Probable Cause

Probable cause is the evidentiary threshold required under the Fourth Amendment of the U.S. Constitution before police may conduct most searches, seizures, or arrests, and before a magistrate may issue a warrant. The text of the amendment specifies that "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 seized."

The standard sits above reasonable suspicion (the lower bar permitting brief investigatory stops under Terry v. Ohio, 1968) but well below beyond a reasonable doubt (the standard for criminal conviction). In practice, probable cause requires more than a hunch but does not demand proof that guilt is more likely than not.

The Supreme Court refined the modern test in Illinois v. Gates (1983), replacing the rigid two-pronged Aguilar-Spinelli framework with a "totality of the circumstances" approach. A magistrate must make a practical, common-sense decision whether, given everything in the affidavit—including informant tips, corroboration, and officer experience—there is a fair probability that contraband or evidence will be found in a particular place.

Probable cause is also the standard applied at:

• Preliminary hearings, to determine whether charges should proceed to trial
• Grand jury indictments, for issuing a true bill
• Warrantless arrests, which Atwater v. Lago Vista (2001) confirmed are permissible for even minor offenses if probable cause exists

Evidence obtained without probable cause is generally subject to the exclusionary rule (Mapp v. Ohio, 1961), though exceptions exist for good-faith reliance on a defective warrant (United States v. Leon, 1984), inevitable discovery, and independent source.

Outside the United States, comparable thresholds exist under different names—"reasonable grounds to suspect" in UK police powers under PACE 1984, for instance—but the specific doctrinal architecture is distinctly American.