Some folks just don't want to get involved. They know of criminal activity and are willing to report it, but they don't want to experience any blowback. Others may simply feel like causing trouble for certain people who haven't done anything wrong. The task for police officers and courts is teasing out the good tips from the bad.
(For more on a related topic, see Learning the Identity of a Confidential Informant.)
An anonymous tip can provide law enforcement with reasonable suspicion to detain someone. The police might, for example, get a call from someone who won’t provide a name but reports a vehicle moving erratically on the freeway. If the report is sufficiently reliable, an officer has a basis to stop the vehicle and briefly investigate. During the detention, the officer may develop probable cause to arrest the suspect. (For more on anonymous tips about motorists, see Supreme Court Rules on Anonymous Reports of Drunk Driving.)
Ultimately, whether an anonymous tip gives the police enough to detain someone depends on the circumstances.
In one case, an anonymous caller reported to the police that a woman named Vanessa White would leave a particular apartment building “at a particular time in a brown Plymouth station wagon with the right taillight lens broken, that she would be going to Dobey's Motel, and that she would be in possession of about an ounce of cocaine inside a brown attaché case.” (Alabama v. White, 496 U.S. 325 (1990).) Two police officers waited at the apartment building, where they saw White leave the building and get into a Plymouth station wagon that had a broken right taillight. She had nothing in her hands. They followed White as she drove the station wagon in the direction of Dobey’s Motel.
The officers had another patrol unit stop the station wagon. White consented to the officers searching her car for cocaine. In it they found a brown attaché case with marijuana inside. They arrested her and eventually found three milligrams of cocaine in her purse.
In ruling on what it called a “close case,” the Supreme Court found that the police corroborated the tip enough for there to be reasonable suspicion for the traffic stop. The tipster's accurate prediction of the suspect's future behavior gave the officers reason to trust that the information about drug possession was accurate. The Court therefore found that the police had behaved lawfully.
In another case, the police received an anonymous call that a young black male wearing a plaid shirt was standing at a specific bus stop. (Florida v. J.L., 529 U.S. 266 (2000).) The tip didn’t provide detail or any kind of prediction of the suspect’s behavior. Police officers went to the relevant bus stop and saw three black males, one of whom was wearing a plaid shirt. The officers didn’t notice anything suspicious about the three males, but one officer approached and frisked the one in plaid. The officer found a gun in the suspect's pocket. (See What’s the difference between an arrest and a detention or “stop and frisk”?)
This time, the Supreme Court held that the simplistic tip didn’t offer enough to create reasonable suspicion for a detention. The tipster didn’t explain how he knew of the gun, nor did he predict any of the suspect’s behavior or otherwise indicate that he had any special knowledge about the situation.
The Supreme Court’s take on anonymous tips is the controlling law in most places. But it’s possible for state courts to rely on their own constitutions rather than the federal Constitution on this issue—in that way, they might require more reliability from anonymous tips before allowing officers to act on them.
For example, the Montana Supreme Court said that the tip in the White case would have been insufficient for a detention under state law. It observed that:
(State v. Martinez, 314 Mont. 434 (2003).)