Would you argue that if crimes weren't punished and the police would just catch you and then release you immediately, no hard feelings, it would have no effect on criminality ?
- It's a self-selected sample. Obviously, caught criminals are already people who commit crimes despite deterrence. As such, again obviously, all the people who might have commited the crime but were deterred, aren't part of it.
- Except for a small minority, people don't just harm themselves. It means that people who commit crime, do it because they think they will get away with it (or don't think at all). It means that the severity of the punishment count, but only insofar as the criminal think he's going to be caught. Being caught without punishment would obviously deter no one, but punishment is meaningless if you aren't caught to endure it.
- Related to above and agreeing with your last statement : you can be deterred through the feeling of "too big a risk" if the sentence is harsh, but psychologically it's about very broad categories. Like "I'll pirate, because even if I'm caught, it's just a fine" - if piracy was punished by being dismembered, probably that the gain wouldn't outweight the risks. But once we are in "pretty big risks" (like 15 years in jail), the only way one will do a crime is if he already think he's a good chance of evading justice, and the gains are big enough to take the risk. At this point, yeah, harsher penalties are likely to have little effects.