Such evidence is not generally admissible in court. For example, if a police officer conducted an unconstitutional (Fourth Amendment) search of a home and obtained a key to a locker in a train station, thus obtaining evidence of a crime from the locker, then that evidence would more than likely be excluded in accordance with the fruit of the poisonous tree doctrine. The discovery of a witness is not evidence in itself because the witness is attenuated by separate interviews, in-court testimony and their own statements.
The doctrine is an extension of the exclusionary rule, which, subject to some exceptions, prevents evidence obtained in violation of the Fourth Amendment from being admitted in a criminal trial. Like the exclusionary rule, the fruit-of-the-poisonous-tree doctrine is intended to deter police from using illegal means to obtain evidence.
The doctrine is subject to three main exceptions. The tainted evidence will be admissible if (1) it was discovered in part as a result of an independent, untainted source; (2) it would inevitably have been discovered despite the tainted source; or (3) the chain of causation between the illegal action and the tainted evidence is too attenuated.
The Fruit of the Poisonous Tree doctrine stems from the 1920 case of Silverthorne Lumber Co. v. United States.
The Fruit of the Poisonous Tree phrase is a reference to the biblical passage originally found in .