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Reasonable suspicion is a legal standard of proof in United States law that is less than probable cause, the legal standard for arrests and warrants, but more than an "inchoate and unparticularized suspicion or 'hunch' "; it must be based on "specific and articulable facts", "taken together with rational inferences from those facts". If police additionally have reasonable suspicion that a person so detained is armed and dangerous, they may "frisk" the person for weapons, but not for contraband like drugs. Reasonable suspicion is evaluated using the "reasonable person" or "reasonable officer" standard, in which said person in the same circumstances could reasonably believe a person has been, is, or is about to be engaged in criminal activity; it depends upon the totality of circumstances, and can result from a combination of particular facts, even if each is individually innocuous.
In Terry v. Ohio, the U.S. Supreme Court ruled that a person can be stopped and briefly detained by a police officer based on a reasonable suspicion of involvement in a punishable crime. If the officer has reasonable suspicion that the person is armed and/or dangerous, the officer may perform a search of the person's outer garments for weapons. Such a detention does not violate the Fourth Amendment prohibition on unreasonable searches and seizure, though it must be brief. Reasonable suspicion does not provide grounds for arrest; however, an arrest can be made if facts discovered during the detention provide probable cause that the suspect has committed a crime.
In Hiibel v. Sixth Judicial District Court of Nevada the Court further established that a state may require, by law, that a person identify himself or herself to an officer during a stop; some states (e.g., Colorado) require that a person detained provide additional information, but as of November 2010, the validity of such additional obligations has not come before the Supreme Court.
Most state child abuse reporting laws employ the “reasonable suspicion” standard as the threshold above which mandated reporters must report the case. However, the definition of this term is not widely understood[1–3]. As a result there is large variation in the rates of child abuse reporting in different states .
1. Levi, B.H. and S.G. Portwood, Reasonable suspicion of child abuse: finding a common language. J Law Med Ethics, 2011. 39(1): pp. 62–9. 2. Levi, B.H. and K. Crowell, Child abuse experts disagree about the threshold for mandated reporting. 2011. 50(4): pp. 321–9. 3. Flaherty, E.G. and R. Sege, Barriers to physician identification and reporting of child abuse. Pediatr Ann, 2005. 34(5): pp. 349–56. 4. Kids Count Data. 2009.
New Jersey v. T. L. O. Is the only state that has set the precedent that probable cause is not necessary to search a student; reasonable suspicion is enough to search a student's belongings. Overly intrusive searches, like a body cavity search, require probable cause.
A few years after T.L.O., the Supreme Court held in O'Connor v. Ortega that while government employees do have Fourth Amendment rights in the workplace, administrative investigations by supervisors of alleged work-related misconduct where no criminal offenses are suspected likewise only require reasonable suspicion to justify a search.
U.S. Customs can do routine suspicionless searches of people and effects crossing the border (including passing through airport customs) without establishing reasonable suspicion. This includes even complicated searches such as the disassembly of an automobile's gas tank. United States v. Flores-Montano. However, there are some more intrusive types of searches, such as body cavity searches of a suspect balloon swallower, that require reasonable suspicion. United States v. Montoya De Hernandez.
A brief, non-custodial traffic stop must normally be supported by reasonable suspicion; the investigating officer must weigh the totality of the circumstances to determine whether sufficient objective facts exist to create reasonable suspicion that the driver is engaged in criminal activity. If the investigating officer witnesses the driver committing a traffic violation, the violation supplies reasonable suspicion to stop the vehicle but probable cause to support an arrest only exists if the traffic violation is a misdemeanor violation such as not wearing a seatbelt, reckless driving or in some states, running a red light. A minor traffic violation such as speeding, an infraction and not a misdemeanor, does not give the officer probable cause to support an arrest. In Illinois v. Caballes, the Supreme Court held that a drug dog may sniff the exterior of a vehicle during a traffic stop so long as it does not extend the stop; the use of a drug dog is sui generis and not considered a search. Police may set up roadblocks and stop drivers without particularized reasonable suspicion that the stopped individual is engaged in criminal activity, so long as the stop’s level of intrusion does not exceed the connection of the crime (for instance, driving while intoxicated) to the roadway. 
Most powers applied by police officers in the United Kingdom are done on reasonable suspicion. Unlike in the United States, British police officers can arrest on reasonable suspicion.
A police officer may briefly detain a person, without a warrant, if the officer has reasonable suspicion that the person is involved in a crime (Terry v. Ohio, 392 U.S. 1 (1968)), and the officer can use reasonable force to effect the detention. Courts have recognized that an officer's safety is paramount and have allowed for a "frisk" of the outermost garments from head to toe if the officer reasonably suspects that the detainee is armed, and for an officer to stop an individual at gunpoint if necessary. In the city of New York, once a person is released from a reasonable suspicion stop, a "stop, question and frisk report" is filled out and filed with the command in which the stop occurs.
US Courts have held that a stop on reasonable suspicion may be appropriate in the following cases: when a person possesses unusual items (like a wire hanger) which would be useful in a crime and is looking into car windows at 2 am, when a person matches a description of a suspect given by another officer, or when a person runs away at the sight of a peace officer. However, reasonable suspicion does not apply merely because a person refuses to answer questions, declines to allow a voluntary search, or is of a particular race or ethnicity .
- Reasonable doubt
- Stop and identify statutes (refusing to identify oneself when detained may be a crime in some jurisdictions)
- United States v. Arnold (searches and seizures of electronic media at a United States port of entry by Customs and Border Protection)
- Samson v. California (individualized reasonable suspicion is not required for searches to be conducted of the persons and homes of parolees)
- Terry v. Ohio, 392 U.S. 1, 27 (1968).
- Terry v. Ohio, 392 U.S. at 21
- Terry v. Ohio, 392 U.S. at 21–22
- Colo. Rev. Stat. §16-3-101(1)
- United States v. Arvizu, 534 U.S. 266 (2002).
- Atwater v. City of Lago Vista, 532 U.S. 318 (2001).
- Illinois v. Caballes, 543 U.S. 405 (2005) at 411
- Michigan Department of State Police v. Sitz, 496 U.S. 444 (1990).
- A definition on Dictionary.com