There are two issues there that have been hotly debated in the United States. It seems they've not been researched and just merely assumed here, but it's a hot debate in the U.S. The first one is, does putting more criminals in jail longer reduce the crime rate? And the best evidence appears to be yes, it does.
This is not rehabilitation. This is not deterrents. This is simply incapacitation. When a person is in jail, he does not commit crimes outside. The more violent criminals, the more serious criminals inside, the fewer outside. The Marvell and Moody study is the most thorough and the most recent on that issue. If you want to look at that or have one of your aides look at that, you will see that.
The second related question is, what happens when they get out? There's a theory in criminology that we should keep people in prison for only short periods of time, because prison's not nice and it irritates them to be inside, and when they get out, they get nasty. So in some of these studies we're looking at what happens when people get out. Is there an increase in crime in the year certain numbers get out? And no, there does not seem to be the empirical support for that.
But as I said, these are debatable issues and there are many reputable researchers on all, not just both, sides.