Definition
Probable cause is a reasonable belief, based on articulable facts and circumstances, that a crime has been, is being, or is about to be committed, or that evidence of a crime will be found in a particular place.
It is the constitutional standard required by the Fourth Amendment before police may:
- Make a warrantless arrest
- Conduct a warrantless search
- Obtain a warrant from a judge
Legal Standard
The Supreme Court has defined probable cause as “a fair probability” or “a substantial chance” — something more than a hunch or mere suspicion, but less than proof beyond a reasonable doubt.
Illinois v. Gates, 462 U.S. 213 (1983): The Court adopted a “totality of the circumstances” test. Courts evaluate the totality of what an officer knew, observed, and reasonably inferred at the time — not in hindsight.
What Counts as Probable Cause
- Direct observation of contraband or criminal activity
- Reliable informant tips (with corroboration)
- Flight from police combined with other factors
- Smell of contraband (e.g., marijuana) in some jurisdictions
- Physical evidence visible in plain view
- A suspect’s admissions
What Does Not Count
- A hunch or intuition alone
- A person’s race, appearance, or neighborhood
- Being in a high-crime area alone
- Nervousness during a police encounter alone
Probable Cause vs. Reasonable Suspicion
Reasonable suspicion is a lower standard — sufficient for a brief investigative stop (a “Terry stop”) but not an arrest or full search. Probable cause is required to escalate to an arrest or warrant.
Why It Matters
Evidence obtained without probable cause may be suppressed under the exclusionary rule (Mapp v. Ohio, 367 U.S. 643 (1961)). If the core evidence in a criminal case is suppressed, charges are often dismissed.