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Probable Cause, Probability, and Hindsight
Jeffrey J. Rachlinski
Chris Guthrie
Andrew J. Wistrich

When judges assess probable cause, they must do so either in foresight (when determining whether to issue a warrant) or in hindsight (when determining whether to allow the admission of evidence obtained without a search warrant). Although the legal standard for probable cause is the same, and the facts that might support cause are the same, judges who assess probable cause in hindsight invariably know whether a search produced incriminating evidence or not. Research on the hindsight bias suggests that judges will be unable to set aside this knowledge and judge probable cause as if they were working in foresight. In this paper, we present of three experiments in which we asked 900 state and federal judges to make judgments of probable cause either in foresight or in hindsight, in hypothetical cases. Surprisingly, we found that that judges make similar rulings on probable cause in foresight and in hindsight. We also found that hindsight appears to cloud judges’ abilities to assess the likely outcome of the search, but hindsight does not influence their legal judgments.

The Fourth Amendment’s prohibition against unreasonable searches and seizures creates a decision-making dilemma for judges.1 It requires that judges supervise police investigations. The centerpiece of the Fourth Amendment’s protection of personal privacy lies in the requirement that is that the police must get permission to conduct such searches by obtaining a warrant from a judge.2 But in the common circumstance in which an exigency prevents a police officer from obtaining a warrant, the judge must assess probable while knowing that the search was successful.3 Decades of research on judgment in hindsight suggest that such judgments will be biased, as people cannot suppress the influence of known outcomes on their judgments— a phenomenon known as


The Fourth Amendment states, “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

Chimel v California, 395 U.S. 752, 762 (1969) (“The police must, whenever practical, obtain advance judicial approval of searches and seizures through the warrant procedure”); Katz v. United States, 389 U.S. 347, 357 (1967) (“searches conducted outside the judicial process, without prior approval by judge or magistrate, are per se unreasonable under the Fourth Amendment ”). See William J. Stuntz, Warrants and Fourth Amendment Remedies, 77 VA. L. REV. 881, 882 (1991).

See Craig M. Bradley, Two Models of the Fourth Amendment Remedies, 83 MICH. L. REV. 1468, 1473 (1985).

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the hindsight bias.4 In this paper, we present three experiments in which we asked groups of state and federal judges to make judgments of probable cause either in foresight or in hindsight, in hypothetical cases. Surprisingly, we found when making probable cause determinations, judges do not seem to be influenced by the hindsight bias. Our result is surprising. The nature of probable cause poses a serious cognitive challenge for judges in implementing their role as the guardians of the Fourth Amendment. The cornerstone of reasonableness in searches is the concept of “probable cause.”5 Commonly, a judge will assess probable cause before the search has been conducted because the police normally have to obtain a warrant before conducting a search. But with numerous exceptions to the warrant requirement that arise from the inevitable exigencies of law enforcement, the police conduct a search without obtaining a warrant.6 When faced with such an exigency, the police may avoid the...
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