probable cause

n.
Reasonable grounds for belief that an accused person may be subject to arrest or the issuance of a warrant.

The Fourth Amendment to the U.S. Constitution stipulates that “no [search or arrest] warrants shall issue, but upon probable cause.” The amendment stemmed from colonial objections to the British abuse of general warrants and writs of assistance, legal documents that allowed officers to search and arrest individuals on mere suspicion of smuggling. The framers did not define the meaning of this phrase, although they clearly intended it to prevent the central government from arbitrarily intruding upon personal privacy.

The Fourth Amendment has been among the most heavily litigated of all the Bill of Rights, but the Supreme Court has considered the probable cause requirement in a relatively small number of cases. The Court measures probable cause by the test of reasonableness, a necessarily subjective standard that falls between mere suspicion and certain knowledge. Facts and circumstances leading to an arrest or seizure must be sufficient to persuade a reasonable person that an illegal act has been or is being committed. Always the test involves the consideration of a particular suspicion and a specific set of facts. Hunches or generalized suspicions are not reasonable grounds for concluding that probable cause exists. Judges, not law officers, must determine if probable cause exists, and thus if a warrant should be issued.

In 1968 the Court modified this standard to allow police officers to stop and frisk suspects in order to protect themselves, even without probable cause for arrest (Terry v. Ohio). More recently, the Court has accepted a law officer's experience, a suspect's reputation, and even anonymous tips, when buttressed by other facts, to weigh in the test of reasonableness when determining probable cause.

See also Search Warrant Rules, Exceptions to.

— David J. Bodenhamer

The 4th Amendment to the U.S. Constitution requires that “no [search or arrest] warrants shall issue, but upon probable cause.” Thus, government officials may not obtain a warrant to search or arrest someone unless they have probable cause, or good reasons, to believe that the person may be involved in criminal behavior of some kind. Judges, not law enforcement officers, have the authority to decide if there is probable cause for issuing a warrant to search or arrest someone. In Terry v. Ohio (1968), however, the Supreme Court adjusted this standard to allow police officers to stop and frisk suspects if they consider it necessary to protect themselves, even without probable cause for arrest.

See also Searches and seizures; Terry v. Ohio

This entry contains information applicable to United States law only.

Apparent facts discovered through logical inquiry that would lead a reasonably intelligent and prudent person to believe that an accused person has committed a crime, thereby warranting his or her prosecution, or that a cause of action has accrued, justifying a civil lawsuit.

Probable cause is a level of reasonable belief, based on facts that can be articulated, that is required to sue a person in civil court or to arrest and prosecute a person in criminal court. Before a person can be sued or arrested and prosecuted, the civil plaintiff or police and prosecutor must possess enough facts that would lead a reasonable person to believe that the claim or charge is true.

The probable cause standard is more important in criminal law than it is in civil law because it is used in criminal law as a basis for searching and arresting persons and depriving them of their liberty. Civil cases can deprive a person of property, but they cannot deprive a person of liberty. In civil court a plaintiff must possess probable cause to levy a claim against a defendant. If the plaintiff does not have probable cause for the claim, she may later face a malicious prosecution suit brought by the defendant. Furthermore, lack of probable cause to support a claim means that the plaintiff does not have sufficient evidence to support the claim, and the court will likely dismiss it.

In the criminal arena probable cause is important in two respects. First, police must possess probable cause before they may search a person or a person's property, and they must possess it before they may arrest a person. Second, in most criminal cases the court must find that probable cause exists to believe that the defendant committed the crime before the defendant may be prosecuted.

There are some exceptions to these general rules. Police may briefly detain and conduct a limited search of a person in a public place if they have a reasonable suspicion that the person has committed a crime. Reasonable suspicion is a level of belief that is less than probable cause. A police officer possesses reasonable suspicion if he has enough knowledge to lead a reasonably cautious person to believe that criminal activity is occurring and that the individual played some part in it. In practice this requirement means that an officer need not possess the measure of knowledge that constitutes probable cause to stop and frisk a person in a public place. In any case, an officer may not arrest a person until the officer possesses probable cause to believe that the person has committed a crime.

The requirement of probable cause for a search and seizure can be found in the Fourth Amendment to the U.S. Constitution, which states,

the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be searched.

All states have similar constitutional prohibitions against unreasonable searches and seizures.

The requirement of probable cause works in tandem with the warrant requirement. A warrant is a document that allows police to search a person, search a person's property, or arrest a person. A judicial magistrate or judge must approve and sign a warrant before officers may act on it. To obtain a search or arrest warrant, officers must present to the magistrate or judge enough facts to constitute probable cause. A warrant is not required for all searches and all arrests. Courts have carved out exceptions that allow police to search and arrest persons without a warrant when obtaining a warrant would be impractical.

The precise amount of evidence that constitutes probable cause depends on the circumstances in the case. To illustrate, assume that a police officer has stopped a motor vehicle driver for a traffic violation. In the absence of any other facts indicating criminal activity by the driver, it would be a violation of the Fourth Amendment if the officer conducted a full-blown search of the driver and the vehicle. The mere commission of a traffic violation is not, in and of itself, a fact that supports probable cause to believe that the driver has committed a crime. However, if the officer notices that the driver's eyes are bloodshot or that the driver smells of alcohol, the officer may detain and question the defendant, search him, and place him under arrest. Most courts hold that a driver's commission of a traffic violation combined with the appearance that the driver has used drugs or alcohol constitute sufficient evidence to lead a reasonable person to believe that the person is driving under the influence of drugs or alcohol.

Probable cause is not equal to absolute certainty. That is, a police officer does not have to be absolutely certain that criminal activity is taking place to perform a search or make an arrest. Probable cause can exist even when there is some doubt as to the person's guilt. Courts take care to review the actions of police in the context of everyday life, balancing the interests of law enforcement against the interests of personal liberty in determining whether probable cause existed for a search or arrest.

Legislatures may maintain statutes relating to probable cause. Many such statutes declare that a certain thing constitutes probable cause to believe that a person has committed a particular offense. For example, under federal law, a forfeiture judgment of a foreign court automatically constitutes probable cause to believe that the forfeited property also is subject to forfeiture under the federal racketeering law (18 U.S.C.A. § 981(i)(3) [1986]).

See: automobile searches; criminal procedure.

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