130
Opinion of Stevens, J.
flight, and whether the person's behavior was otherwise unusual might be relevant in specific cases. This number of variables is surely sufficient to preclude either a bright-line rule that always justifies, or that never justifies, an investigative stop based on the sole fact that flight began after a police officer appeared nearby.4
Still, Illinois presses for a per se rule regarding "unprovoked flight upon seeing a clearly identifiable police officer." Id., at 7. The phrase "upon seeing," as used by Illinois, apparently assumes that the flight is motivated by the presence of the police officer.5 Illinois contends that unprovoked flight is "an extreme reaction," id., at 8, because innocent people simply do not "flee at the mere sight of the police," id., at 24. To be sure, Illinois concedes, an innocent person—even one distrustful of the police—might "avoid eye contact or even sneer at the sight of an officer," and that
4 Of course, Terry itself recognized that sometimes behavior giving rise to reasonable suspicion is entirely innocent, but it accepted the risk that officers may stop innocent people. 392 U. S., at 30. And as the Court correctly observes, it is "undoubtedly true" that innocent explanations for flight exist, but they do not "establish a violation of the Fourth Amendment." Ante, at 125. It is equally true, however, that the innocent explanations make the single act of flight sufficiently ambiguous to preclude the adoption of a per se rule.
In Terry, furthermore, reasonable suspicion was supported by a concatenation of acts, each innocent when viewed in isolation, that when considered collectively amounted to extremely suspicious behavior. See 392 U. S., at 5-7, 22-23. Flight alone, however, is not at all like a "series of acts, each of them perhaps innocent in itself, but which taken together warran[t] further investigation." Id., at 22. Nor is flight similar to evidence which in the aggregate provides "fact on fact and clue on clue afford-[ing] a basis for the deductions and inferences," supporting reasonable suspicion. United States v. Cortez, 449 U. S. 411, 419 (1981).
5 Nowhere in Illinois' briefs does it specify what it means by "unprovoked." At oral argument, Illinois explained that if officers precipitate a flight by threats of violence, that flight is "provoked." But if police officers in a patrol car—with lights flashing and siren sounding—descend upon an individual for the sole purpose of seeing if he or she will run, the ensuing flight is "unprovoked." Tr. of Oral Arg. 17-18, 20.
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