Photo courtesy of Zuma Press.
Confusion. That's the best word to describe the state of understanding of the concept of "probable cause," which is often abbreviated as "PC." Not only is there widespread misunderstanding as to the meaning of the term, there's also a deeply entrenched practice among many law enforcement officers, lawyers, and judges of wrongly applying the probable-cause standard where it doesn't belong. Let's see if we can reduce the confusion.
"Probable Cause" Defined
The Fourth Amendment protects a right to be free from unreasonable searches and seizures, and it specifies that "no warrants shall issue, but upon probable cause." This makes it a constitutional requirement that search warrants and arrest warrants be based on probable cause. However, the Constitution does not define "probable cause" or give any examples of what does or does not constitute PC.
That task, as well as the job of figuring out when to apply the same standard to warrantless searches and seizures, was left for the courts to perform. Therefore, our body of law explaining and applying the concept is found in U.S. Supreme Court decisions, and sometimes in lower court decisions applying Supreme Court rulings.
Over the years, the Supreme Court has tried to describe the level of suspicion that would amount to PC, but it has always done so in general wording that leaves it up to courts to apply, case by case. In a 1949 opinion, the court said this about probable cause:
"The rule of probable cause is a practical, nontechnical conception. In dealing with probable cause, we deal with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act. Probable cause exists where the facts and circumstances within the officers' knowledge and of which they have reasonably trustworthy information are sufficient in themselves to warrant a man of reasonable caution in the belief that an offense has been or is being committed." (Brinegar v. U.S.)
The essence of this discussion is that it is not possible for judges and lawyers ("legal technicians") to assign some mathematical probability value for PC. Instead, the "technicians" must try to measure the facts of each case from a practical viewpoint and assess whether those facts would justify a reasonable officer in believing criminal activity was afoot.
Because some lower courts persisted in an effort to create a legal checklist of complicated "prongs" or "factors" to be evaluated, the Supreme Court made another effort to define PC in a 1983 case, saying the following:
"Perhaps the central teaching of our decisions bearing on the probable cause standard is that it is a practical, nontechnical conception. The process does not deal with hard certainties, but with probabilities. The evidence must be seen and weighed not in terms of library analysis by scholars, but as understood by those versed in the field of law enforcement. Probable cause is a fluid concept, turning on the assessment of probabilities in particular factual contexts-not readily, or even usefully, reduced to a neat set of legal rules." (Illinois v. Gates)
Translation: lower courts should stop using a law school approach to analyzing PC issues and simply consider how an officer in the field would understand the situation he or she confronted.
In a later decision, the court admitted that it had been unable to give a precise definition of probable cause but gave this guidance for lower courts engaged in assessments of probable-cause issues:
"The probable-cause standard is incapable of precise definition or quantification into percentages because it deals with probabilities and depends on the totality of the circumstances. The substance of all the definitions of probable cause is a reasonable ground for belief of guilt, and that the belief of guilt must be particularized with respect to the person to be searched or seized." (Maryland v. Pringle)
Translation: PC is a substantial suspicion, based on all the circumstances, that someone is a criminal.
"Probable Cause" Illustrated
Perhaps the best way to understand probable cause is by comparing it with other levels of suspicion or proof that are more easily understood.
PC is much less than proof "beyond a reasonable doubt," which is the much higher standard the prosecutor must meet in order to convict a defendant.
PC is much less than "clear and convincing" proof, which is the higher standard that must be met as to certain legal rulings, such as whether jury challenges were improperly based on group bias.
PC is less than a "preponderance of the evidence" (meaning 50 percent-plus), which is the plaintiff's burden of proof to win a civil case.
But PC is something more than the "reasonable suspicion" required to justify a temporary investigative detention.