Putting Probability Back into Probable Cause

Abstract

When deciding whether baseball players are likely to get a hit, we look at their history of success at the plate. When deciding if a stock price is likely to rise or fall, we look at its past performance. But when police officers claim that they have probable cause to believe a certain location contains evidence, we do not look at whether they have been right or wrong when they have made the same claim in the past. Law enforcement search success rates vary widely, even when the same legal standard applies. Searches pursuant to warrants issued on a probable cause standard recover evidence at very high rates, usually exceeding 80%. By contrast, warrantless searches, even when officers allege they have probable cause, succeed at far lower rates, recovering evidence as infrequently as 12% of the time. Similarly, some officers are far better than others when they conduct probable cause searches. Some almost never succeed; some almost always find evidence.

What role should these differential success rates play in the probable cause analysis? The current answer is none. Judges are not presented with the success rates of the law enforcement officers who appear before them. I argue that this is a mistake. Law enforcement should be forced to present success rate data to judges when making probable cause claims and judges should be allowed to consider the data when deciding whether to issue a warrant or whether to approve a previously conducted search. These success rates capture information not currently analyzed in the search process and their addition would improve the accuracy of search decisions. Most significantly, we would learn private information in the possession of law enforcement not currently presented to judges.

Keywords: probable cause, warrants, statistical evidence, checkpoints, police perjury