Confessions may be obtained by interrogation techniques that violate the defendant's free will or procedural rights. A defense attorney should be prepared to argue that these confessions are inadmissible as evidence against their client.
Confessions may be inadmissible for a variety of reasons but generally these can be classified into two categories. The first category of reasons asks the factfinder to determine whether the confession was voluntary in nature. If it is voluntary, then it may be admissible. If it was involuntary, then it should be inadmissible because it is inherently unreliable.
Defense counsel may argue that a confession is involuntary if it is the product of torture, coercion, duress or even police trickery. Each interrogation must be examined on its own facts. The most obvious case to argue for the inadmissibility of evidence is when a confession is the direct result of torture or other inhuman and degrading treatment. Cases of coercion or duress fall in the middle category where international jurisdictions differ as to what level of coercion or duress is necessary to trigger inadmissibility of a confession. Finally, in some jurisdictions police deception or trickery, even if not coercive, may lead the confession to be inadmissible.
In common law/adverserial criminal justice systems the defense attorney should argue that the confession should be exclusion. In a civil law / inquisitorial system the defense attorney should argue that the confession shoul be nullified.
In either case, failure to get a confession excluded from consideration at trial does not prevent the defense attorney from arguing that the confession is inherently unreliable and that the factfinder should consider the circumstances surrounding the interrogation when deliberating at the conclusion of the trial.
The second category of reasons asks the fact finder to examine the procedural aspects of the interrogation to determine if police followed procedures that are required under the law. Under this test the confession may be inadmissible even if was given voluntarily.
International Covenant on Civil and Political Rights
Article 14, Section 3 -
- In the determination of any criminal charge against him, everyone shall be entitled to the following minimum guarantees, in full equality:
- (a) To be informed promptly and in detail in a language which he understands of the nature and cause of the charge against him;
- (b) To have adequate time and facilities for the preparation of his defence and to communicate with counsel of his own choosing; **(c) To be tried without undue delay;
- (d) To be tried in his presence, and to defend himself in person or through legal assistance of his own choosing; to be informed, if he does not have legal assistance, of this right; and to have legal assistance assigned to him, in any case where the interests of justice so require, and without payment by him in any such case if he does not have sufficient means to pay for it;
- (e) To examine, or have examined, the witnesses against him and to obtain the attendance and examination of witnesses on his behalf under the same conditions as witnesses against him;
- (f) To have the free assistance of an interpreter if he cannot understand or speak the language used in court;
- (g) Not to be compelled to testify against himself or to confess guilt.
Examples of standards for the admissibility of confessions
No confession made to a police officer is valid as evidence at a trial. All confessions must be made to a Magistrate not below the rank of Judicial Magistrate. The Magistrate taking the confessions must give the accused due time out of the custody of the police, and make an effort to ensure that the accused was not coerced or intimidated in anyway, before receivint the confession. At the bottom of the confession the Magistrate must write out that he has informed the accused that this confession may be used against him and he is not obligated in any way to imcriminate himself.
Voluntariness Test - In the United States, a confession is admissible if a judge deems it to have been made voluntarily. In Brown v. Mississippi, 297 U.S. 278 (1936), the U.S. Supreme Court first excluded confessions procured and introduced in a state criminal case concluding that admission of the confessions would violate due process. The rule is grounded in three principals. First, exclusion of involuntary confessions tends to deter police misconduct. Second, a confession should be freely made by a rational person. Finally, confessions obtained with duress are inherently unreliable.
The admissibility of incriminating statements made at the time a defendant had his "rational intellect" and/or "free will" compromised by mental disease or incapacity is to be governed by state rules of evidence and not by the Supreme Court's decisions regarding coerced confessions and Miranda waivers. Also, coercive police activity is a necessary predicate to the finding that a confession is not "voluntary" within the meaning of the Due Process Clause of the 14th Amendment. 
When determining whether a confession was voluntary, the court should look at the "totality of the circumstances".  In certain circumstances, police misconduct may be so egregious that the confession evidence should be excluded without regard for how that conduct the defendant. However, when conduct is less egregious, a court should consider if the conduct actually induced an involuntary confession.
McNabb-Mallory Rule - A second line of cases in the U.S. Federal Courts holds that a confession obtained during Federal custody is inadmissible if the defendant is not promptly produced in court after arrest. If argued under this line of cases, it is unnecessary to prove that the confession was, in fact, involuntary. It is enough to show that the police or prosecution unlawfully detained the suspect for a prolonged period, extracted a confession, and then attempted to use the confession against the defendant in court. See Federal Rule of Criminal Procedure 5(a):
Rule 5. Initial Appearance
(a) In General. (1) Appearance Upon an Arrest. (A) A person making an arrest within the United States must take the defendant without unnecessary delay before a magistrate judge, or before a state or local judicial officer as Rule 5(c) provides, unless a statute provides otherwise. (B) A person making an arrest outside the United States must take the defendant without unnecessary delay before a magistrate judge, unless a statute provides otherwise.
- India Criminal Procedure Code Section 51
- Colorado v. Connelly, 479 U.S. 157 (1987)
- Haynes v. Washington, 373 U.S. 503 (1963)
- McNabb v. United States, 318 U.S. 332 (1943), Mallory v. United States, 354 U.S. 449 (1957)