Opponents of marijuana legalization, particularly members of law enforcement, frequently claim that liberalizing cannabis laws will lead to an increase in incidences of criminal activity, such as burglary, robbery, and driving under the influence. But two recent scientific papers report that just the opposite is true.

In the most recent paper, published online in March in the scientific journal PLoS ONE, researchers at the University of Texas at Dallas reported that the enactment of laws legalizing people’s access to medical marijuana is not associated with any rise in statewide criminal activity, and that it may even be related to reductions in incidences of violent crime.

Investigators tracked crime rates across all 50 states in the years between 1990 and 2006, during which time 11 states—Alaska (1998), California (1996), Colorado (2000), Hawaii (2000), Maine (1999), Montana (2004), Nevada (2000), Oregon (1998), Rhode Island (2006), Vermont (2004), and Washington (1998)—legalized the use, home cultivation, and (in some cases) the retail dispensing of marijuana for medical purposes. (A total of 20 states and the District of Columbia have now approved similar laws.) Authors reviewed FBI Uniform Crime Report data to determine whether there exists any association between the enactment of medicinal cannabis laws and rates of statewide criminal activity, specifically the number of reported crimes involving homicide, rape, robbery, assault, burglary, larceny, and auto theft. Their analysis is the first to look at multiple offenses across multiple states and time periods to determine whether medical marijuana legalization impacts state crime rates.

Authors reported that the passage of medical marijuana laws is not associated with an increase in any of the seven crime types assessed, but that liberalized laws are associated with decreases in certain types of violent crime. Authors wrote:

“The central finding gleaned from the present study was that MML (medical marijuana legalization) is not predictive of higher crime rates and may be related to reductions in rates of homicide and assault. Interestingly, robbery and burglary rates were unaffected by medicinal marijuana legislation, which runs counter to the claim that dispensaries and grow houses lead to an increase in victimization due to the opportunity structures linked to the amount of drugs and cash that are present.”

They 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 notfound to have a crime exacerbating effect on any of the seven crime types. On the contrary, our findings indicated that MML (medical marijuana legalization) 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. 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.”

Commenting on the findings in an accompanying news release, the study’s lead author, Robert Morris, associate professor of criminology, said, "The results are remarkable.… It takes away the subjective comments about the link between marijuana laws and crime so the dialogue can be more in tune with reality."

The paper’s results, specifically the notion that cannabis liberalization may lead to a reduction in alcohol-induced criminal activity, are similar to those previously documented in a study published last year in the Journal of Law and Economics. In that study, researchers at Montana State University, the University of Oregon, and the University of Colorado assessed whether the enactment of medical cannabis laws was associated with a reduction in incidences of alcohol-related traffic fatalities for the years 1990 to 2010. It was.

Authors wrote: “[T]he legalization of medical marijuana is associated with a 13.2 percent decrease in fatalities in which at least one driver involved had a positive BAC level. … Why does legalizing medical marijuana reduce traffic fatalities? Alcohol consumption appears to play a key role. … Using individual-level data from the Behavioral Risk Factor Surveillance System (BRFSS) …, we find that MMLs (medical marijuana laws) are associated with decreases in the probability of [an individual] having consumed alcohol in the past month, binge drinking, and the number of drinks consumed.

“The negative relationship between the legalization of medical marijuana and traffic fatalities involving alcohol lends support to the hypothesis that marijuana and alcohol are substitutes,” authors concluded.

Separate studies have also failed to link the establishment of marijuana retail dispensaries with any alleged increases in criminal activity. A 2011 study of crime rates in Los Angeles published by the Rand Corporation "found no evidence that medical marijuana dispensaries in general cause crime to rise." (Disturbingly, Rand withdrew the paper shortly following its publication because its findings were publicly criticized by the Los Angeles city attorney's office, which had long claimed that these operations were magnets for various street crimes.) A followup study commissioned by the federal government in 2012 similarly reported that the proliferation of cannabis dispensaries is not associated with elevated rates of either violent crimes or property crimes.

It concluded: "There were no observed cross-sectional associations between the density of medical marijuana dispensaries and either violent or property crime rates in this study. These results suggest that the density of medical marijuana dispensaries may not be associated with crime rates or that other factors, such as measures dispensaries take to reduce crime (i.e., doormen, video cameras), may increase guardianship such that it deters possible motivated offenders."

Most recently, localized crime data from Colorado once again reinforced the notion that pot legalization isn’t associated with increased criminal activity. According to a report in Vox.com, incidences of both violent crime and property crimes have dipped slightly during the three-month period following that state’s enactment of full-scale marijuana legalization. Yet members of law enforcement are still continuing to publicly claim otherwise. Some things never change.