The paper “The Cannabis Effect on Crime: Time-Series Analysis of Crime in Colorado and Washington State” compares crime rates in two states that have legalized marijuana and 21 states that have not between 1999 and 2016. The researchers found:

Crime rates did not substantially change in Washington and Colorado after the legalization of marijuana, matching trends in states with stricter pot laws, according to a study conducted by Washington State University researchers.

The report, which used crime statistics reported to the FBI over a 20-year period and will be published in the journal Justice Quarterly, is intended to be the first empirical look at a question that has gained more prominence as Congress considers new banking laws for marijuana and complete legalization of the drug: What’s the effect on crime?

But it’s unlikely to settle the debate over legal pot’s influence on public safety, said Dale Willits, an assistant professor of criminal justice at WSU and one of the paper’s authors.

“I think it will be pretty clear evidence that, at a minimum, the sky isn’t falling,” said Willits.

The paper’s authors, which also include researchers at Stockton University in New Jersey and the University of Utah, argue that early studies on crime and marijuana legalization have been largely based on anecdotal evidence and evaluation of data over too brief of a time period. Those results have been used by policymakers and public safety officials to argue either that the drug has led to increased property crime in areas surrounding stores and grow operations fueled by substance abuse, or an abatement of violent crime based on marijuana’s calming influence. Some studies have shown the opposite to be true.

Researchers gathered crime data from 1999 to 2016 for 23 states, then compared rates between Washington and Colorado with the average rate in 21 other states that outlawed marijuana entirely. Washington and Colorado legalized possession of the drug through a vote of the public in 2012, and state-licensed retail stores opened up shop in both states in 2014.

The study found that, in Washington and Colorado, the rates of both violent and property crime stayed close to the average of other states prohibiting the possession or sale of marijuana after 2014. For the two-year interim between Initiative 502, which legalized the drug, and recreational stores opening in summer 2014, there was an increase in property crime, burglary and aggravated assault in Washington, according to the study, but that trend changed once shops opened. Burglary rates have actually declined more sharply in Washington since 2014 than states that haven’t legalized pot, according to the study.

Willits said the anomalies directly after legalization in crime rates are likely due to the short window of time (about 18 months) which can skew results. That’s also a limitation of the study as a whole, which only included data for two years of the legal marijuana market operating in Washington.

“We really need to see where this goes,” Willits said. “Right now we said, no short-term effects. And that’s really all we can say with the data we have. But I wouldn’t feel comfortable saying, in 10 years, we won’t see some benefit or cost from this we didn’t anticipate.”

The study has other limitations. It looks only at crimes that were categorized under what the FBI calls its uniform crime reporting system, which often times lists only the most serious offense when multiple infractions are committed. Spokane has since switched to the National Incident-Based Reporting System, or NIBRS, which incorporates multiple, lower-level charges in a single criminal incident into the data.

The research also doesn’t include driving under the influence charges, which some studies have shown increase in areas that have legalized the drug as law enforcement tries to crackdown on driving while high. It also doesn’t include data on intervention in the school system for underage users, a limitation that Spokane County Sheriff Ozzie Knezovich said should give readers pause when considering the WSU results.

“I can tell you, I’ve lost count of the number of pastors, business owners and citizens that have been in and out of my conference room talking about the quality of life when those stores open,” said Knezovich, the Republican sheriff who’s long been critical of marijuana’s legalization in the state. “It’s one thing to look at pure statistics and numbers, it’s another thing to come down and talk to people about life on the street.”

The research team has conducted interviews with law enforcement and policy makers in the state as part of other studies they hope to publish soon, Willits said. The National Institute of Justice, the research arm of the Justice Department, provided a $996,000 grant to the team in 2016 to conduct their studies, which Willits said he hoped would soon include data from schools about discipline and more localized results comparing, say, Spokane to Seattle over the same time period.

“We have heard, from not just the folks in Spokane, but all across the state, there is a general concern about access to youth,” Willits said. “You hear that from both liberal and conservative departments.”

Still, the paper’s publication should be used to inform what will inevitably become more specific arguments about whether remaining states should legalize marijuana for medical or recreational use, Willits said, or if Congress will take up the question under pressure from the states acting on their own.

“I hope that we put this out there, people look at it and say, ‘Oh, these are some parts that could be done better,’” he said. “Then they publish more papers, and it turns out somebody did something differently and it turned out a different result, that would be a great sort of debate and discussion to have.”