This paper proposes two theories for distinguishing scientific from non-scientific evidence. Under one theory, scientific evidence is based on verifiable experimentation, while non-scientific evidence is based on experience, training, and experience. Under the second theory, scientific evidence provides a generalizable theory, while non-scientific testimony applies only to a particular case. Most psychiatric and psychological testimony has scientific and non-scientific elements.

Courts in common-law jurisdictions have developed special strict rules for the admissibility of scientific evidence, and commentators are starting to urge the development of similarly strict rules for the admissibility of non-scientific evidence. The overarching trend is for courts to exclude questionable evidence.

The most common tests used to screen expert testimony are the "general acceptance" and "reliability" tests. Depending on which test is applied in a particular jurisdiction, and how stringently it is applied, much of the psychiatric and psychological evidence that has traditionally been admitted in trials may be at risk of exclusion in the future.

Acknowledgments

The author acknowledges the financial assistance of the Mellon Foundation, which funded this research. The author thanks Solveig Bernstein, Bert Black, Harold Edgar, and Ian Freckelton for their comments.

Many common-law jurisdictions are struggling to establish standards for the admissibility of expert opinion evidence, including psychiatric and psychological (P & P) evidence. Courts in the United States,⁽¹⁾ Australia,⁽²⁾ and Canada,⁽³⁾ have developed special strict tests for the admissibility of scientific expert evidence. To the extent that P & P evidence is deemed to be "scientific" (i.e., it is deemed to be making scientific claims), those tests will be applied to P & P evidence.

With regard to non-scientific expert evidence, courts--particularly in the U.S.--have traditionally been extremely lenient in allowing qualified experts to testify without examining the basis of their testimony. That lenient attitude has recently attracted criticism. Critics argue that speculative, unsupported non-scientific testimony can be just as damaging to the legal process as unreliable scientific evidence, and should therefore be subjected to similarly strict scrutiny.⁽⁴⁾

Despite this criticism, courts have not yet promulgated coherent standards for the admissibility of non-scientific expert evidence. Some legal scholars have argued that courts should apply the same tests to non-scientific expert testimony as they do to scientific testimony. Others have argued for more lenient rules to govern the admissibility of non-scientific evidence. The test for the admissibility of P & P evidence therefore may depend on whether P & P evidence is considered scientific or non-scientific.

1. Possible Distinctions Between Scientific and Non-Scientific Evidence

If the tests for the admissibility of scientific and non-scientific evidence do diverge, courts must be able to distinguish between scientific and non-scientific expert testimony. Unfortunately, there has been little judicial or scholarly attention to this issue.

There are several possible ways to distinguish between scientific and non-scientific expert testimony. To the extent that courts have attempted to do so, they have generally used a layman's definition of science. Thus, for example, testimony by chemists within their area of expertise would be scientific evidence, while testimony by historians would not. Such a rule can resolve the easy cases, but it cannot resolve more ambiguous issues, including the issue of whether P & P testimony is scientific.

Some American scholars have attempted to distinguish between "hard" and "soft" scientific evidence, particularly in the context of mental health evidence.⁽⁵⁾ Hard scientific evidence is based on objective testing and the traditional standards of scientific verification and falsifiability. "Soft" scientific evidence, in contrast, is an opinion based on the professional's education, training, and experience, and is therefore not verifiable or falsifiable.

These scholars fail to explain what makes so-called "soft" scientific evidence scientific at all. Professor Edward Imwinkelried takes a more sophisticated approach. He argues that evidence based on education, training, and experience may be admissible as non-scientific evidence, but it is simply not scientific.⁽⁶⁾ Imwinkelried contends that only evidence based on repeatable and verifiable experiments should be considered scientific.⁽⁷⁾

Applying Imwinkelried's theory, P & P testimony could have both scientific and non-scientific elements. Testimony regarding rape trauma syndrome, for example, would fall into the scientific category if an expert wished to testify that studies have shown that the syndrome exists. On the other hand, an expert's testimony regarding a diagnosis of a particular woman whom the expert believes suffers from rape trauma syndrome would be considered non-scientific testimony.

A third possible distinction between scientific and non-scientific evidence is that scientific evidence purports to provide a generalizable theory, while non-scientific evidence only speaks to the case before the court.⁽⁸⁾ Again under this theory, a psychologist testifying that a psychological condition known as rape trauma syndrome exists is presenting scientific evidence. Evidence based on a clinical diagnosis that a particular woman suffers from rape trauma syndrome would be non-scientific.

