On 27 February 2017, United States Attorney General Jeff Sessions suggested that there is a link between legalized marijuana and violence, and that his conversations with Nebraska Attorney General Douglas Peterson — a long-time legalization opponent — helped bring this issue to his attention.

As reported by Huffington Post:

“I don’t think America is going to be a better place when people of all ages, and particularly young people, are smoking pot,” Sessions said to reporters Monday at the Department of Justice. “I believe it’s an unhealthy practice, and current levels of THC in marijuana are very high compared to what they were a few years ago, and we’re seeing real violence around that.” Sessions said he had a meeting on Monday with the attorney general of Nebraska, who is very concerned about marijuana flowing in from Colorado, which legalized weed in 2012. “Experts are telling me there’s more violence around marijuana than one would think and there’s big money involved,” he said. […] “You can’t sue somebody for drug debt; the only way to get your money is through strong-arm tactics, and violence tends to follow that,” Sessions said.

These comments offer two claims that require investigation. First is the specific information that Sessions attributes to Nebraska attorney general Peterson regarding “more violence around marijuana than one would think” as a result of legalization in neighboring Colorado. The second is that there exists a broad association between marijuana legalization and violent crime. Both claims lack credible data to support these assertions.

Peterson has already attempted unsuccessfully (along with then Oklahoma Attorney General and now EPA commissioner Scott Pruitt) to seek permission from the United States Supreme Court to sue the state of Colorado over alleged increases to crime. It is likely that the matters highlighted by this case are what Sessions is referencing.

On 18 December 2014, Oklahoma and Nebraska filed a motion for permission from the Supreme Court to sue the state of Colorado over damages they alleged to have suffered after their legalization of recreational marijuana, as reported by SCOTUSBlog.com at the time:

Two of Colorado’s neighboring states, arguing that the legalization of marijuana for Coloradans is causing crime problems across state borders, asked the Supreme Court on Thursday to allow them to file a lawsuit directly before the Justices. If the suit goes forward, Nebraska and Oklahoma’s filing said, the Court should rule that the commercial part of the Colorado scheme is unconstitutional and could no longer be enforced. Under the Constitution, states with legal complaints against other states have a right to sue them in the Supreme Court without first going through a lower court, but they need the Justices’ permission to do so. Nebraska and Oklahoma chose that route, their filing said, because no other court can protect neighboring states from the impact of Colorado’s marijuana marketing law and rules.

This filing alleges a number of damages suffered by the two neighboring states, which are best summarized in the written congressional testimony provided by Nebraska Attorney General Peterson to the United States Senate Caucus On International Narcotics Control on 5 April 2016, after his state’s attempt to sue Colorado was halted by the Supreme Court:

The diversion of marijuana into my state has been fierce. I have learned from consulting with law enforcement officials in Nebraska’s largest urban areas that significant amounts of Colorado marijuana have been diverted into our state. As lower quality marijuana has been displaced by high-potency products from Colorado, the street price has spiked. This, in turn, has contributed to an increase in drug trade-related violence, which law enforcement ascribes to higher prices and increased currency flows. […] In addition to marijuana being shipped out of Colorado by mail and commercial carrier, Nebraska residents involved in the drug trade are increasingly traveling to Colorado, making large purchases, and returning to arbitrage the higher prices available on the black market at home.

Of the above claims, the only statement backed up by corroborating evidence (as opposed to the more general “consultation with law enforcement officials”) in either this document or any of the prior court filings is that marijuana from Colorado is entering Nebraska in higher numbers since legalization. No publicly available empirical evidence has been introduced to demonstrate that this Colorado marijuana has led to any increase in drug-trade related violence in Nebraska.

In terms of violent crime in general, Peterson’s testimony did introduce one document, written by the federal drug enforcement task force known as the Rocky Mountain High Intensity Drug Trafficking Area Program, which reported on the law enforcement ramifications of recreational marijuana legalization within Colorado. Peterson used this document to imply (but not demonstrate) such issues could carry over (or already have) into Nebraska:

I have attached the executive summary of the September 2015 report by the Rocky Mountain High Intensity Drug Trafficking Area Program on the impact of Colorado’s marijuana legalization. The data contained in this report – gleaned from a broad and diverse array of federal, state, and local agencies – paints a grim picture of the results of Colorado’s scheme […]

Though this document does raise concerns about about a variety of issues, including unexpected challenges to law enforcement, drugged driving, increased potency of marijuana, and marijuana related hospital visits within Colorado (though these data come with caveats as well), the entirety of data related to the relationship between violent crime and legalization in that edition of the report, as well as the most recent report, is contained below:

