In United States constitutional law, the exclusionary rule is a legal principle holding that evidence collected or analyzed in violation of the U.S. Constitution is not admissible in a court of law (that is, it cannot be used in a trial). The rule is designed to provide a disincentive for prosecutors and police to illegally gather evidence in violation of the Fourth and Fifth Amendments in the Bill of Rights, which provides for protection from unreasonable searches and seizure and compelled self-incrimination.

The exclusionary rule was created in the case of Weeks v. United States. This decision, however, only created the rule on the federal level. It was not until Mapp v. Ohio, 367 U.S. 643 (1961) that the exclusionary rule was also binding on the states through the Fourteenth Amendment, which guarantees due process. This landmark decision was considered by former Justice Potter Stewart as "the most important search and seizure decision in [American] history."

The exclusionary rule applies to all citizens or aliens (illegal or documented) that reside within the United States. It is not applicable to aliens residing outside of U.S. borders. United States v. Alvarez decided that property owned by aliens in a foreign country is admissible in court. Certain persons in the U.S. receive limited protections, such as prisoners, probationers, parolees, and persons crossing U.S. borders. Corporations, by virtue of being, also have limited rights under the Fourth Amendment (see corporate personhood).

The Fruit of the Poisonous Tree doctrine holds that any evidence discovered indirectly through an illegal search or seizure is inadmissible in court. The rationale is that the evidence would not have been found were it not for the violation of the Fourth Amendment, and therefore must be suppressed, because it is the "fruit of the poisonous tree."

Exceptions to the Rule

The inevitable discovery doctrine was adopted in People v. Fitzpatrick in 1973. It holds that evidence obtained through an unlawful search or seizure is admissible in court if it can be established, to a very high degree of probability, that normal police investigation would have inevitably led to the discovery of the evidence. This decision was upheld because, given the fact that the exclusionary rule was created specifically to deter police and state misconduct, excluding evidence that would inevitably (hypothetically) have been discovered otherwise would not serve to deter police misconduct. In People v. Stith , the Court stated that this doctrine may not be used to admit primary evidence , only secondary evidence i.e. evidence found as a result of the primary evidence.

The attenuation exception to the exclusionary rule is that evidence may only be suppressed if there is a clear causal connection between the illegal police action and the evidence. The evidence must result from the unlawful conduct. A three-pronged test was created in People v. Martinez to determine whether there was sufficient attenuation of this connection (i.e. the lack of connection between the disputed evidence and the unlawful conduct): (1) the time period between the illegal arrest and the ensuing confession or consensual search; (2) the presence of intervening factors or event; and (3) the purpose and flagrancy of the official misconduct.

The indepedent source exception allows evidence to be admitted in court if knowledge of the evidence is gained from a separate, or independent, source that is completely unrelated to the illegality at hand. this rule was formally accepted in People v. Arnau .

The good-faith exception may allow some evidence gathered in violation of the Constitution if the violation results in only a minor or technical error. If a magistrate is erroneous in granting a police officer a warrant, and the officer acts on the warrant in good faith, then the evidence resulting in the execution of the warrant is not suppressible.