The Fourth Amendment

For the more than 100 years after the ratification of the Fourth Amendment, it was of little value to criminal defendants because evidence seized by law enforcement in violation of the warrant or reasonableness requirements was still admissible during the defendant's prosecution. The U.S. Supreme Court dramatically changed the Fourth Amendment jurisprudence when it handed down its decision in the trial of Weeks v. United States. In the landmark case of Weeks v. United States in 1914, the U.S. Supreme Court announced the far-reaching legal doctrine that has come to be known as today as the exclusionary rule, which generally bars the use in court of evidence that is illegally obtained. In the Weeks case, federal marshals kicked down his front door, scoured his home without a search warrant, and discovered several incriminating documents, which were later used against him at his federal trial. The Supreme Court said such evidence must be tossed out because it was illegally obtained. Five decades later, in Mapp v. Ohio (1961), the Court extended the exclusionary rule to criminal trials held at the state level. The Mapp decision put an end to that federal-state discrepancy and the courts declared that the fruits of an unconstitutional search are now inadmissible in both state and federal courts.

There are some exceptions to the exclusionary rule, these rules could be for many different reasons but can be used in different ways. One good example is while under the good-faith exception, evidence is not excluded if it is obtained by officers who reasonably rely on a search warrant that turns out to be invalid. The Independent Source Doctrine states that evidence initially obtained during an unlawful search or seizure may later be admissible if the evidence is later obtained through a constitutionally valid search or seizure. The Inevitable Discovery Doctrine is related to the independent source doctrine, and allows admission of evidence that was discovered in an unlawful search or seizure if it would have be discovered in the same condition anyway, by an independent line of investigation that was already being pursued when the unlawful search or seizure occurred.

Attenuation Doctrine is used in cases where the relationship between the evidence challenged and the unconstitutional conduct is too remote and attenuated, the evidence may be admissible Evidence Admissible for Impeachment is the exclusionary rule does not prevent the government from introducing illegally gathered evidence to impeach, or attack the credibility of, defendants’ testimony at trial. The Supreme Court recognized this exception in Harris vs. New York as a truth-testing device to prevent perjury. Even when the government suspects perjury, however, it may only use tainted evidence for impeachment, and may not use it to show guilt. The last example is called Qualified Immunity. This exclusionary rule is often a defendant's only remedy when police officers conduct an unreasonable search or violate their Miranda rights. Even if officers violate a defendant's constitutional or statutory rights, qualified immunity protects the officers from a lawsuit unless no reasonable officer would believe that the officers' conduct was legal.

The "fruit of the poisonous tree" doctrine goes together with the exclusionary rule. As we have seen earlier it takes the protection given by the exclusionary rule one step further. Under this doctrine, the state cannot use evidence against you in trial if it was discovered through other evidence that was obtained in violation of the Fourth Amendment. This doctrine was established in 1920 by the decision in Silverthorne Lumber Co. v. United States, and the phrase "fruit of the poisonous tree" was coined by Justice Frankfurter in his 1939 opinion in Nardone v. United States. The "poisonous tree" is considered the evidence first seized or discovered, but in the same act was seized or discovered in violation of the Fourth Amendment. The "fruit" is considered all following evidence that is found later in the investigation because of the initial information or evidence obtained from that illegal search, seizure or arrest. The exclusionary rule and the fruit of the poisonous tree doctrine were designed to curb or deter misconduct by police and other government agents. This doctrine work on the basis that the police will do their best to make sure that they are not violating the Fourth Amendment if they know that the investigation cannot benefit from illegally seized or discovered evidence.