The study looked at crime data from 11 medical marijuana states from 1990-2006 and concluded:
In sum, these findings run counter to arguments suggesting the legalization of marijuana for medical purposes poses a danger to public health in terms of exposure to violent crime and property crimes. To be sure, medical marijuana laws were not found to have a crime exacerbating effect on any of the seven crime types. On the contrary, our findings indicated that MML precedes a reduction in homicide and assault. While it is important to remain cautious when interpreting these findings as evidence that MML reduces crime, these results do fall in line with recent evidence  and they conform to the longstanding notion that marijuana legalization may lead to a reduction in alcohol use due to individuals substituting marijuana for alcohol [see generally 29, 30]. Given the relationship between alcohol and violent crime , it may turn out that substituting marijuana for alcohol leads to minor reductions in violent crimes that can be detected at the state level. That said, it also remains possible that these associations are statistical artifacts (recall that only the homicide effect holds up when a Bonferroni correction is made).
The important finding is that adopting medical marijuana doesn’t increase crime. This is good news for the efforts to advance medical marijuana and potentially good news for the marijuana legalization in general.
While you can’t guarantee that these results on medical marijuana will carry over into legalization of recreational marijuana, there is legitimate reasons to believe they should. There are several medical marijuana states, like California, where the laws are permissive so they have functioned almost as a de facto legalization. Ultimately, we will need to wait for some recent data from Colorado and Washington State on the effect of full legalization but these are some very reassuring results.
Photo by Sonya Yruel/Drug Policy Alliance used with permission