Probable Cause

The Fourth Amendment specifies that warrants should not issue except “upon probable cause,” but it does not define the term probable cause, nor does it specify whether all warrantless searches are necessarily “unreasonable.”

In determining whether police had established probable cause to search a car and remove a panel over a rear passenger armrest in Ornelas v. United States (1996), the U.S. Supreme Court observed that neither the term reasonable suspicion nor the term probable cause were subject to precise definition, but observed that the former was “a particularized and objective basis for suspecting the person stopped of criminal activity,” whereas the latter occurred “where the known facts and circumstances are sufficient to warrant a man of reasonable prudence in the belief that contraband or evidence of a crime will be found.” After indicating in Maryland v. Pringle (2003)—a case involving the arrest of occupants of a vehicle where police found drugs—that “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 U.S. Supreme Court went on to say that “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.”

Professor Craig S. Lerner has argued that while there is a perception that probable cause is required for searches, most take place without it. He concludes that the constitutionality of most searches depends less on probable cause than on “the reasonableness of the search, factoring in the degree of the intrusion and the gravity of the investigated offense” (2003, 956).

Lerner further believes that early U.S. law and cases—epitomized by such forfeiture cases as Locke v. United States (1813) and The Appolon (1924)—tended to equate probable cause with “reasonable cause” or “reasonable belief.” He sees a similar pattern in such malicious prosecution cases as Bacon v. Towne (Mass., 1849). Lerner believes that the Court's pathbreaking decision in Boyd v. United States (1886) resulted in somewhat greater scrutiny. Lerner traces the current distinction between probable cause and reasonable suspicion to the Court's decision in Terry v. Ohio (1968), which allowed for warrantless stop and frisk searches in cases where police reasonably suspected individuals of being armed or about to commit crimes. He notes, however, that the government allows for a lesser level of proof when conducting administrative searches versus those collecting evidence for criminal prosecution. He also notes the rise of “special needs exceptions” and “exigent circumstances,” for which courts also accept lesser levels of proof. Ultimately, Lerner concludes that such exceptions are consistent with the Framers' intent to leave “a standard [probable cause] that itself could adapt to changed circumstances” (2003, 1029).