General deterrence comes merely from the general perception that the guilty are likely to be caught and punished. This perception doesn't come from the actual sentences that get handed down, because nobody ever hears about them. This perception comes more from TV, movies, and urban legend than from any actual data. Meanwhile, the specific deterrence of a given sentence is also negligible. Those who are deterred are not affected by the length of the sentence, but rather the desire to avoid ANY sentence in the future. But recidivists choose to re-offend regardless of the sentence they received, often ratcheting up longer and longer sentences without any deterrent effect.