Some P & P experts have presented testimony in such a way that it appears that their testimony is completely non-scientific under the last two definitions described above. For example, a recent book describes a case in which a prosecutor asked a psychiatrist why the psychiatrist believed that a defendant suffered from an abnormality of mind when the defendant committed a killing. The psychiatrist responded: "On the reason of my diagnosis and my assessment and my experience of 32 years. That is enough reason, I suggest."⁽⁹⁾ But while the psychiatrist's was making a non-scientific claim based on his diagnosis and experience, he was implicitly relying on the underlying scientific proposition that it is possible to distinguish between "normal" and "abnormal" minds. A court reviewing the psychiatrist's testimony would have to consider his scientific and non-scientific claims separately.

2. The Admissibility of Scientific Psychological and Psychiatric Evidence

Simply because proposed expert testimony comes within the "scientific" category because the expert is deemed to be making a scientific claim does not mean that the testimony is good science, or that it is admissible for legal purposes. Rather, a court must apply its jurisdiction's exclusionary rule for scientific evidence to any proposed scientific testimony. The two most popular and significant tests for the admissibility of scientific expert testimony are the general acceptance test, represented by the venerable American Frye case,⁽¹⁰⁾ and the reliability test, recently adopted by the United States Supreme Court in the Daubert case.⁽¹¹⁾ The Frye test has been particularly influential in Australia,⁽¹²⁾ while the reliability test is becoming the dominant test in Canada.⁽¹³⁾

The Frye test, in its classic form, states as follows:

Somewhere in this twilight zone the evidential force of the principle must be recognized and while the 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 to which it belongs.⁽¹⁴⁾

Thus, under Frye, only evidence based on scientific techniques or theories generally accepted by the relevant scientific communities is deemed admissible.

In Daubert, the U.S. Supreme Court held that the appropriate test for the admissibility of scientific evidence is a "flexible" test focusing on the underlying reliability and scientific relevance of proffered evidence. Daubert outlines several factors to help judges in determining whether proffered scientific evidence is reliable and therefore admissible. These factors include (1) testability of the underlying theory or technique; (2) peer review and publication; (3) the known or potential rate of error of the technique in question; and (4) general acceptance.⁽¹⁵⁾

Some have argued that Frye should not be applied to scientific P & P evidence because it is too restrictive. Professor David McCord, for example, argues that most theories in psychology and psychiatry are subject to dispute among the various members of the scientific community, thus making general acceptance virtually impossible.⁽¹⁶⁾ But if this is so, McCord begs the question of why a court should rely on evidence that is in serious dispute within its field.

Moreover, contrary to McCord's critique, Frye is not a particularly exacting test for the admissibility of scientific P & P evidence. If the American Psychiatric Association or a similar body has accepted a theory or technique, it is presumptively generally accepted. For example, under Frye scientific testimony regarding rape trauma syndrome is presumptively admissible because the American Psychiatric Association recognized it as a form of chronic and acute post-traumatic stress disorder in its third Diagnostic and Statistical Manual of Mental Disorders (DSM-III).⁽¹⁷⁾

A more compelling criticism of Frye is it that it is too forgiving. Frye allows the admission of generally accepted but unreliable testimony.⁽¹⁸⁾ For example, battered woman syndrome is arguably generally-accepted by the psychiatric community, and therefore can pass muster under Frye. Indeed, in Runjanjic v. R,⁽¹⁹⁾ the South Australian Court of Criminal Appeal reversed a trial court opinion refusing to admit evidence of battered woman syndrome on behalf of the defendants. King CJ found that battered woman syndrome is "accepted by experts competent in the field of psychology or psychiatry as a scientifically established facet of psychology."⁽²⁰⁾ Nevertheless, there are reasons to be suspicious of the validity of the theory behind battered woman syndrome, including the fact that aspects of the theory are based on dog studies, the results of which may not be readily translatable to humans.⁽²¹⁾

In contrast to the relatively liberal nature of the Frye rule, Daubert forces courts to go beyond determining the consensus in the relevant scientific community in favor of a determination of the "evidentiary reliability" and "scientific validity" of proffered evidence. Scientific P & P evidence rarely meets several criteria for reliability proposed in Daubert. P & P is often not peer reviewed, has no known or a high error rate, and is not falsifiable. Indeed, Freudians explicitly disavow the scientific method promoted by Daubert.⁽²²⁾ One scholar argues, with only slight exaggeration, that court should not apply Daubert to P & P evidence because read literally it "would dictate the end of the receipt of [scientific] psychiatric and psychological testimony in federal courts."⁽²³⁾ Nevertheless, the trend in American jurisdictions where Daubert reigns is to apply Daubert to such evidence.⁽²⁴⁾

3. The Admissibility of Non-Scientific Psychological and Psychiatric Evidence

Just as "scientific" P & P evidence is not necessarily sufficiently accepted or reliable to be admissible, application of the moniker "non-scientific" to P & P evidence is not a commentary on the quality of the evidence, and does not suggest that the evidence should not be deemed admissible. Unfortunately, the literature on what standards courts should force non-scientific testimony to meet is quite sparse, and not always sophisticated.

