7.3: Identifying Probable Cause

Although there is certainly more to probable cause and reasonable suspicion than just principles, it’s a good place to start. It is ordinarily a bad idea to begin a module by admitting that the subjects to be discussed cannot be usefully defined. But when the subjects are probable cause and reasonable suspicion, and when the readership is composed of people who have had some experience with them, it would be pointless to deny it. Consider that the Seventh Circuit once tried to provide a good legal definition but concluded that, when all is said and done, it just means having “a good reason to act.” Even the Supreme Court— whose many powers include defining legal terms— decided to pass on the probable cause because, said the Court, it is “not a finely-tuned standard” and is actually an “elusive” and “somewhat abstract” concept. As for reasonable suspicion, the uncertainty is even worse. For instance, in United States v. Jones, the First Circuit would only say that it “requires more than a naked hunch.”

But this imprecision is actually a good thing because probable cause and reasonable suspicion are ultimately judgments based on common sense, not technical analysis. Granted, they are important judgments because they have serious repercussions. But they are fundamentally just rational assessments of the convincing force of information, which is something the human brain does all the time without consulting a rule book. So instead of being governed by a “neat set of rules,” these concepts mainly require that officers understand certain principles— principles that usually enable them to make these determinations with a fair degree of consistency and accuracy.

First, however, it is necessary to explain the basic difference between probable cause and reasonable suspicion, as these terms will be used throughout this module. Both are essentially judgments as to the existence and importance of evidence. But they differ as to the level of proof that is required. In particular, probable cause requires evidence of higher quality and quantity than reasonable suspicion because it permits officers to take actions that are more intrusive, such as arresting people and searching for things. In contrast, reasonable suspicion is the standard for lesser intrusions, such as detentions and pat searches. As the Supreme Court explained:

Reasonable suspicion is a less demanding standard than probable cause not only in the sense that reasonable suspicion can be established with information that is different in quality or content than that required to establish probable cause but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause.

What Probability is Required?

When people start to learn about probable cause or reasonable suspicion, they usually want a number: What probability percentage is required? Is it 80%? 60%? 50%? Lower than 50? No one really knows, which might seem strange because, even in a relatively trivial venture such as sports betting, people would not participate unless they had some idea of the odds.

Nevertheless, the Supreme Court has refused to assign a probability percentage to these concepts because it views them as nontechnical standards based on common sense, not mathematical precision. “The probable cause standard,” said the Court, “is incapable of precise definition or quantification into percentages because it deals with probabilities and depends on the totality of circumstances.” Similarly, the Tenth Circuit observed, “Besides the difficulty of agreeing on a single number, such an enterprise would, among other things, risk diminishing the role of judgment based on situation-sense.” Still, based on inklings from the United States Supreme Court, it is possible to provide at least a ballpark probability percentage for probable cause. Reasonable suspicion, on the other hand, remains an enigma.

Pin It! Privacy Probable Cause vs. Reasonable Suspicion

Learn more about the difference between probably cause and reasonable suspicion in this video: probable cause vs. reasonable suspicion.

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Probable Cause

Many people assume that probable cause requires at least a 51% probability because anything less would not be “probable.” While this is technically true, the Supreme Court has ruled that, in the context of probable cause, the word “probable” has a somewhat different meaning. Specifically, it has been said that probable cause requires neither a preponderance of the evidence nor “any showing that such belief is correct or more likely true than false,” and that it requires only a “fair” probability, not a statistical probability. Thus, it is apparent that probable cause requires something less than a 50% chance. How much less? Although no court has tried to figure it out, we suspect it is not much lower than 50%.

Reasonable Suspicion

As noted, the required probability percentage for reasonable suspicion is a mystery. Although the Supreme Court has said that it requires “considerably less [proof] than a preponderance of the evidence” (which means “considerably less” than a 50.1% chance), this is unhelpful because a meager 1% chance is “considerably less” than 51.1% but no one seriously thinks that would be enough. Equally unhelpful is the Supreme Court’s observation that, while probable cause requires a “fair probability,” reasonable suspicion requires only a “moderate” probability. What is the difference between a “moderate” and “fair” probability? Again, nobody knows. What we do know is that the facts need not rise to the level that they “rule out the possibility of innocent conduct.” As the Court of Appeal explained, “The possibility of an innocent explanation does not deprive the officer of the capacity to entertain a reasonable suspicion of criminal conduct. Indeed, the principal function of his investigation is to resolve that very ambiguity.” We also know that reasonable suspicion may exist if the circumstances were merely indicative of criminal activity. In fact, the California Supreme Court has said that if the circumstances are consistent with criminal activity, they ‘demand’ an investigation.”