A common argument is that an armed population will deter criminal activity as the potential criminal takes into account the probability of having his head blown off. The evidence, however, suggests such effects aren't significant (e.g. Hoskin, 2011, Household gun prevalence and rates of violent crime: a test of competing gun theories, Criminal Justice Studies, Vol. 24, pp. 125-136).
Simple question: Why aren't deterrence effects supported?