Amicus Brief - Strict Scientific Method in Criminal Cases


     There is a tremendous amount of scientific study regarding the ability of psychologists and psychiatrists to predict the likelihood that a specific individual will commit future acts of violence. Most of the early studies that were done failed to find any reliable way that such predictions could be made.  Indeed, an early review of this "first generation" research found that when mental health professionals assess risk for future danger, they will be accurate only one out of three times when they predict that an individual will be violence in the future.  Based largely upon early studies of this kind, the American Psychiatric Association concluded in an amicus brief filed in 1982 that "[p]sychiatrists should not be permitted to offer a prediction concerning the long-term future dangerousness of a defendant in a capital case, at least in those circumstances where the psychiatrist purports to be testifying as a medical expert possessing predictive expertise in this area."

     One important reason that early studies were unable to establish a reliable basis for predictions of "future dangerousness" was that the predictions being reviewed generally had not been developed using scientific methods, but rather had been based on largely unstructured subjective judgments. The subsequent application of more rigorous scientific methods yielded modestly more accurate predictions of violence risk potential
in certain settings, as confirmed by later studies.  Several features of these more rigorous scientific methods that are important to the reliability of any prediction of violence risk potential are noteworthy. First, consideration of the "base rate" of violence is critical. Research has established that error rates for predictions of future violence vary significantly depending on the -base rate," or the known frequency that a particular behavior will occur within a specified population over a set period of time.

     The "base rate" of violent behavior in a given group represents the single most important piece of information in any risk assessment, because it indirectly affects the accuracy of predictions of future dangerousness. Studies have shown that if the "base rate" is low (in other words, if the overall occurrence of the event in question is very infrequent), it is particularly difficult to predict a specific occurrence of the event with reliability, and there is a tendency to over-predict."

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