The Fourth Amendment to the United States Constitution provides that:
“The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and 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.”
Thus, the Fourth Amendment is designed to protect persons against unreasonable government searches and seizures of persons and property. And government agents (the police) generally must secure a warrant (a written order) from a judge or magistrate to search persons or private property in places where persons have a reasonable expectation of privacy, or to seize (arrest or take) persons or private property.
The U.S. Supreme Court has held that a person has a reasonable expectation of privacy when (1) the person has exhibited an actual, subjective expectation of privacy, and (2) the expectation of privacy is one that society is prepared to recognize as reasonable—by reference to concepts of real or personal property law or to understandings that are recognized and permitted by society. If both of these conditions for a reasonable expectation of privacy are present and the government has taken an action that violates this expectation of privacy, the government has violated the person’s Fourth Amendment rights.
This means that most warrantless searches of persons or private property are prohibited under the Fourth Amendment unless a specific exception to the warrant requirement applies. Exceptions to the warrant requirement include: (1) if a police officer asks for and is given consent or permission to search the property by a person with authority to give consent to search; (2) if the search is incident to a lawful arrest; (3) if there is probable cause to search and there are exigent circumstances requiring an immediate search—such as when persons are in imminent danger, where there is imminent danger that evidence will be destroyed (illegal drugs flushed down the toilet), or imminent danger that a suspect will escape; and (4) if items are in plain view, the police are not required to secure a warrant to search and seize the items. Thus, a gun lying in plain view on the seat of a motor vehicle stopped for a routine traffic violation may be searched and seized without a warrant if there is probable cause to believe the gun may be evidence of a crime.
Similarly, a warrantless seizure of a person (an arrest) is generally prohibited under the Fourth Amendment unless (1) the police officer has probable cause to believe a crime has been committed or is about to be committed, and (2) there are exigent circumstances requiring an immediate arrest—such as when persons are in imminent danger, where there is imminent danger that evidence will be destroyed (illegal drugs flushed down the toilet), or imminent danger that a suspect will escape. And the Fourth Amendment requires persons arrested without a warrant to be promptly brought before a magistrate or judge to determine whether there was probable cause for the arrest.
Another exception to the general requirement of a warrant to search private property in which persons have a reasonable expectation of privacy is the automobile exception. Under the automobile exception, a motor vehicle may be searched without a warrant when the evidence or contraband may be removed from the scene due to the mobility of the motor vehicle, and it is not practical to secure a search warrant without jeopardizing the potential evidence. This exception may also apply to other motor vehicles such as motorcycles, boats, and private aircraft.
Any evidence the police discover from an illegal warrantless search—directly or indirectly—will not be admissible in evidence in a criminal prosecution against the person. This rule of evidence is known as the exclusionary rule, and it extends to evidence the police would not have uncovered without the illegal warrantless search—and such derivative evidence is known as the fruit of the poisonous tree.
There are exceptions to the exclusionary rule—including (1) the good-faith exception; (2) the independent source doctrine; (3) the inevitable discovery doctrine; (4) the attenuation doctrine; and (5) impeachment evidence.