[s-v]

That study only looks at the deterrent effect on existing criminals, not the general populace. I think there is confusion about the word "deterrent". I always thought about 'deterrent' in this context as: "Prisons are a deterrent.. just the thought of serving time in jail will deter anyone from committing a crime." Whereas the study you linked reports: "We have found that criminals who serve longer sentence are no more deterred from committing another crime than those who served short sentences."

[Pwnguinz]

Fair enough. There are plenty of studies out there regarding this topic, though a definitive conclusion is probably beyond any one single study. See papers Justin McCrary have published (he's done quite a bit of work in this area).

One such paper takes an interesting approach, combing the criminal records to see if the fact that deterrence is stiffened after 18 has an deterrent effect on crime rate by comparing juvenile offenders that are months pre-18 and 'adult' offenders that have just recently turned 18. The finding is 'surprisingly' a negative. Only very minor drop off rate is seen.

You can read the paper here: http://www.nber.org/papers/w11491.pdf