From 2014 to 2015 [in all of Colorado]: Property crime increased 6.2 percent Violent crime increased 6.7 percent All crime increased 6.2 percent From 2014 to 2015 [In Denver]: Crimes against persons increased 7.5 percent Crimes against property increased 6 percent Crimes against society increased 15.6 percent All other offenses decreased 5.7 percent All Denver crimes increased 4.1 percent From 2013 to 2014 [In Denver]: Crimes against persons increased 15.1 percent Crimes against property decreased 3 percent Crimes against society increased 23 percent All other offenses increased 41 percent

While the statistics are not in dispute, the numbers offer no avenue to test causation — they are merely observations of year-to-year changes in crime rates. In the latest edition of this report, the task force makes this point explicitly (and in all caps) in a disclaimer:

NOTE: SOME OF THE DATA REPORTED IN THIS SECTION IS BECAUSE THERE HAVE BEEN SO MANY INQUIRIES ON THE PARTICULAR SUBJECT, SUCH AS CRIME AND SUICIDES. THIS IS NOT TO INFER THAT THE DATA IS DUE TO THE LEGALIZATION OF MARIJUANA.

In terms of published evidence, this report from Colorado is all that Nebraska (or Oklahoma) has officially cited in defense of the claim that marijuana legalization will lead (or has led) to an increase in violent crime in their state(s). Even if proper data existed, the adoption of recreational marijuana in Colorado is likely too recent to provide enough data to demonstrate anything statistically significant.

That means that the best data the public can rely on in terms of an association between liberalization of marijuana laws and violent crime comes from states which have legalized medical marijuana and have a significant marketplace for medical dispensaries. The most thorough investigation of that relationship is a 26 March 2014 study published in the journal PLOS One. That study investigated changes to crime rates in states before and after policies legalizing medical marijuana, concluding:

Results did not indicate a crime exacerbating effect of MML on any of the Part I offenses. Alternatively, state MML may be correlated with a reduction in homicide and assault rates, net of other covariates. […] 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.

In fact, this same study tested the very premise Sessions proposed in his statements to reporters (that the high amounts of money involved in the marijuana industry will attract further violent or property crime):

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.

This large-scale study echoes the findings of more preliminary or limited research, as well. Results from an investigation between medical marijuana legislation and violent crime was presented at the 2015 Fall Research Conference of the Association for Public Policy Analysis and Management. This study utilized state-level crime reports from 1994 to 2012 to test for an association between medical marijuana legalization and different forms of violent and nonviolent crime. They reported:

Preliminary results suggest MMLs lead to a significant decrease in arrest rates for violent crimes among both juveniles and adults. Initial estimates also point to a reduction in arrest rates for property crime, which is likely driven by a decrease in burglary and theft arrests among juveniles. Adults living in states with MMLs experience a significant decline in drug abuse violation arrests. Potential mechanisms to explain the decline in arrest rates include increased security at dispensaries and homes, the decreased level of alcohol consumption that accompanies the implementation of MMLs, and the role of law enforcement.

Another study, published on 13 January 2016 in the Journal of Drug Issues, compiled data from 11 states in the Western United States to “test hypotheses about potential effects on rates of violent and property crime”. These researchers concluded:

There is no evidence of negative spillover effects from medical marijuana laws (MMLs) on violent or property crime. Instead, we find significant drops in rates of violent crime associated with state MMLs.

Ultimately, the inherent weakness in the arguments attempting to make connections between marijuana legalization and violent crime is even betrayed by Peterson’s own congressional testimony, where he implores congress to “envision” the trouble that could potentially come:

With law enforcement in Colorado now confronting a “wave of illicit marijuana cultivation” on public lands, it is not difficult to envision linkages between a relaxed overall enforcement environment within Colorado and emboldened drug gangs increasing their production activities on public lands.

Historically, “envisioning” terrible things coming from marijuana (without demonstrating the reality of those things) has been a big part of campaigns against ending federal marijuana prohibition. Harry Anslinger, the first Commissioner of the U.S. Federal Bureau of Narcotics and the driving force behind marijuana prohibition in the 1930s, wrote a July 1937 editorial for The American Magazine that echoed similar speculative fears:

How many murders, suicides, robberies, criminal assaults, holdups, burglaries and deeds of maniacal insanity it causes each year, especially among the young, can only be conjectured…

Conjecture aside, no credible data exists that supports a significant association between increased violent crime and marijuana legalization. Furthermore, studies suggest that — so far — violent crime decreases in states with legalized medical marijuana. Until new research credibly suggests otherwise, the claim that a demonstrable link between the two exists will remain classified as false.