"Given that the current results failed to uncover a crime exacerbating effect attributable to MML [Medical Marijuana Legalization], it is important to examine the findings with a critical eye. While we report no positive association between MML and any crime type, this does not prove MML has no effect on crime (or even that it reduces crime). It may be the case that an omitted variable, or set of variables, has confounded the associations and masked the true positive effect of MML on crime. If this were the case, such a variable would need to be something that was restricted to the states that have passed MML, it would need to have emerged in close temporal proximity to the passage of MML in all of those states (all of which had different dates of passage for the marijuana law), and it would need to be something that decreased crime to such an extent that it ‘‘masked’’ the true positive effect of MML (i.e., it must be something that has an opposite sign effect between MML [e.g., a positive correlation] and crime [e.g., a negative correlation]). Perhaps the more likely explanation of the current findings is that MML laws reflect behaviors and attitudes that have been established in the local communities. If these attitudes and behaviors reflect a more tolerant approach to one another’s personal rights, we are unlikely to expect an increase in crime and might even anticipate a slight reduction in personal crimes."
Robert G. Morris, Michael TenEyck, JC Barnes, and Tomislav V. Kovandzic, "The Effect of Medical Marijuana Laws On Crime: Evidence From State Panel Data, 1990-2006," PLoS ONE 9(3): e92816. March 2014. doi: 10.1371/journal.pone.0092816.