Opponents of medical marijuana envision all kinds of insidious ways that legalizing the drug might lead to crime. Make marijuana more accessible, and more people will use it. If more people use it, more will tumble through the weed "gateway" to cocaine, or worse. Those people will then engage in crime to fund their hard-drug habits, or violence in the service of getting the stuff.
Furthermore: Once word gets out about medical dispensaries, those locations will become hotspots for criminals who now know exactly where to find prey carrying cash and drugs. Same goes for grow houses, which just invite property crime.
Pondering all of these dark possibilities, it's no wonder anyone suspects mayhem in medical marijuana laws. Actual historic crime data, however, suggest there's no evidence that legalizing the drug for medicinal purposes leads to an increase in crime. In fact, states that have legalized it appear to have seen some reductions in the rates of homicide and assault.
These findings come from a nationwide study published Wednesday in the journal PLOS One (which is notable for the fact that no one seems to have done this crucial analysis before). Researchers at the University of Texas at Dallas looked at the FBI's Uniform Crime Report data across the country between 1990 and 2006, a span during which 11 states legalized medical marijuana. Throughout this time period, crime was broadly falling throughout the United States. But a closer look at the differences between these states – and within the states that legalized the drug before and after the law's passage – further shows no noticeable local uptick among a whole suite of crimes: homicide, rape, robbery, assault, burglary, larceny, and auto theft.
The robbery and burglary findings are particularly interesting, as those are the crimes we'd most likely expect to see outside of medical dispensaries. But what about the apparent declines in homicide and assault?
The researchers, Robert G. Morris, Michael TenEyck, J.C. Barnes and Tomislav V. Kovandzic, caution that this may be a mere statistical artifact of their analysis. But there's also a plausible explanation:
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.
Their analysis controlled for other potentially confounding factors: employment and poverty rates in each state, income and education levels, age and urban demographics, per-capita rates of prison inmates and police officers, as well as per-capita rates of beer consumption (per the Beer Institute).
The results don't definitely prove that medical marijuana has no effect on crime (or that it might even reduce it). Maybe the researchers failed to account for some other crucial variable here, some common factor that further depressed crime in precisely these 11 states, precisely after the moment that each passed a medical marijuana law, masking the actual crime increase caused by the policy. Or, there's this interpretation, from the authors:
Perhaps the more likely explanation of the current findings is that [medical marijuana] laws reflect behaviors and attitudes that have been established in those societies. If these attitudes and behaviors reflect a more tolerant populace that is less likely to infringe on one another’s personal rights, we are unlikely to expect an increase in crime and might even anticipate a slight reduction in personal crimes.