roll over on sb

roll over on sb (to inform the authorities of a person's involvement in a crime, for personal benefit; to go to the police about someone doing something illegal) — донести на кого-л., сдать

...The basic building block of game theory starts with the Prisoner's Dilemma in which two criminals are caught and encouraged roll over on the other. The possible scenarios are that both do so, one does so and the other doesn't, and that neither does so. Prisoner A gains the most benefit from ratting out Prisoner B when B has not ratted him out. Second most benefit is if neither rats each other out, then if both rat each other out , and the worst outcome is if A keeps quiet and B rats him out (if both do he gains cooperation points).

Used mainly on television and in movies. I don't think I've ever heard it in real life. Any fan of police dramas or related stories will be very familiar with the term. Another TV/move term is 'I ratted him out', ie I told the cops about his illegal actibvities. Again, ordinary people don't say this.
Note this.
If Person A rolls over on Person B, it usually means that A and B were both involved in the crime.
In other words, if you simply see someone rob a bank and then you call the police, you are not 'rolling over' on the bank robber. (

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