"[3] In the United Kingdom and the Commonwealth realms, the term is to turn Queen's or King's evidence, depending on the sex of the reigning monarch.
In the 1980s, about 30 members of paramilitary groups (both loyalist and republican) gave the authorities evidence against their former comrades in exchange for a more lenient sentence (or immunity from prosecution), as well as a new identity to protect them from retribution.
[14] Among the highest-ranking Mafia members to turn state's evidence was Salvatore Gravano ("Sammy the Bull"), an underboss of the Gambino crime family who pleaded guilty to 19 murders and agreed to testify against family boss John Gotti; as a result, Gravano was sentenced to 5 years and Gotti was sentenced to life imprisonment in 1992.
The Justice Minister at that time, Diether Posser, wanted to enable witnesses to testify in return for a lenient sentence if there was little chance of successfully prosecuting a criminal.
[18] Additionally, Germany lacked a witness protection program such as that found in the United States; Ulrich Schmücker was murdered after he informed on his former associates in the 2 June Movement.
[26][27] The incentives to turn state's evidence, or to not to do so, are explored in the famous prisoner's dilemma, created by Merrill Flood and Melvin Dresher.