Oh, no, deterrence has no effect. Certainly general deterrence has no effect. What affects individuals is not actually the notion of the sentence; it's the likelihood of being caught. It's like the kitten or puppy, right? There's absolutely no point in swatting them on the nose, if that's what people still do with puppies. What's effective is a guarantee that they'll be caught and dealt with really fast. There is evidence that this type of deterrence works, but not the kind of specific deterrence we have.
On June 3rd, 2010. See this statement in context.