In United States criminal law, probable cause is the standard by which an officer or agent of the law has the grounds to make an arrest, to conduct a personal or property search, or to obtain a warrant for arrest, etc. when criminal charges are being considered. It is also used to refer to the standard to which a grand jury believes that a crime has been committed. This term comes from the Fourth Amendment of the United States Constitution:
"Probable" in this case may relate to actual statistical probability, or to a general standard of common behavior and customs. The context of the word "probable" here is not exclusive to community standards and does not predate statistics, as some have suggested.
Definitions in the United States of America
The best-known definition of probable cause is "a reasonable belief that a person has committed a crime". Another common definition is "a reasonable amount of suspicion, supported by circumstances sufficiently strong to justify a prudent and cautious person's belief that certain facts are probably true". Notable in this definition is a lack of requirement for public position or public authority of the individual making the recognition, allowing for use of the term by citizens and/or the general public.
In the context of warrants, the Oxford Companion to American Law defines probable cause as "information sufficient to warrant a prudent person's belief that the wanted individual had committed a crime (for an arrest warrant) or that evidence of a crime or contraband would be found in a search (for a search warrant)". "Probable cause" is a stronger standard of evidence than a reasonable suspicion, but weaker than what is required to secure a criminal conviction. Even hearsay can supply probable cause if it is from a reliable source or supported by other evidence, according to the Aguilar–Spinelli test.
Probationers and parolees
In early cases, the Supreme Court held that when a person is on probation, the standard required for a search to be lawful is lowered from "probable cause" to "reasonable grounds" or "reasonable suspicion." Specifically, the degree of individualized suspicion required of a search was a determination of when there is a sufficiently high probability that criminal conduct is occurring to make the intrusion on the individual's privacy interest reasonable. The U.S. Supreme held that "[a]lthough the Fourth Amendment ordinarily requires the degree of probability embodied in the term 'probable cause,' a lesser degree satisfies the Constitution when the balance of governmental and private interests makes such a standard reasonable...When an officer has reasonable suspicion that a probationer subject to a search condition is engaged in criminal activity, there is enough likelihood that criminal conduct is occurring that an intrusion on the probationer's significantly diminished privacy interests is reasonable."
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