Many will find the answer easy—though they may disagree with each other on which the answer is. A standard line on the prisoner's dilemma rests on dominance: What you do won't affect what Twin does. Twin may rat or keep mum, but in either case, you yourself will do better to rat. Whichever Twin is doing, you would spend less time in jail if you were to rat than if you were to keep mum. Therefore the rational way to minimize your own time in jail is to rat.