Fruit of the poisonous tree
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Fruit of the poisonous tree is a legal metaphor in the United States used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well.
The doctrine underlying the name was first described in Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920). The term's first use was by Justice Felix Frankfurter in Nardone v. United States (1939).
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 train station locker, and evidence of a crime came from the locker, that evidence would most likely be excluded under the fruit of the poisonous tree legal doctrine. The testimony of a witness who is discovered through illegal means would not necessarily be excluded, however, due to the "attenuation doctrine", which allows certain evidence or testimony to be admitted in court if the link between the illegal police conduct and the resulting evidence or testimony is sufficiently attenuated. For example, a witness who freely and voluntarily testifies is enough of an independent intervening factor to sufficiently "attenuate" the connection between the government's illegal discovery of the witness and the witness's voluntary testimony itself. (United States v. Ceccolini, 435 U.S. 268 (1978))
The "fruit of the poisonous tree" 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 four main exceptions. The tainted evidence is admissible if:
- it was discovered in part as a result of an independent, untainted source; or
- it would inevitably have been discovered despite the tainted source; or
- the chain of causation between the illegal action and the tainted evidence is too attenuated; or
- the search warrant was not found to be valid based on probable cause, but was executed by government agents in good faith (called the good-faith exception).
This doctrine was also used by the European Court of Human Rights in Gäfgen v. Germany [GC], no. 22978/05, § 25, ECHR (2010). In certain cases continental European countries have similar laws (e.g. in cases of torture), while the doctrine itself is generally not known. Illegally obtained evidence is used by the courts to ensure that the judgment is factually correct, however the person obtaining the illegal evidence typically faces independent consequences.[clarification needed]
Outside of the United States
Admissibility of an evidence in courts, obtained by unlawful means, has been a contention in courts throughout the world. Examples would include stolen documents or tapped phones.
India and United Kingdom
English courts have relied on an 1861 verdict, R v Leathem, to admit evidence irrespective of the legality of the source. This is the general stance.
R v Leathem (1861) 8 Cox CC 498
Drawing on the English tradition, the doctrine does not have a parallel in India and courts will admit evidence, even if it is illegally obtained (stolen etc), especially if it will help prove guilt or innocence. While the quality of the evidence may be suspect, the position that the evidence should not be taken cognisance of at all, is not a position Indian courts take.
Note that there are other considerations as to the admissibility of the evidence, such as whether it was extracted under duress or other violation of human rights including privacy in modern times, "if its prejudicial effect on the jury was likely to outweigh its probative value”  However, this article deals only about cognisance in case the source of the evidence itself may be unlawful.
The Indian Supreme Court has dealt with the matter multiple times, decisively so in 1971, ruling against the applicability of this doctrine in India.
In 2019 the Attorney General argued in front of a three bench of the court which included the sitting Chief Justice, that official/classified documents stolen from the government - which happened to be integral to the case in question - should not be taken cognisance of by the court, as they were classified, and the stealth and subsequent leakage to a newspaper was crime under the Official Secrets Act. K M Joseph from the bench, noted that "even stolen evidence can be looked into by the Court. It is well settled under Evidence Act.", while the Chief Justice queried if the plea of alibi (of an accused) was based on stolen evidence, would it right for the court to ignore it. The third constituent of the bench, Sanjay Kaul further noted that even if the Attorney General's argument were to be true, any evidence would be admissible if it would shock the conscience of the court.
Admissibility of evidence in Indian courts hinges majorly on relevance, and then to the source. The Indian Supreme Court, especially, is empowered by the Constitution to have any document produced before it. In fact, in the 1971 verdict touched upon above, the Supreme Court decision relies on R v Leathem (1861).
However, considerations of protection against self-incrimination - granted in the Constitution - are taken into account and evidence obtained under duress will be grounds to reject its validity, but not the legality of the source alone. 
The Indian Supreme Court, which as the highest appellatte court and the constitutional court, sets precedent for the courts throughout the country.
- Commonwealth v. Matos
- Silverthorne Lumber Co. v. United States
- Mapp v. Ohio, 367 U.S. 643 (1961)
- Wong Sun v. United States, 371 U.S. 471 (1963)
- Nix v. Williams, 467 U.S. 431 (1984)
- Ex turpi causa non oritur actio
- Parallel construction
- Sugar bowl (legal maxim)
- Section 24(2) of the Canadian Charter of Rights and Freedoms
- Dressler, Joshua (2002). Understanding Criminal Procedure (3rd ed.). Newark, NJ: LexisNexis. ISBN 0-8205-5405-7.
- Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920)
- "Fruit of a poisonous tree legal definition of Fruit of a poisonous tree. Fruit of a poisonous tree synonyms by the Free Online Law Dictionary". Legal-dictionary.thefreedictionary.com. 1959-06-04. Retrieved 2014-03-04.
- See also Bransdorfer, Mark S. (1987). "Miranda Right-to-Counsel Violations and the Fruit of the Poisonous Tree Doctrine". Indiana Law Journal. 62: 1061. ISSN 0019-6665.
- "Nardone v. United States, 308 U.S. 338 (1939)". Justia Law. Retrieved 2019-05-27.
- Gaines, Larry; Miller, LeRoy (2006). Criminal Justice In Action: The Core. Belmont, CA: Thomson/Wadsworth. ISBN 0-495-00305-0.
- "attenuation doctrine". Retrieved 2018-12-16.
- Gibson, Meg. "Illegally or Improperly Obtained Evidence: does it matter how you get it?". Cambridge University Law Society. Cambridge University Law Society. Retrieved 8 June 2019.
- Varma, Sanjay. "Column: Fruit of the Poisonous Tree". barandbench.com. Retrieved 8 June 2019.