Well there is the google machine. Go take a stab at that if you like. 
Deterrence efficacy is inconclusive -- and that's with harsh punishments for crimes that harm others. Here we have a 'crime' that is really just self-abuse. Mostly deterrence only works when people think they are going to get caught. Since it is nearly impossible to police drug possession it probably has a negligible impact. 
As for the second point, we have plenty of countries and regions to study and the evidence strongly supports this statement. Where decriminalization/legalization takes place, locals do not suddenly start using.
If heroin was made legal tomorrow, would you suddenly be grabbing a syringe?!