal courts examining handwriting and hair analysis have questioned the admissibility of testimony based on these traditional techniques, thereby undercutting the notion that Daubert is a more liberal standard.

Technical evidence. In interpreting Rule 702, the Supreme Court in Daubert dealt only with "scientific" evidence. The epidemiological testimony offered in Daubert was purportedly scientific. Rule 702, however, also includes the terms "technical" and "specialized" knowledge. That alternate phrasing raised two issues: (1) whether Daubert 's reliability requirement extended to these other types of nonscientific expert testimony, and (2) if so, whether the factors set forth in Daubert for judging reliability applied in this context. In 1999, the Supreme Court in Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999), answered both questions in the affirmative. Kumho involved a civil casea tire blowout accident, in which the Court upheld the trial court's decision to exclude engineering testimony concerning the cause of the blowout. The Court acknowledged that some of the Daubert factors might prove inappropriate in assessing the reliability of certain types of nonscientific testimony. The Court accorded trial judges discretion to select the factors to employ in evaluating the specific type of nonscientific expertise. Kumho applies to criminal cases as well. Accordingly, the Daubert reliability requirement applies across the board to all expert evidence, for example, handwriting and hair comparisons.

Other developments

At the same time that DNA evidence was being closely scrutinized by the courts and Kumho was prompting renewed judicial interest in nonscientific expert testimony, several notorious cases of scientific malfeasance received extensive treatment in the news media (Giannelli, 1997). One case is illustrative. Fred Zain, the former head serologist of the West Virginia State Police crime laboratory, falsified conventional serological test results in as many as 134 cases from 1979 to 1989. Defendants, since exonerated, were sentenced to long prison terms based upon his testimony. A judicial inquiry concluded that "as a matter of law, any testimonial or documentary evidence offered by Zain at any time in any criminal prosecution should be deemed invalid, unreliable, and inadmissible" (In re Investigation of the W. Va. State Police Crime Lab., Serology Div. , 438 S.E.2d 501, 520 (W. Va. 1993)). In 1989, Zain accepted a serologist position with the County Medical Examiner's laboratory in San Antonio, where he performed DNA profiling and testified in death penalty cases. Another instance of misconduct involved New York State Troopers who tampered with fingerprint evidence.

Despite the publicity that the cases of intentional misconduct generated, there is good reason to believe that in at least some crime laboratories simple incompetence is a more prevalent problemoften traceable to inadequate funding (Giannelli, 1988). There is a widespread feeling that courts should rigorously police the reliability of proffered expert testimony to give the laboratories an even greater incentive to select trustworthy scientific tests and to use meticulous care in conducting the tests.


Scientific evidence is often superior to other types of evidence, such as eyewitness identifications and confessions. The Justice Department report of twenty-eight convicts released from prison during the 1990s due to exculpatory DNA evidence dramatizes this point. Most of these convictions were based on eyewitness identifications. One involved a mentally limited defendant who falsely confessed and then pled guilty to avoid the death penalty.

However, if the introduction of expert testimony is to enhance the fact-finding process, the courts must separate the wheat from the chaff. Forensic science is being scrutinized as never before. The "junk science" debate emanating from the civil docket is having a spillover effect on criminal litigation. The Supreme Court in Dau- bert and Kumho affirmed its determination to improve the quality of expert testimony in federal trials. In Kumho, the Court declared that the "objective of [Daubert 's gatekeeping] requirement is to ensure the reliability and relevancy of expert testimony. It is to make certain that an expert, whether basing testimony upon professional studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field" (526 U.S. at 152). One of the benefits of the DNA litigation is that these cases set high standards for the evaluation of scientific proof. Finally, Zain and similar misconduct cases have further fueled the drive for crime laboratory accreditation, examiner certification, and the standardization of forensic procedures.

Paul C. Giannelli

Edward J. Imwinkelried

See also Civil and Criminal Divide; Crime Causation: Biological Theories; Crime Causation: Psychological Theories; Diminished Capacity; Domestic Violence; Eyewitness Identification: Psychological Aspects; Rape: Behavioral Aspects; Legal Aspects; Violence.


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. "The Admissibility of Laboratory Reports in Criminal Trials: The Reliability of Scientific Proof." Ohio State Law Journal 49 (1988): 671701.

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Giannelli, Paul, and Imwinkelried, Edward. Scientific Evidence. 3d ed. Charlottesville, Va.: Lexis Law Publishing, 1999.

Huber, Peter W. Galileo's Revenge: Junk Science in the Courtroom. New York: Basic Books, 1991.

Imwinkelried, Edward. "The Standard for Admitting Scientific Evidence: A Critique from the Perspective of Juror Psychology." Villanova Law Review 28 (1983): 554571.

