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The doctrine of the "fruit of the poisonous tree" is a fundamental legal principle that holds that any evidence obtained unlawfully—specifically, through the violation of fundamental rights—must be considered tainted and, therefore, inadmissible in a judicial process. This concept originated in the jurisprudence of the United States Supreme Court in 1920, specifically in the case of Silverthorne Lumber Co. v. United States. It is illustrated by a metaphor: if the source, the "tree," from which the evidence comes is tainted, then the "fruit" of that "tree" will be similarly tainted.
The name of the doctrine finds inspiration in biblical texts, such as Matthew 7:17-20, which states: "Likewise, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them."
The first ruling explicitly referencing the "fruit of the poisonous tree doctrine" occurred in the case "Nardone v. United States" on December 11, 1939. In this case, related to the wiretapping of an alcohol smuggler, it was established that the judge must allow the defendants the opportunity to demonstrate that a substantial part of the accusation against them derived from a source tainted by illegality.
In legal terms, this principle implies that evidence obtained unlawfully is not only inadmissible but must also be declared null and void. In the Spanish legal system, the doctrine of the "fruit of the poisonous tree" was enshrined in 1984, following a ruling by the Constitutional Court (STC 114/1984), which established that evidence obtained by violating fundamental rights, such as freedom and privacy, should not be considered.
From this ruling of the Constitutional Court, an important legal provision emerged: Article 11.1 of the Organic Law of the Judiciary of 1985, which states that "in all types of proceedings, the rules of good faith shall be respected, and evidence obtained, directly or indirectly, by violating fundamental rights or freedoms shall have no effect." According to this provision, any evidence obtained through an illegal act is considered invalid as a means of proof in a judicial process, and the nullity of such evidence extends to all other evidence that directly or indirectly derives from it.
A significant case in which this doctrine was applied took place in January 2023, when two individuals accused of drug trafficking were acquitted because the search warrant issued by the investigating judge was based on insufficient information provided by the police, which amounted to mere suspicions. Magistrate Fernández-Montells, in his ruling 172/2022 of September 21, held that "the enabling order does not meet the requirements for its procedural regularity, as there are no rational indications to consider and evaluate on which the restrictive measure of rights should be based. This evidence, obtained in this manner, is contrary to constitutional requirements and, therefore, is illegal evidence obtained in violation of fundamental rights."
The "fruit of the poisonous tree" doctrine posits that any evidence obtained directly or indirectly from an unlawful source must also be considered null and void. This means that tainted evidence becomes illegitimate and carries with it the nullity of all other evidence that directly derives from it. Cases where fundamental rights such as the right to privacy, the secrecy of communications, or the inviolability of the home, as established in Article 18 of the Spanish Constitution, are violated also fall within this doctrine.
However, the application of this doctrine is not absolute, and there are exceptions such as the inevitable discovery doctrine, the independent source doctrine, and the attenuation doctrine.
The inevitable discovery doctrine applies when the circumstances would have led to the same result, making it impossible to causally link the second piece of evidence to the previous one. Circumstances would inevitably have led to the same outcome. The Supreme Court ruling STS 885/2002 of May 21 states: "Any result that would have occurred, even if one of its conditions had not occurred, is not the result of that condition."
The independent source doctrine refers to the existence of a different, uncontaminated investigative channel that allows evidence to be obtained by a means other than that used to obtain the elements of evidence considered illegal.
Finally, the attenuation doctrine, also known as the "taint attenuation doctrine," assumes the Constitutional Court's doctrine on the connection of unlawfulness or prohibition of evaluation, understood as a legal link between one piece of evidence and another. The prohibition is constitutionally reflected, preventing the use of evidence obtained by violating constitutional rights, and legally reflected in Article 11.1 of the LOPJ: "In all types of proceedings, the rules of good faith shall be respected. Evidence obtained directly or indirectly, by violating fundamental rights or freedoms, shall have no effect."
The doctrine of the "fruit of the poisonous tree" has recently gained considerable relevance in the Spanish legal field, due to a case that has garnered notable attention. Specifically, the Civil and Criminal Chamber of the Madrid High Court (TSJM) overturned a sentence of 15 years, 1 month, and 1 day in prison imposed on Daniel H., concluding that the main incriminating evidence, consisting of images obtained from his mobile phone, was obtained unlawfully. This ruling is based on the rigorous application of the "fruit of the poisonous tree" doctrine, according to which any evidence derived from a source tainted by illegality must be considered null and inadmissible in a judicial process.
The original sentence, issued by the Provincial Court of Madrid, was based on photographs and frames found on Daniel H.'s mobile device, which were obtained without his prior consent, thus constituting a violation of his fundamental rights to privacy and intimacy. The appellate court, composed of magistrates Celso Rodríguez Padrón, Francisco José Goyena Salgado, and María Prado Magariño, determined that evidence obtained unlawfully cannot be used to support a conviction, in accordance with the jurisprudence of the Supreme Court and the doctrine of the Constitutional Court.
The events leading to this case date back to 2021, when Daniel H.'s adopted daughter informed her mother that her adoptive father entered her bed at night and photographed himself with her. The mother, without Daniel H.'s consent, accessed his mobile device and discovered the images, leading to police intervention and the subsequent extraction of 24 photographs by the Computer Security Group of the Judicial Police Brigade of Madrid. However, during the judicial process, it was not proven that Daniel H. had consented to access to his mobile device, a fact that was decisive in the TSJM's decision.
In this context, the TSJM declared the nullity not only of the images obtained unlawfully but also of all evidence derived from them, including subsequent investigations and identifications. This decision made the original sentence untenable, leading to Daniel H.'s acquittal, as all the evidence against him was irreparably tainted by the illegality of access to his mobile device.
In conclusion, the doctrine of the "fruit of the poisonous tree" is an essential legal principle that guarantees the protection of fundamental rights in the judicial process by establishing the inadmissibility of evidence obtained unlawfully. This principle, deeply rooted in both Spanish jurisprudence and law, holds that any evidence derived from a source tainted by illegality must be declared null. The recent rigorous application of this doctrine by the Civil and Criminal Chamber of the Madrid High Court, which resulted in the annulment of a conviction by considering that the evidence obtained violated fundamental rights, reaffirms its importance in safeguarding due process and justice.
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