Probable Cause

Probable cause is the standard of proof required, in most instances, for a valid search, seizure, or arrest. The text of the Fourth Amendment of the Constitution prohibits the issuance of warrants except upon probable cause. The Supreme Court has extended the requirement of probable cause to warrantless searches, seizures, and arrests. There are some exceptions to the probable cause requirement. In some instances, a reduced level of proof will justify a brief detention of a person or property. Some searches and seizures require no suspicion at all, although the Supreme Court has limited such suspicionless searches to situations involving special need.

In Brinegar v. United States, 338 U.S. 160 (1949), the Supreme Court explained the competing policy considerations involved with the probable cause requirement. Like the warrant requirement, it serves as a check on the power of the government to intrude on the privacy of the people. The American colonial experience with general warrants and writs of assistance— which could be issued without a showing of cause and without naming the thing or person to be searched or seized—in effect allowed the British government to search and seize any person or piece of property. The Fourth Amendment codified the intention of the Framers to end these practices and to permit intrusions into people’s lives and liberties only upon a showing of more than mere suspicion. However, probable cause has emerged as a flexible standard that gives police the ability to investigate crime, arrest criminals, and seize evidence without being required to meet an overly burdensome standard. Probable cause is therefore a compromise. Astricter standard of proof would leave police powerless to investigate crime and arrest criminals. A lesser standard would give police nearly limitless authority to intrude on the private affairs of law-abiding citizens.

The Supreme Court has tried to avoid defining probable cause in a technical or legalistic fashion. An assessment of probable cause must sometimes be made in a split second. Accordingly, probable cause requires the officer or court to look at probabilities, not conclusive proof. Its substance is reasonableness, judged on the basis of the facts of each individual encounter and case. While probable cause requires more than mere suspicion, it does not require the same amount or quality of evidence as would be needed at trial. In hindsight, an officer or magistrate may have been incorrect in his or her belief as to the existence of an offense. However, probable cause is a standard that permits mistakes to be made, provided they are reasonable. It is a practical and commonsense standard that is flexible and does not require formal modes of proof.

To arrest a person, a police officer must have probable cause that a crime has been committed and that the arrestee is the person who committed it. An officer’s subjective belief, hunch, or suspicion of wrongdoing is not sufficient. Rather, there must be objective evidence that would lead a reasonably prudent person to believe that the arrestee has committed a crime. If the officer does not have a warrant at the time of the arrest, Gerstein v. Pugh (420 U.S. 103, 1975) requires a judicial determination of probable cause if the arrestee will continue to be detained pretrial or if he or she is released with burdensome conditions.

A search or seizure of property likewise requires probable cause in most instances. The proof relates to the property, its connection to an alleged crime, and its present location. There must be evidence that would lead a reasonable person to believe that a crime has been committed and that the property sought is evidence of that crime. The police cannot violate a person’s privacy in order to engage in a suspicionless ‘‘fishing expedition.’’ Nor can they use an after-the-fact discovery of incriminating evidence or contraband to justify their initial, suspicionless intrusion. Furthermore, there must be proof that would lead a reasonable person to conclude that the property is where the officer believes it to be. The timeliness of the information used to support probable cause is therefore critical.

The fruits of an arrest or search made without probable cause will, in most instances, be subject to suppression under the exclusionary rule. A defendant challenging the probable cause for an arrest, search, or seizure will ordinarily file a motion to suppress. The court’s duty in hearing the motion is to assess the quality and quantity of evidence known to the officer at the time of the search or arrest or presented to the magistrate at the time of the issuance of the warrant.

One of the Supreme Court’s most recent pronouncements about probable cause occurred in 1983 when it decided Illinois v. Gates (462 U.S. 213, 1983). In Gates, a magistrate issued a warrant for the search of the defendants’ home and car based on an anonymous letter alleging that the defendants were engaged in drug trafficking. The police corroborated major portions of the letter, including predictions the writer had made about the defendants’ upcoming activities. The Court held that sufficient probable cause existed based on the ‘‘totality of the circumstances.’’ It noted that probable cause is a ‘‘fluid concept,’’ depends on the facts of a particular case, does not readily lend itself to rigid rules, and requires nontechnical, factual, and practical considerations. The Court rejected the so-called Aguilar–Spinelli test, which had required an issuing magistrate to assess the informant’s veracity or reliability, as well as his or her basis of knowledge. Instead, the Court stated that these considerations are mere factors that should be considered in an analysis of the totality of the circumstances. The Court thus reaffirmed the principle that probable cause is a flexible standard.

The value of probable cause as a check on governmental oppression necessarily depends on the definition courts ascribe to it. Courts’ adherence to a flexible standard based on probabilities—in deference to the societal value of enabling police to fight crime—means that there is a risk that innocent people and their property may be searched and detained.


References and Further Reading

  • Black’s Law Dictionary, 8th ed., s.v. ‘‘probable cause.’’
  • LaFave, Wayne R. ‘‘Probable Cause.’’ In Search and Seizure: A Treatise on the Fourth Amendment, 4th ed., vol. 2, St. Paul, MN: Thomson/West, 2004, Ch. 3.

Cases and Statutes Cited

  • Aguilar v. Texas, 378 U.S. 108 (1964)
  • Brinegar v. United States, 338 U.S. 160 (1949)
  • Gerstein v. Pugh, 420 U.S. 103 (1975)
  • Illinois v. Gates, 462 U.S. 213 (1983)
  • Spinelli v. United States, 393 U.S. 410 (1969)

See also Administrative Searches and Seizures; Airport Searches; Arrest; Arrest Warrants; Automobile Searches; Exclusionary Rule; General Warrants; Gerstein v. Pugh, 420 U.S. 103 (1975); Illinois v. Gates, 462 U.S. 213 (1983); Proof beyond a Reasonable Doubt; Search (General Definition); Search Warrants; Seizures; Warrant Clause (IV); Warrantless Searches; Writs of Assistance Act


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