Rules governing searches and seizures by the police or other law enforcement agents arise from the Fourth Amendment to the U.S. Constitution. This provision protects the privacy rights of citizens against excessive intrusions by the government. However, law enforcement has a right to conduct searches and seizures that are reasonable. A search or seizure is reasonable if the police have a warrant from a judge based on probable cause to believe that a suspect has committed a crime. Also, a search may be reasonable without a warrant if an exception applies under the circumstances. The Fourth Amendment does not protect citizens from searches by private security guards, unless they are acting for or with the police.
Protections under the Fourth Amendment apply only to items and locations in which a citizen has a legitimate expectation of privacy. Determining whether an expectation of privacy existed requires a court to consider subjective and objective expectations. In other words, it must decide whether the person actually expected privacy and also whether a reasonable person would expect privacy. There is no expectation of privacy when an item or location is in plain view, while there may be a strong expectation of privacy in a person’s home or in a space that society traditionally considers to be private, such as a restroom.
The Exclusionary Rule
There are two main legal doctrines that can apply when a search or seizure violates the Fourth Amendment. The first is known as the exclusionary rule. It provides that any evidence obtained through an unreasonable search or seizure cannot be introduced against a defendant at a criminal trial. The defendant or their attorney will need to bring a motion to suppress this evidence, though, rather than relying on the judge to exclude it automatically. Sometimes the prosecution depends heavily on the evidence from the search to prove its case, so a successful motion to suppress may defeat the case altogether.
It may seem counterintuitive or unnecessarily lenient to allow a defendant to escape liability for a crime that they committed, based on a procedural error. The justification for the exclusionary rule is that the police would have an incentive to conduct unconstitutional searches and seizures if they could introduce the evidence anyway. This would undermine the privacy of many citizens. Certain exceptions to the exclusionary rule also limit its scope.
As with most legal rules, there are exceptions to the exclusionary rule. For instance, the exclusionary rule generally does not apply to violations of the knock-notice rule unless a particular state has decided differently.
The prosecution sometimes can use the unconstitutionally seized evidence to impeach the credibility of the defendant as a witness at trial. If the prosecution is able to get a conviction without that evidence, the judge may still consider the evidence in determining the sentence. It also may be admissible in a deportation proceeding against the defendant in the immigration system.
Fruit of the Poisonous Tree
The second main doctrine in the search and seizure context affects evidence that is obtained because law enforcement obtained the unconstitutionally seized evidence. This evidence also is not admissible against the defendant under the fruit of the poisonous tree doctrine. (The idea is that the tree is the unconstitutionally seized evidence, and the fruit is the evidence obtained through it.)
The fruit of the poisonous tree doctrine may affect multiple pieces of evidence. For example, Officer Bill stops Megan without cause, searches her purse, and finds a note indicating that she is hiding stolen property in her closet. Officer Bill enters Megan’s apartment, seizes the stolen property, and arrests Megan for theft. Neither the note nor the hidden property is admissible evidence because the search of Megan’s purse was illegal (the poisonous tree), and the seizure of the hidden property (the fruit) resulted from the illegal search.
An exception to the fruit of the poisonous tree doctrine applies when law enforcement would have found the evidence anyway. Also, if an officer does not have a legitimate reason to stop and search someone, but a legitimate reason arises during the stop, evidence that they find may be admissible in some situations. There is another exception for voluntary statements by defendants that are provided without Miranda warnings. While the statements cannot be admitted because of the Miranda violation, evidence obtained from the statements can be admitted.