I'm nearly certain that a pair of students cheated on my final exam—the probability they had so many identical answers on the multiple-choice exam is infinitesimal. If I pursue them, it takes me time, and there’s no assurance they will be found guilty. If I don’t, I’ll feel badly about giving them an undeserved grade. Even for fairly risk-averse students, cheating seems like a good idea. I doubt that most cheating is caught; and unless the penalty is very severe (expulsion) and/or the students’ costs of contesting the accusation are high, and both are very well-publicized, the incentive to cheat for students with weak consciences seems overpowering. To salve my own conscience I’ll report them, although it’s probably a waste of my time; but I doubt that reporting them will deter their future cheating or deter others very much.