The Exclusionary rule is exemplified in the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, which was adopted in 1984:
Each State Party shall ensure that any statement which is established to have been made as a result of torture shall not be invoked as evidence in any proceedings, except against a person accused of torture as evidence that the statement was made.
The Convention is interpreted through authoritative comment from the Committee against Torture. In its General Comment number two, on the implementation of article 2 by States parties, the Committee observed the non-derogable nature of the article 15 obligation:
6. The Committee reminds all States parties to the Convention of the non-derogable nature of the obligations undertaken by them in ratifying the Convention. [Among them are article] 15 (prohibiting confessions extorted by torture being admitted in evidence, except against the torturer). [s]uch provisions […] “must be observed in all circumstances. The Committee considers that articles 3 to 15 are likewise obligatory as applied to both torture and ill-treatment.
Even prior to the Convention prohibition, the international community had accepted that the rule reflected international law. The 1975 Declaration on the Protection of All Persons from Being Subjected to Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, provides:
Any statement which is established to have been made as a result of torture or other cruel, inhuman or degrading treatment or punishment may not be invoked as evidence against the person concerned or against any other person in any proceedings.
More recently, the General Assembly has reaffirmed the strength of the exclusionary rule as a customary standard in resolutions against torture. In its resolution 13/19 dated 15 April 2010, the General Assembly:
“Strongly urges States to ensure that no statement that is established to have been made as a result of torture is invoked as evidence in any proceedings, except against a person accused of torture as evidence that the statement was made, and calls upon States to consider extending that prohibition to statements made as a result of other cruel, inhuman or degrading treatment or punishment, and recognizes that adequate corroboration of statements, including confessions, used as evidence in any proceedings constitutes one safeguard for the prevention of torture and other cruel, inhuman or degrading treatment or punishment” (UN Doc. A/HRC/Res/13/19).
Though the International Covenant for Civil and Political Rights does not explicitly set out a rule of exclusion for evidence tainted by torture and other ill-treatment, the rule has been recognised as an inherent part of the absolute prohibition of torture, and part of the necessary guarantees to ensure fairness of judicial proceedings. In its General Comment number 32, the Human Rights Committee provides:
6. ... [A]s article 7 [prohibition of torture and other ill-treatment] is also non-derogable in its entirety, no statements or confessions or, in principle, other evidence obtained in violation of this provision may be invoked as evidence in any proceedings covered by article 14, including during a state of emergency, except if a statement or confession obtained in violation of article 7 is used as evidence that torture or other treatment prohibited by this provision occurred. Deviating from fundamental principles of fair trial, including the presumption of innocence, is prohibited at all times.
41. […] Domestic law must ensure that statements or confessions obtained in violation of article 7 of the Covenant are excluded from the evidence, except if such material is used as evidence that torture or other treatment prohibited by this provision occurred, and that in such cases the burden is on the State to prove that statements made by the accused have been given of their own free will.
The Inter-American Convention to Prevent and Punish Torture describes the duty to prohibit information obtained by torture in a similar way to the Convention against Torture. It provides:
No statement that is verified as having been obtained through torture shall be admissible as evidence in a legal proceeding, except in a legal action taken against a person or persons accused of having elicited it through acts of torture, and only as evidence that the accused obtained such statement by such means.
The Robben Island Guidelines for the Prohibition and Prevention of Torture in Africa, formally adopted by resolution of the African Commission in October 2002 and approved by Heads of State and Governments of the African Commission in July 2003, provides a wider prohibition to exclude evidence obtained as a result of torture, as well as other forms of ill-treatment:
29. [States should] [e]nsure that any statement obtained through the use of torture, cruel, inhuman or degrading treatment or punishment shall not be admissible as evidence in any proceedings except against persons accused of torture as evidence that the statement was made.
Various standards have also given the exclusionary rule greater meaning, by providing guidance to the different actors in the criminal justice system, such as prosecutors and judges.
