Expert Psychological Testimony

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 [Page 268]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.”

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.

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