Expert Psychological Testimony

Expert psychological testimony comes in many types and concerns a vast array of subjects. Psychological expertise ranges widely both in scientific subject areas and the breadth of the legal landscape covered. Indeed, there are few, if any, legal contexts in which expert testimony on psychology does not sometimes have an impact. This is not surprising, because law shares with psychology an abiding interest in human behavior. Because of the large number of areas in which psychology and law intersect, any summary will be somewhat incomplete. This research paper, therefore, is intended to illustrate the range of expertise and the legal contexts in which it is put to use. It first reviews expert testimony according to its subject, with sections on testimony concerning past mental states, past behavior, future behavior, and current mental states. The entry concludes with a discussion of the probative value of other evidence. Many of these categories of evidence appear in both civil and criminal cases, and the basic admissibility standards in these two legal domains are the same. Hence, for example, predictions of future violence might be used in civil cases (e.g., civil liability for failing to predict violence), civil cases that are quasi-criminal (e.g., sexually violent predator commitments), and criminal cases (e.g., capital sentencing). Also, many subjects of expert psychological testimony are used by both prosecutors and criminal defendants (e.g., the battered-woman syndrome [BWS]) and by plaintiffs and civil defendants (e.g., polygraphs).

Expert Psychological Testimony on Past Mental States

In the popular imagination, the principal use of psychological expertise occurs in the context of discerning past mental states in criminal cases. The central legal context in which this subject arises is insanity. Insanity in the law is a construct that relates to responsibility or what might be termed moral culpability. The law presumes that behavior is freely willed and the product of a rational mind. A person might be excused under the law if these presuppositions are demonstrated not to be so in a particular case. Most jurisdictions employ an insanity defense based on the 19th-century case of Daniel M’Naghten, who attempted to assassinate Sir Robert Peel, the British Prime Minister, but shot and killed Peel’s assistant, Edward Drummond, by mistake. Under the test, a defendant should be acquitted if he “was under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know he was doing what was wrong.”

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As a practical matter, the subject of past mental states is complicated by the very different vocabularies that lawyers and scientists bring to the subject. Lawyers speak in terms of insanity and diminished capacity, whereas psychologists employ an expansive vocabulary designed to account for the wide variation in behaviors observed. The law, therefore, presumes, and has constructed, a world in which mental capacity to reason exists largely in two-dimensional space: A person was sane or insane when he or she committed a particular act. Psychology, in contrast, presumes, and has constructed, a world in which mental capacity to reason varies widely in multidimensional space: A person might suffer from a disability with multiple etiologies and with varying effects on his or her capacity to reason.

Although insanity occupies much of the scholarly attention regarding past mental states, much of the syndrome literature similarly involves the effort to explain preexisting thought processes. For example, the BWS is used in many jurisdictions to demonstrate that battering victims did not behave unreasonably when they used deadly force against their batterers. Part of the factual inquiry for triers of fact in these cases, as defined by substantive law, is whether the battered woman believed that she was in imminent danger of harm at the time that she killed the batterer. Since many of these cases involve circumstances in which the defendant acted at a time when she did not confront an immediate objective harm—if, say, the killing occurred when the victim was sleeping—the psychological proof is offered to support her claim that she was reasonable in believing that harm was nonetheless imminent. According to BWS advocates, this inference follows from available research in two possible ways. First, as a general matter, the data suggest that prolonged abuse renders battered women constantly fearful, a psychological outcome that is a natural consequence of the violence. Second, advocates argue that specific clinical observations can support the individual defendant’s claim that she was in constant fear and, thus, honestly and reasonably believed that harm was imminent when she killed.

A fundamental challenge for psychologists regarding past mental state concerns the inherent difficulty in assessing a phenomenon that cannot be observed even indirectly. In effect, when the law asks psychologists to assess past mental states, it puts them in the role of forensic investigator. Little research is available to suggest that psychologists can fulfill this role in a reliable fashion. Nonetheless, some scholars, most notably Christopher Slobogin, argue that the inherent difficulty of the task should lead courts to relax the usual rules of admissibility. According to this view, psychologists can still “assist the trier of fact” regarding past mental states, even if the phenomenon defies direct observation or straightforward test.

Expert Psychological Testimony on Past Behavior

Possibly the most controversial use of psychological expertise is behavioral profile evidence or psychological expertise that is offered as proof that a person committed some act, typically one that he or she is charged with a crime for having committed. Most evidence codes proscribe the use of past bad acts—referred to as “character evidence”—and thus ostensibly prohibit behavioral profiling for the purpose of proving that the defendant probably committed the alleged crime because he or she has a propensity to commit such crimes. Nonetheless, courts still often admit such evidence in one form or another. The most egregious examples of this practice involve courts’ admission of evidence such as rapist profiles to prove the substantive offense.

