Researchers. There have been innumerable studies on the subject - but really it's common sense. If you're a criminal you're unlikely to be particularly adept at the art of deliberation - otherwise you would choose a different way of life. But for evidence here's a somewhat recent review: http://www.jstor.org/discover/10.2307/1147698?uid=3739920&ui...

Also, interestingly, deterrence is not the primary reason most people push for harsher penalties. Rather it is punishment for its own sake: http://www.law.asu.edu/files/!NoTemplate/why%20do%20we%20pun...