. "The Next Step after Daubert: Developing a Similarly Epistemological Approach to Ensuring the Reliability of Nonscientific Expert Testimony." Cardozo Law Review 15 (1994): 22712294.

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Levy, Harlan. And the Blood Cried Out: A Prosecutor's Spellbinding Account of the Power of DNA. New York: Basic Books, 1996.

Lewontin, Richard C., and Hartl, Daniel L. "Population Genetics in Forensic DNA Typing." Science 254 (1991): 1745.

Maletskos, Constantine J., and Spielman, Stephen J. "Introduction of New Scientific Methods in Court." In Law Enforcement, Science & Technology. Edited by S. A. Yefsky. U.S. Office of Law Enforcement Assistance. Washington, D.C.: Thompson Book Company, 1967. Pages 957964.

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. The Evaluation of Forensic DNA Evidence. Washington, D.C.: National Academy Press, 1996.

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Andrews v. State, 533 So.2d 841 (Fla. App. 1988), rev. denied, 542 So. 2d 1332 (Fla. 1989).

Barefoot v. Estelle, 463 U.S. 880 (1983).

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Scientific Evidence


Evidence presented in court that is produced from scientific tests or studies.

Scientific evidence is evidence culled from a scientific procedure that helps the trier of fact understand evidence or determine facts at issue in a judicial proceeding. Under rule 702 of the federal rules of evidence and similar state court rules of evidence, "a witness qualified as an expert by knowledge, skill, experience, training, or education" may testify and offer opinions in court if "scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue." Article VII of the Federal Rules of Evidence contains other rules on expert testimony and scientific evidence. All states have rules on expert testimony and scientific evidence that are similar to the rules in article VII.

Expert testimony on scientific evidence is different from ordinary testimony from laypersons. A lay witness may testify to inferences and give opinions only if they are rationally based upon the witness's perceptions of the subject of the testimony. Experts, by contrast, may give opinions and testify about possible inferences based in part on information obtained from secondhand sources and not from observation of the object of the testimony. For example, a layperson would not be allowed to take the witness stand and offer an opinion on a plaintiff's injury unless the individual had witnessed relevant information regarding the injury. However, a doctor who is certified as a specialist in the particular injury could take the stand and offer opinions on the injury based not only on an examination of the plaintiff but also on secondhand information that is normally relied on by experts in that particular field of medical study.

One of the most important issues that arises in expert testimony is which scientific procedures a court should accept as evidence. Many scientific procedures are not seriously in dispute and are accepted by courts with little or no inquisition into their validity. Examples include fingerprint tests for purposes of identification, blood tests, breathalyzer tests for alcohol consumption, and ballistics tests of bullets and their impact areas. These scientific procedures are so widely accepted that a court may take judicial notice of the procedure's validity. Judicial notice means that the parties in the case do not have to present evidence to the court to establish the validity of the scientific procedure. In some instances legislatures have specifically authorized the use of scientific tests, such as breathalyzer tests for suspected drunk drivers.

Whether they are judicially noticed or legislatively mandated, scientific tests that are universally accepted must be presented by a qualified expert. A person is established as a qualified expert before the court through questioning by the attorney who is using the witness as an expert. The attorney asks a series of questions to establish that the witness has adequate education and training to testify as an expert—a process called laying a foundation for the witness. Once the court is convinced that the witness is an expert on the procedure or subject matter that will be presented as evidence, the witness gives an expert opinion to the exact procedures that were used or the factual circumstances that arose in the case at hand. For example, assume that a person sues a doctor for medical malpractice, arguing that the defendant failed to set a broken bone properly. If the plaintiff offers a bone specialist as an expert witness on the issues surrounding the care he received from the defendant, the expert witness must testify about the witness's credentials and give details about the plaintiff's treatment.

Some scientific tests and examinations that are not universally accepted are nevertheless generally considered reliable. Some examples are neutron activation analysis to determine the identity of goods, voiceprints to determine a person's identity, and genetic testing or DNA analysis. These types of scientific procedures may be accepted in the medical communities, but they are not so established that they may be judicially noticed as automatically valid sources of scientific evidence. They may be admitted as evidence, but only after an expert witness has testified to the validity of the test. In determining whether to admit scientific evidence from procedures that are not universally accepted, a court must ask whether the test is reliable. A technique's reliability depends on a number of factors, including whether the technique can be or has been tested, whether it has been subjected to peer review, whether the test procedures have been published, whether the test has a margin of error and, if so, at what rate, and whether the technique, as applied, conformed to existing standards for the test (Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579, 113 S. Ct. 2786, 125 L. Ed. 2d 469 [1993]).

In Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137, 119 S. Ct. 1167, 143 L. Ed. 2d 238 (1999) the U.S. Supreme Court ruled that the Daubert standards govern not just the admissibility of scientific evidence in federal court, but should be applied to all witnesses seeking federal court approval to testify as an expert. Thus, the Supreme Court found that a purported expert on tire failure was subject to a Daubert inquiry before he could be permitted to testify on the subject in a products liability trial, even if some of his proffered testimony was not wholly "scientific." The lower court had attempted to draw a distinction between scientific expert for which the Daubert standards did apply and a technical expert for which the Daubert standards did not apply. By expanding the daubert test, the Court reemphasized the trial court's broad discretion in matters of expert testimony.

In some instances courts are reluctant to admit certain scientific evidence because the procedures yield results that are not considered sufficiently reliable to be used as evidence. Such procedures include polygraph and chemical tests that have been created to determine whether a person is telling the truth. If all parties agree that testimony derived from such procedures shall be admissible, however, a court is free to allow the evidence to be introduced.

In any case, regardless of the level of acceptance of a particular scientific procedure, the scientific evidence presented must be relevant to the issue at hand. Furthermore, the scientific evidence must have been obtained in a manner that is consistent with the way such evidence is normally obtained. For instance, assume that a physicist intends to testify to the speed of the defendant's vehicle in a personal injury case stemming from a car accident. If the physicist used different methods from those used by other physicists in determining a vehicle's speed, the court may refuse to allow the physicist to testify as to the vehicle's speed.

An expert witness giving testimony on scientific evidence may offer opinions on issues related to that evidence. An expert witness may also give an opinion on the ultimate issue in the case. Under rule 704 of the Federal Rules of Evidence, however, an expert witness testifying with respect to the mental state or condition of a criminal defendant may not state "an opinion or inference as to whether the defendant did or did not have the mental state or condition constituting an element of the crime charged or of a defense thereto." This rule, which is applied by courts only in criminal cases, was approved by the U.S. Congress in 1984, largely in response to the outcome of the criminal prosecution of John Hinckley, who attempted to assassinate President ronald reagan in 1981. Hinckley was charged with attempted assassination, assault on a federal officer, and use of a firearm in the commission of a federal offense, but was found not guilty by reason of insanity after the jury heard testimony from a psychiatrist who declared that Hinckley could not be found guilty because he lacked the knowing mental state required for a conviction on the charges.

The weight given to scientific evidence may vary according to the particular test that yielded the evidence. One party's expert testimony may be convincing, but it may not be dispositive of the case because the other party may have experts from the same field who have studied the same evidence and come to different conclusions. Experts have become indispensable to the vast majority of litigated cases, and many cases, civil and criminal alike, come down to a battle between experts. One notable exception to this trend is the paternity case, in which blood test results or DNA test results can establish the ultimate issue in the case. This is true, however, only if the parties in the paternity case agree that the particular tests will be conclusive and if the tests show that the individual named as the father could not be a parent of the child in question. If the tests show that the individual named as the father could be the parent, the test results will not dispose of the case, and the parties will have to present further evidence.

Courts have the discretion to appoint an expert witness to testify to scientific evidence. Under rule 706 of the Federal Rules of Evidence and similar state court rules of evidence, a court may appoint an expert to present evidence on a particular topic and order compensation for the expert's time and effort. Typically, in a civil case, the parties must apportion the costs as the court directs. In just compensation cases under the fifth amendment and in criminal cases, the court orders payment for the expert out of government funds.

One of the most well-known experts on scientific evidence in the United States is Barry Scheck, a criminal defense lawyer who rose to prominence during the 1995 o.j. simpson murder trial as a member of Simpson's so-called "Dream Team." In 1992 he and fellow Dream-Team member Peter Neufeld opened the National Association of Criminal Defense Lawyers' Innocence Project, a nonprofit legal clinic at the benjamin n. cardozo School of Law in New York. Through testing of dna evidence, the Innocence Project has helped exonerate 127 wrongly convicted inmates. Scheck, 43, has chronicled the stories of his exonerated clients in books and on the lecture circuit. He also assisted Colorado prosecutors and police officers investigating the Jon-Benet Ramsey murder case.

further readings

"Evidence." 1994. SMH Bar Review.

Faigman, David L., et al. 2002. Modern Scientific Evidence: The Law and Science of Expert Testimony. 2d ed. St. Paul, Minn.: West Group.

Guidotti, Tee L., and Susan G. Rose. 2001. Science on the Witness Stand: Evaluating Scientific Evidence in Law, Adjudication, and Policy. Beverly Farms, Mass.: OEM Press.

Masters, Brooke A. 2000. "Helping Those Who Might Be Wrongfully Imprisoned; Lawyers, Students Pursue New Trials in Questionable Cases." Washington Post (December 7).


Fingerprints; Forensic Science; Insanity Defense.

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