The American College of Trial Lawyers, for example, argues that courts should apply Daubert to non-scientific expert testimony.⁽²⁵⁾ The College uses a layman's definition of science, contrasting "scientific" evidence with evidence concerning non-scientific subjects such as accounting or economic principles.⁽²⁶⁾ As discussed above, however, such evidence may be considered scientific to the extent that it is based on research and experimentation or that it asserts generalizable theorems. And when it comes to such truly non-scientific evidence as a psychiatric diagnosis, the criteria for determining the reliability of scientific evidence suggested in Daubert, such as peer review, obviously cannot be applied

Another potential test for the admissibility of non-scientific evidence is the Frye general acceptance rule. A court applying Frye to non-scientific testimony must ask whether the expert used a generally-accepted methodology in reaching his or her conclusion. In the context of P & P testimony, such methodologies could include interviews with the subject, analysis of a personality profile test completed by the subject, a medical examination, and other standard diagnostic techniques.

The influential California Supreme Court has gone beyond the original Frye test, and requires not only that experts use a generally-accepted methodology be used, but that "correct scientific procedures [a]re used in the particular case."⁽²⁷⁾ Similarly, New York's highest court requires that an expert show that he or she "performed the technique in an accepted manner in the particular case."⁽²⁸⁾ Thus, for example, a psychologist relying on a personality profile test must not only demonstrate that this test is generally accepted as evidence regarding the issue at hand, but also must show that he or she is interpreting the results of the test in a generally-accepted way.

Some critics of applying Frye to non-scientific P & P evidence would argue that the test is too forgiving in this context. They would contend that many methodologies used by mental health professionals are generally accepted within the profession, but have rather low reliability. Frye skeptics would argue the legal system should strive for objective truth, particularly in criminal cases, where both individual rights and the safety of society are at stake. They might urge that in place of Frye, courts should extract from Daubert the requirement that a particular P & P methodology has a known and low error rate for a particular diagnostic purpose before allowing a mental health expert to rely on the methodology for that purpose.⁽²⁹⁾ If, for example, professionals using the same technique to diagnose a group of patients cannot generally agree on their diagnoses, then the technique itself is flawed, and should be banned from the courtroom.

The counter-argument in favor of a general acceptance test is that Frye's purpose is to ensure that the legal system is using the best available expert evidence. One cannot expect the legal system to achieve a greater understanding of mental health phenomena than experts in the field have. Courts must do the best they can with contemporary knowledge, just as private parties do outside the legal system.

While some would argue that the Frye test is not sufficiently demanding for non-scientific expert testimony, Professor Imwinkelried argues that it is too demanding. Imwinkelried would allow an expert to present non-scientific testimony if the expert has sufficient experience regarding the subject at issue.⁽³⁰⁾ Imwinkelried's standard for the admissibility of non-scientific evidence would therefore be far less demanding than the current rules for the admissibility of scientific evidence. In fact, his proposed rule for admitting non-scientific evidence is essentially the same as the traditional, liberal rules that governed the admissibility of scientific evidence in many common-law jurisdictions before the modern trend toward strict scrutiny.

Conclusion

To conclude, the appropriate standards for the admissibility of P & P evidence is an important but neglected issue, and one that is likely to attract increased attention as the debate over Daubert and Frye grows louder. This author will not hazard a guess regarding how the rules governing the admissibility of P & P evidence will ultimately develop. Few if any courts have approached the issue in a systematic way, instead preferring to deal with proffered P & P evidence on an ad hoc, case-by-case basis. Legal scholars, meanwhile, have often treated P & P evidence as if it were a separate species of evidence, having little if any relation to the ongoing debates over the admissibility of scientific and non-scientific expert evidence.

But simply because courts and commentators have punted on the issue does not mean that appropriate solutions cannot be found. As this article shows, an approach to the admissibility of P & P evidence can be systematized, although further work on the issue is undoubtedly needed. Moreover, P & P evidence does not need its own special rules; general rules governing scientific and non-scientific evidence can be applied.

Depending on the rules selected, however, much of the P & P evidence that has traditionally been admitted in trials could henceforth be suspect. One wonders whether forensic psychiatrists and psychologists, and the legal system in general, are prepared for such a drastic result.

1. Daubert v Merrell Dow Pharmaceuticals (1993) 113 S Ct 2786; Frye v United States (1923) 293 F 1013.

2. For example, Runjanjic v R (1991) 53 A Crim R 362.

3. For example, R v Mohan (1994) 89 CCC3d 402, 29 CR4th 239.

4. For example, David L. Faigman, "To Have and Have Not: Assessing the Value of Social Science to the Law As Science and Policy" (1989) 38 Emory LJ 1005; Samuel R. Gross, "Expert Evidence" (1991) Wisconsin Law Review 1113.

