Terry stop

A Terry stop in the United States allows the police to briefly detain a person based on reasonable suspicion of involvement in criminal activity. Reasonable suspicion is a lower standard than probable cause which is needed for arrest. When police stop and pat-down a pedestrian, this is commonly known as a stop and frisk. When police stop an automobile, this is known as a traffic stop. If the police stop a motor vehicle on minor infringements in order to investigate other suspected criminal activity, this is known as a pretextual stop.

In the United States at the federal level, the Supreme Court has decided many cases that define the intersection between policing and the Fourth Amendment protection against unreasonable searches and seizures. However, Congress has not defined a baseline for police behavior. There has been some state action at both the legislative and judicial levels, and also some cities have passed laws on these issues.

Some law academics are concerned that jurisprudence permitting Terry stops does not account for possible implicit bias of officers, and that this possibly results in racially skewed decision-making. Communities that have high rates of incarceration may experience more intense and punitive policing and surveillance practices even during periods of time when general crime rates are decreasing.

The concept of a Terry stop originated in the 1968 Supreme Court case Terry v. Ohio, in which a police officer detained three Cleveland men on the street behaving suspiciously, as if they were preparing for armed robbery. The police conducted a pat down search and discovered a revolver, and subsequently, two of the men were convicted of carrying a concealed weapon. The men appealed their case to the Supreme Court, arguing that the search in which the revolver was found was illegal under the Fourth Amendment. This brief detention and search were deemed permissible by the court, judging that the officer had reasonable suspicion which could be articulated (not just a hunch) that the person detained may be armed and dangerous. This was not mere "suspicion" but "reasonable suspicion" which could be articulated at a later date.

This decision was made during a period of great social unrest in the United States in the 1960s, with rising crime, opposition to U.S. involvement in the Vietnam War and the civil rights movement, and race riots. It was thought that law enforcement needed to be provided with tools to deal with the unrest and new issues of urban crime. Some criticized the decision for watering down the prohibition against unreasonable searches and seizures; others praised it for balancing safety and individual rights.

The United States Supreme Court held that where: a police officer observes unusual conduct by a subject; the subject's conduct leads the officer reasonably to conclude that criminal activity may be afoot, and that the subject may be armed and presently dangerous; the officer identifies themselves as a police officer; the officer makes reasonable inquiries; and nothing in the initial stages of the encounter serves to dispel the officer's reasonable fear for safety, the officer may conduct a carefully limited search of the outer clothing of the subject in an attempt to discover weapons, and that such a search is a reasonable search under the Fourteenth Amendment, so that any weapons seized may properly be introduced in evidence.

To possess reasonable suspicion that would justify a stop, police must have "specific and articulable facts" that indicate that the person to be stopped is, or is about to be, engaged in criminal activity. Because officers usually do not have supervision when they encounter civilians, they have discretion regarding whom to stop. Reasonable suspicion depends on the "totality of the circumstances". Reasonable suspicion is a vague term, and the Supreme Court concluded that it is to be decided on a case-by-case basis. It often arises from a combination of facts, each of which would, in itself, not be enough justification for the stop.

The suspicion must be that of an individual person. Police officers primarily use situational factors based on criminal behavior to determine whether a stop is needed. In essence, when they witness a person behaving suspiciously or violating the law, they are constitutionally permitted to stop them. Other factors informing the decision include personal attitudes and the decision-making model in effect where the officer works. These subjective influences naturally create the opportunity for bias on the part of police officers who possess animus toward a certain class of people.