In the 1990 UN Guidelines on the Role of Prosecutors, article 16 provides
16. When prosecutors come into possession of evidence against suspects that they know or believe on reasonable grounds was obtained through recourse to unlawful methods, which constitute a grave violation of the suspect's human rights, especially involving torture or cruel, inhuman or degrading treatment or punishment, or other abuses of human rights, they shall refuse to use such evidence against anyone other than those who used such methods, or inform the Court accordingly, and shall take all necessary steps to ensure that those responsible for using such methods are brought to justice.
The Special Rapporteur on torture has also published standards applicable to the police or other detaining authorities, and judges, in his general recommendations. In relation to the role of judges, the Special Rapporteur recommends that:
“Where allegations of torture or other forms of ill-treatment are raised by a defendant during trial, the burden of proof should shift to the prosecution to prove beyond reasonable doubt that the confession was not obtained by unlawful means, including torture and similar ill-treatment. Serious consideration should also be given to the creation of witness protection programmes for witnesses to incidents of torture and similar ill-treatment which ought to extend fully to cover persons with a previous criminal record. In cases where current inmates are at risk, they ought to be transferred to another detention facility where special measures for their security should be taken.”
After conducting visits to countries, the Special Rapporteur has also made specific recommendations which have general application beyond the jurisdiction visited. In a visit to Tunisia, the Special Rapporteur gave additional assistance and advice to judges to ensure effective exclusion of torture-tainted evidence:
"[The Special Rapporteur] encourage[s] judges and prosecutors to routinely ask persons arriving from police custody how they have been treated, and to order an independent medical examination in accordance with the Istanbul Protocol if they suspect that detainees have been subjected to ill-treatment; an ex-officio investigation should be initiated whenever there are reasonable grounds to believe that a confession was obtained through torture and ill-treatment" (SRT, Visit to Tunisia, A/HRC/19/61/Add.1, 2 February 2012).
After a visit to Kyrgyzstan, the Special Rapporteur gave further justification for the exclusion of evidence that might cause the criminal trial to be abandoned:
"[The Special Rapporteur] recall[s] that evidentiary rules – and their incorrect interpretation -- should not reward police and investigator misconduct; the exclusion of evidence at trial is one effective means to combat misconduct and abuses in the course of a criminal investigation" (SRT, Visit to Kyrgyzstan, A/HRC/19/61/Add.2, 21 February 2012).
Finally, after a visit to Turkey, the Special Rapporteur gives valuable instruction on how to proceed with an allegation, or suspicion, that evidence is obtained by torture or ill-treatment. He recommended:
"Prosecutors and judges should not require conclusive proof of physical torture or ill-treatment (much less final conviction of an accused perpetrator) before deciding not to rely as against the detainee on confessions or information alleged to have been obtained by such treatment; indeed, the burden of proof should be on the State to demonstrate the absence of coercion" (SRT, Visit to Turkey, E/CN.4/1999/61/Add.1, 27 January 1999).
The Subcommittee on Prevention of Torture has also considered the risks involved when States rely too heavily on confessional evidence in securing a conviction to promote investigations led by evidence, rather than confessions. In their report on a visit to the Maldives, the SPT provided:
"The SPT considers that the possibility of criminal conviction solely on confession may open the way for certain individuals to abuse the process by trying to extract confessions by ill-treating persons deprived of their liberty. In this respect the SPT would like to highlight the prohibition against taking undue advantage of the situation of a detained or imprisoned person for the purpose of compelling him to confess, to incriminate himself otherwise or to testify against any other person and the principle that no detained person while being interrogated shall be subject to violence, threats or methods of interrogation which impair his capacity of decision or his judgment. The SPT considers an evidence-led and not confession-led approach to criminal investigation to be one of the fundamental safeguards, as it would render having recourse to extracting confessions by means of ill-treatment meaningless and thus reduce considerably the risk of ill-treatment of persons in police custody" (SPT, visit to the Maldives, CAT/OP/MDV/1).