For a defendant’s confession to be admissible at trial, it must be “voluntary.” The notion is that the legal system should tolerate only those statements that are a product of free will. So, physical or psychological pressure by law enforcement agents can render a confession “involuntary,” and therefore inadmissible.
It’s a widely accepted principle that statements produced by way of law enforcement promises of leniency or immunity are inadmissible. For example, a defendant’s statement will likely be inadmissible if it was the product of an offer or promise, either explicit or implicit, of:
(For a different kind of immunity offer, see Immunity in Exchange for Testimony.)
The statement may be inadmissible even if the officer is sincere in the offer or promise. The New Hampshire Supreme Court, for example, found a confession inadmissible even though the officers followed through on their promises not to charge the defendant with additional crimes. (State v. Rezk, 150 N.H. 483 (2004); also see Foster v. State, 283 Ga. 484 (2008).)
In order for a promise or suggestion of leniency to invalidate a confession, the party making the promise or suggestion must be a “person in authority”—generally, a prosecutor or member of law enforcement. If, for example, the defendant’s cellmate, acting completely on his own, convinces the defendant to fess up by guaranteeing that a confession will lead to a reduced sentence, the defendant's eventual statement will probably be admissible.
Conversely, if the defendant reasonably believes that someone who doesn't have authority to follow through on an offer of leniency does have such authority, then any resulting statement may be inadmissible. Such a person might include someone who visits the defendant in jail representing himself to be a member of the prosecution, but who really isn’t.
Further, the law of the state in question may provide even more protection to defendants. In an Oregon case, for example, the state supreme court found that the actions of private investigators tainted the defendant’s incriminating statements. The investigators promised the defendant that the case wouldn’t go to the police if he confessed; the court ordered that his statements to the investigators be suppressed. (State v. Powell, 352 Or. 210 (2012).)
In many states, an offer or promise by a person in authority regarding a collateral benefit—and not a benefit concerning the present charges—has no effect on the admissibility of a confession. A collateral benefit would include an officer’s offer to help a robbery defendant’s sister find a job. In states subscribing to the “collateral benefit rule,” any statement the defendant were to give in response to the officer’s promise would be presumptively admissible because the promise didn’t relate to the robbery case.
On the other hand, the promise of a collateral benefit—when combined with other coercive elements—may render a statement inadmissible. Such elements might include hours upon hours of incessant questioning and intricate, harrowing lies. (Note, however, that the police are generally allowed to lie during interrogations—indeed, lying is one of their most powerful tools.) Further, in some states, promises of certain collateral benefits related to the defendant’s loved ones may make a statement inadmissible; an example could be a promise to free the defendant’s incarcerated daughter.
To strictly enforce the no-promises-of-leniency rule would be to deprive police officers of one their fundamental interrogation tactics. (For more on such tactics, see Tactics Police Use to Get a Confession.)
Consider these examples of situations in which courts have found defendants’ statements admissible:
Some courts won’t suppress statements by defendants that are premised on certain kinds of offers. (See, for example, U.S. v. Harris, 914 F.2d 927 (7th Cir. 1990), where the court said that the police can lawfully offer to reduce charges in order to obtain confessions.) And, an offer of leniency doesn’t automatically make a confession inadmissible—rather, the offer must have contributed to the confession. If a court determines that, given all the circumstances, the offer didn’t cause the confession, then the statement will likely be admissible.
In addition, an offer or promise of leniency may not invalidate a subsequent statement if the defendant requested the benefit, rather than the police volunteering it. Likewise, a defendant’s statements to the authorities will probably be admissible if they are part of a properly bargained-for plea deal where the defendant has legal counsel.