The Limited Diagnosticity of Criminal Trials

A fundamental function of the criminal trial is to determine the facts correctly in order to distinguish between guilty and innocent defendants, and between strong and weak prosecutions. This Article seeks to answer a simple question: How good is the criminal trial at reaching accurate factual conclusions? The Article applies a body of experimental psychology to examine the ability of factfinders to assess the evidence and draw correct inferences from it. The psychological research indicates that the mental processes involved in determining facts in criminal trials are more complex and fickle than generally believed. Part I exposes the difficulties in deciphering the human testimony that is frequently encountered in criminal trials, including eyewitness identification, witness memory for the event, confessions, alibis, and witness demeanor. The task is further hindered by two systemic problems with the integrity of the evidence: false corroboration and the paucity of the investigative record. Part II demonstrates that the inference-making process is susceptible to distortion from the context of legal decisionmaking. Intrusive factors include courtroom persuasion, exposure to impermissible information, emotional arousal, racial prejudice, and the decisionmaker's cognitive process itself. In sum, the accuracy of the criminal trial falls short of the system's high epistemic demands and the certitude it exudes. The findings contribute to our understanding of the causes of mistaken verdicts, particularly wrongful convictions. The Article proposes ways to improve the diagnosticity of the process. INTRODUCTION Few political institutions play as palpable, ubiquitous, and solemn a role in the U.S. public life as the criminal justice system. The task of determining the defendant's criminal liability with a high degree of certitude is performed through the ritualized and highly proceduralized adjudicative process, with the trial at its core. The United States Supreme Court has portrayed the criminal trial as a "decisive and portentous"1 and "paramount" event.2 Trials are considered "the central institution of law as we know it,"3 the "crown jewel" of the legal system.4 Amidst its multiple purposes, an essential objective of the criminal trial is to determine facts: which human events constitute crimes and who perpetrated them. Specifically, the trial is designed to serve the diagnostic function of distinguishing between prosecutions of guilty and innocent people, or at least between compelling prosecutions and those that do not meet the requisite certitude. The prevailing sentiment within the American polity and legal profession is that the trial is indeed acutely diagnostic.5 Naturally, the potential for accurate criminal verdicts depends on the ability of the factfinders - typically juries - to ascertain the facts accurately. The Supreme Court routinely lauds the process's factfinding capabilities. Even for the toughest of factual questions, the Court is "content to rely upon the good sense and judgment of American juries."6 This Article seeks to answer a simple question: How good are factfinders in determining facts? In other words, how well does the criminal trial serve its diagnostic function of distinguishing between factual guilt and innocence? The need to examine this question is increasing with the mounting revelations of false convictions.7 Although the dust has yet to settle, the newfound cognizance of the system's capacity to err is beginning to make some cracks in the prevailing view of the criminal justice process.8 One of the most distinctive features of the criminal justice process is that it is operationalized predominantly through people: witnesses, detectives, prosecutors, suspects, defense attorneys, forensic examiners, judges, and jurors. These actors turn the wheels of the system through their mental operations: perceptions, memories, recognitions, assessments, inferences, judgments, and decisions - all tied in with emotions, affective states, motivations, role perceptions, and institutional commitments. …