More common, however, is court allowance of evidence that serves dual purposes: one permissible and the other not. For example, there is a growing use of BWS by states to prosecute alleged abusers. BWS would not ordinarily be allowed simply to support the inference that because the defendant abused the witness in the past, he is probably guilty of the assault for which he is on trial. This is prohibited character evidence. However, in many cases, women who were abused and filed a police complaint subsequently testify at trial that the defendant was not the source of her injuries. Prosecutors have successfully introduced BWS for the purpose of impeaching the witness’s testimony and thus explaining why she changed her story. But evidence of past battering, which is symptomatic of BWS, is likely to be used by the trier of fact substantively—that is, for the prohibited purpose of proving that the defendant assaulted the witness on the occasion in question.

Expert Psychological Testimony on Future Behavior

The subject of predicting future behavior raises a host of issues involving both the reliability of the claimed expertise and the scope of the substantive and procedural rules that apply to that expertise. The most usual prediction involves a person’s likelihood of behaving violently. Courts call for expert predictions of future violence in a wide assortment of legal contexts, including ordinary civil commitment hearings, capital sentencing hearings, commitment hearings following a verdict of not guilty by reason of insanity, commitment hearings following a determination of incompetency to stand trial, parole and probation hearings, and hearings under community notification laws for “sexual predators.” Yet courts regularly remark that predicting future behavior is inherently difficult, and most research indicates that psychiatrists and psychologists do not do it very well. Indeed, this area of the law presents a paradox in which judges seemingly take the most lenient approach toward scientific evidence involving some of the most controversial science to enter the courtroom.

As a general procedural matter, courts ordinarily do not apply evidentiary rules of admissibility to predictions of violence. In many areas, such as capital sentencing or probation hearings, rules of evidence do not apply. In other areas, such as commitment hearings or community notification determinations, evidence rules ostensibly apply, but courts proceed, either implicitly or explicitly, on the basis that the substantive law requires psychological expert testimony. Hence, the expertise is admissible not because it is deemed relevant and reliable but because it is deemed necessary under the substantive law that applies to the case.

Although evidence rules might not apply to predictions of violence, constitutional safeguards do. The Supreme Court, however, rejected a constitutional challenge to predictions of violence in Barefoot v. Estelle (1983), the only case it has heard on the subject. The Barefoot Court rejected the defendant’s contention, backed by an amicus brief submitted by the American Psychiatric Association (APA), that predictions of future violence were unreliable. The Court argued that “neither petitioner nor the [American Psychiatric] Association suggests that psychiatrists are always wrong with respect to future dangerousness, only most of the time.”

Despite the APA’s statement that psychiatrists cannot distinguish accurate from inaccurate predictions, the Court believed juries could do so. “We are unconvinced . . . that the adversary process cannot be trusted to sort out the reliable from the unreliable evidence and opinion about future dangerousness, particularly when the convicted felon has the opportunity to present his own side of the case.” Yet as Justice Blackmun pointed out in dissent, this observation “misses the point completely,” for “one can only wonder how juries are to separate valid from invalid expert opinion when the ‘experts’ themselves are so obviously unable to do so.”

Expert Psychological Testimony on Current Mental State

A seemingly less daunting task for psychologists than describing past mental states, characterizing past behavior, or predicting future behavior involves assessing current mental states. The law seeks such mental assessments in a variety of contexts, including the competency of defendants to be tried and the competency of those convicted of capital offenses to be executed. In the former category, the issue involves whether a defendant is able to assist in his or her own defense and comprehends the nature of the proceedings and the charges against him or her. In the latter category, the issue involves whether the condemned person comprehends the State’s reasons for executing him or her and understands what is about to occur.

Competency assessments are typically decided by judges as a matter of law. Experts usually rely on a mixture of clinical judgment and standardized tests, which may range widely in terms of reliability and construct validity. Indeed, this area has not been the subject of close or critical review by the courts, and there appear to be few guidelines for courts to ensure the receipt of relevant and valid scientific opinion. In general, evidence rules do not apply to this subject and courts do not employ evidentiary standards of reliability to expert opinion regarding current mental states. Indeed, this task, perhaps more than any other, tends to be handled by court-appointed experts.

Commentary on the Probative Value of Other Evidence

A large segment of psychological expertise is devoted to the subject of how people ordinarily respond, mentally and behaviorally, to different sets of circumstances. For example, research on domestic violence indicates that many victims of such abuse fail to leave battering relationships for a variety of psychological and sociological reasons. Similarly, research on victims of rape and sexual assault indicates that many of them do not report the crime immediately, again for a variety of reasons. In the law, this kind of research might be relevant on a couple of related issues. Specifically, it is sometimes offered to buttress the credibility of a witness. For instance, this research paper might suggest that an alleged victim of sexual assault is not an untruthful witness because he or she failed to report the crime in a timely fashion, since it is not unusual for sexual assault victims to delay reporting. This sort of testimony is also proffered for the more general purpose of giving triers of fact background information regarding the usual circumstances that surround similar situations. For example, triers of fact might be informed in a case involving a defendant who was battered that a large percentage of battered women do not leave abusive situations. Courts’ receptivity to these uses varies.

