I don't remember the statistics off the top of my head (it has been a long time since I looked them up) but most people who enter the prison system are "rehabilitated" in that they don't get caught committing another crime and the majority of people serving out prison sentences are repeat offenders ...
The explaination I have heard for this is that most people "break the rules" under the belief that they will never get caught and/or there will be no serious consequences for their actions, and the act of being caught and the threat of consequences is enough to prevent future crimes by these individuals; while a smaller portion of the population is driven to "break the rules" because of some form of compulsion and no consequence will make these individuals act responsibly.
If this theory is correct, giving people probation on their first offence (for non-violent/sexual crimes), 1/2 sentences for their second offence, a full sentence for their third offence, and progressively longer sentences after that would probably be the best approach; and the underlying principle of this system would be to protect society from crime rather than punish criminals, and the only real punishment is the seperation from society (in other words, besides considerations for the safety of other prisoners and guards prisoners don't have to be really denied much).
