What Is Probable Cause?
The legal standard police must meet before making an arrest or conducting a search. It requires more than a hunch — there must be a reasonable basis to believe a crime occurred and the person committed it (for arrests) or that evidence will be found in the place searched (for searches).
Quick Answer
The legal standard police must meet before making an arrest or conducting a search. It requires more than a hunch — there must be a reasonable basis to believe a crime occurred and the person committed it (for arrests) or that evidence will be found in the place searched (for searches).
Full Explanation
Probable cause is a constitutional standard derived from the Fourth Amendment, which protects against 'unreasonable searches and seizures.' It is the minimum justification required for a warrant and, in many cases, for warrantless arrests.
For an arrest, probable cause exists when police have trustworthy facts sufficient to lead a reasonable person to believe that the suspect has committed or is committing a crime. This is more than suspicion but less than proof beyond a reasonable doubt — it is sometimes described as more than a 50% likelihood.
For a search, probable cause requires a reasonable belief that contraband or evidence of a crime will be found in the specific place to be searched. A warrant must particularly describe the place to be searched and the items to be seized — general warrants are prohibited.
Probable cause can be established by direct police observation, reliable informant tips (assessed under the totality of circumstances test from Illinois v. Gates, 1983), or other trustworthy evidence. The determination is made by a neutral magistrate reviewing a sworn affidavit — or, in the case of warrantless searches and arrests, reviewed afterward by a court.
Evidence obtained in violation of probable cause requirements is generally suppressed under the exclusionary rule established in Mapp v. Ohio (1961).
Real-World Example
Police receive a tip from a reliable informant that a specific house contains drugs and stolen merchandise. An officer corroborates some of the informant's details through independent investigation. A magistrate issues a search warrant based on the affidavit. If the officer's affidavit contains enough specific, corroborated detail, there is probable cause.
If a police officer sees someone running from a store where an alarm just went off, holding what appears to be a bag from the store, that might supply probable cause to arrest — but a mere hunch or the person's race or neighborhood does not.
Why It Matters for Law Students
Probable cause is one of the most tested concepts in criminal procedure. It governs when police can act without a warrant, what a warrant must show, and what happens when police get it wrong. The exclusionary rule makes probable cause practically significant — evidence obtained without it often cannot be used at trial.