Social scientists point to studies showing that prison sentences longer than, say, 20 years do not have any deterring effect on crime. Since prison does not rehabilitate (quite the opposite) it makes no logical sense to have sentences longer than that, the argument goes.
This is a short-sighted view. Harsher prison sentences make sense from a eugenic point of view. Get rid of the criminal element in the gene pool and the society will prosper. Europe had been doing this for hundreds if not thousands of years--until recently. They have forgotten what it was that made their countries into what they are today.