In most jurisdictions, evidence cannot be introduced for the specific purpose of buttressing the credibility of a witness. Courts consider the use of evidence to support a witness’s credibility to be an invasion of the province of the jury. Juries are entrusted with the task of assessing credibility. At the same time, however, many courts allow expert testimony regarding how people tend to respond to particular situations for the purpose of educating the jury regarding the background context confronted by the testifying witness. Three basic contexts arise with regard to this kind of evidence. In some cases, such as sexual assault cases, psychological evidence is proffered not to show that a witness is truthful but to demonstrate that, in light of how other people respond in similar situations, the witness’ account is not unbelievable. In the second, the expert does not speak to the credibility of a witness but is offered to inform the jury regarding the likely accuracy of a witness’s testimony. In the third group, experts are offered to speak to credibility directly, usually through the use of a test such as a polygraph. These three contexts will be considered in turn.

In many cases, especially including rape cases and sexual assault of children, psychological expertise is proffered to show that the alleged victim’s behavior is not inconsistent with that of others who have experienced similar trauma. Evidence, for example, that an alleged rape victim failed to immediately report the crime might be put into perspective by expert testimony that many sexual assault victims behave similarly. This can be important evidence in a case in which the defendant claims consent because the witness’s behavior might otherwise appear inconsistent with having been assaulted. In this view, evidence on the reporting rates of sexual assault are not offered to support the credibility of the witness but to inform the jury about how people tend to respond to similar situations. As many courts and commentators have observed, however, there is a very fine line—if any line at all—between the permitted purpose of rebutting the defendant’s contention that the victim’s behavior is inconsistent with having been assaulted and the prohibited purpose of supporting the prosecution’s contention that the victim’s behavior is consistent with having been assaulted. It is illogical to permit expert evidence to prove nonconsent but not to prove that a rape occurred (i.e., in most cases, non-consent). But most courts adhere to this distinction.

The second group of cases is not overly controversial as regards invading the trier of fact’s province to decide witness credibility, because it principally concerns the general accuracy of similarly situated witnesses. The lion’s share of this type involves expert opinion on the reliability of eyewitness identification. (Expert testimony on implantation of false memories in repressed-memory cases would be another example.) Courts rarely raise the credibility objection to this evidence for a couple of reasons. First, a principled line can be drawn between credibility and accuracy, and the latter seems less invasive of the jury function. Second, eyewitness experts usually do not testify regarding the accuracy of a particular witness but, instead, only to the general factors that might interfere with eyewitness accuracy. Again, this form of testimony is less invasive. Nonetheless, courts have not been enthusiastic admirers of psychological expertise on the unreliability of eyewitnesses. Objections to this sort of expert testimony typically involve the question whether the information is “beyond the ken” of the average layperson. Under most modern evidence codes, the subject need not be entirely “beyond the ken,” because expert testimony need only “assist the trier of fact” (and be reliable) to be admitted. Nonetheless, much psychological expertise has been excluded on the basis that it simply reflects common sense. Although courts have become somewhat more receptive to expert testimony regarding the unreliability of eyewitness identifications recently, it is still looked on by many courts as only marginally helpful to triers of fact and largely a waste of the court’s time.

In a third group of cases, the specific import of proffered expert testimony is the credibility of a specific witness. Today, this evidence is best represented by testimony regarding the results of polygraph machines. Surprisingly, perhaps, modern courts do not object to this evidence on the ground that it invades the province of the trier of fact. Indeed, courts, including a majority of justices on the U.S. Supreme Court (United States v. Scheffer, 1998), have questioned whether this rationale would be sufficient to exclude a reliable lie detector. Instead, courts have focused on the lack of demonstrated reliability of these tests to support exclusion. Most courts agree that polygraphs are not admissible, at least absent stipulation of the parties prior to administration of the test. But their rationale for exclusion— lack of proven validity—leaves open the possibility that future tests, such as fMRI, might be admitted if validity is adequately demonstrated.


  1. Barefoot v. Estelle, 463 U.S. 880, 887 (1983).
  2. Monahan, J., & Loftus, E. F. (1982). The psychology of law. Annual Review of Psychology, 33, 441—475.
  3. Faigman, D. L. (1989). To have and have not: Assessing the value of social science to the law as science and policy. Emory Law Journal, 38, 1005-1095.
  4. Faigman, D. L, & Monahan, J. (2004). Psychological evidence at the dawn of the law’s scientific age. Annual Review of Psychology, 56, 631-659.
  5. Special Theme: Expert testimony in the courts: The influence of the Daubert, Joiner, and Kumho decisions. Psychology, Public Policy, and Law, 8,
  6. United States v. Scheffer, 523 U.S. 303 (1998).

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