5. Charles Bleil, "Evidence of Syndromes: No Need for a 'Better Mousetrap'" (1990) 32 South Texas Law Review 37, at 67-69; Cathleen C Herasimchuk, A Practical Guide to the Admissibility of Novel Expert Evidence in Criminal Trials Under Federal Rule 702 (1990) 22 St Mary's LJ 181 at 205, 216-17. David McCord, "Syndromes, Profiles & Other Mental Exotica: A New Approach to the Admissibility of Nontraditional Psychological Evidence in Criminal Cases" (1987) 66 Oregon Law Review 19 at 35, 38-41.

6. Edward Imwinkelried, "The Next Step After Daubert: Developing a Similarly Epistemological Approach to Ensuring the Reliability of Nonscientific Expert Testimony" (1994) 15 Cardozo Law Review 2271 at 2291.

7. Similarly, David Faigman disputes the notion that social science should be admissible as "soft science." Instead, he argues that social science evidence should be admissible only to the extent that it has the attributes of science. David L Faigman, "To Have and Have Not: Assessing the Value of Social Science to the Law As Science and Policy" (1989) 38 Emory LJ 1005 at 1009.

8. The author is grateful to Peter Huber of the Manhattan Institute for Policy Research for suggesting this distinction.

9. Malcolm Brown & Paul Wilson, Justice and Nightmares: Successes and Failures of Forensic Science in Australia and New Zealand (1992) p 155.

10. Frye v United States (1993) 293 F 1013.

11. Daubert v Merrell Dow Pharmaceuticals (1993) 113 S Ct 2786.

12. See Ian Freckelton, "Expert Evidence and the Role of the Jury" (1994) 12 Australian Bar Review 73.

13. See David Paciocco, "Evaluating Expert Opinion Evidence for the Purpose of Determining Admissibility: Lessons from the Law of Evidence," (1994) 27 CR(4th) 302.

14. 293 F at 1014.

15. 113 S Ct at 2797.

16. McCord, op cit, n _, ante at 83.

17. American Psychiatric Association, Diagnostic and Statistical Manual on Mental Disorders (1980).

18. See Bert Black, "A Unified Theory of Scientific Evidence" (1988) 56 Fordham Law Review 595.

19. (1991) 53 A Crim R 362.

20. 56 SASR at 119; 53 A Crim R at 366.

21. For a recent summary of criticisms of the theory behind battered woman syndrome, see Anne M. Coughlin, "Excusing Women" (1994) 82 California Law Review 1.

22. Ronald J. Allen, "Expertise and the Daubert Decision" (1994) 84 Journal of Criminal Law and Criminology 1157 at 1172; Ralph Underwager & Hollida Wakefieldm "A Paradigm Shift for Expert Witnesses" (1993) 5 Issues In Child Abuse Accusations 156 at 159. See Adolf Grunbaum, The Foundations of Psychoanalysis: A Philosophical Critique (1984).

23. Michael H. Gottesman, "The Randolph W. Thrower Symposium: Scientific and Technological Evidence: Admissibility of Expert Testimony after Daubert: The 'Prestige' Factor: (1994) 43 Emory LJ 867. Ronald Allen makes a similar argument. Allen, op cit, n _, ante at 1172.

24. Paul Isely v Capuchin Province, 1995 WESTLAW 104310 (D. Mich.) (admitting evidence regarding post-traumatic stress disorder and repressed memories); Gier v Educational Service Unit No. 16 (1994) 845 FSupp 1342 (excluding evidence regarding suspected child abuse); State v Alberico (1994) 861 P2d 192 (1993) (partial exclusion of evidence of post-traumatic stress disorder); State v Foret (1993) 628 So2d 1116 (1993).

25. American College of Trial Lawyers, Standards and Procedures for Determining the Admissibility of Expert Evidence After Daubert (April 15, 1994) p 9.

26. Ibid.

27. People v Kelly (1976) 549 P2d 1240.

28. People v Castro (1989) 545 NYS2d 985 at 988.

29. Compare Lisa R Askowitz & Michael H Graham, "The Reliability of Expert Psychological Testimony in Child Sexual Abuse Prosecutions" (1994) 15 Cardozo Law Review 2027 at 2098. The authors suggest that courts should not permit experts to use statement validation techniques as the basis for an opinion regarding child sexual abuse until the psychological community has established the reliability of these techniques.

30. Imwinkelried, op cit, n _, ante at 2290-2293. A U.S. trial court recently applied a similar test to a diagnosis of post-traumatic stress disorder. Paul Isely v Capuchin Province, 1995 WESTLAW 104310 (D. Mich.).