A study analyzing crime rates in Colorado and Washington suggests legalizing recreational marijuana has a minimal effect on crime rates, if any.

One argument for legalizing marijuana is the assumption that it would lead to lower crime rates—decriminalizing the drug, so the theory goes, removes illegal trade and the criminal activities that go along with it. Indeed, a study published in The Economic Journal in 2017 found that states on the US-Mexico border that legalized medical marijuana saw a decrease in violent crimes of 5.6 to 12.5 percent.

However, if a new study is anything to go by, this drop in crime is not consistent. In Colorado and Washington, legalizing recreational use of the drug appeared to have little to no effect on the number of violent and property crimes.

Researchers from Washington State University, Stockton University, and the University of Utah came to this conclusion after comparing monthly crime rates in Colorado and Washington to those of 21 states that haven't legalized pot for medical or recreational use, both before and after legalization, to see if there were any trends that deviated in Colorado and Washington. The stats were collected from the FBI's Uniform Crime Report from 1999 to 2016.

The state of Colorado was one of the first to legalize marijuana. Citizens took to the polls on November 6 2012 to vote on a ballot initiative that would permit possession of up to an ounce of marijuana for those 21 and older. (Photo by Marc Piscotty/Getty Images) Marc Piscotty/Getty

The two states were chosen as subjects because they were the first to legalize the recreational use of marijuana and the industry that supports it, passing laws to that effect in 2012. Since then, nine more states have followed their lead—including Alaska, California, Illinois, Maine, Massachusetts, Michigan, Nevada, Oregon and Vermont—as well as DC. Several others look set to do so.

"In many ways, the legalization of cannabis constitutes a grand ongoing experiment into how a major public policy initiative does or does not accomplish its expected outcomes," said Ruibin Lu, assistant professor of criminal justice at Stockton University. "Given the likelihood of more states legalizing recreational marijuana, we felt it was important to apply robust empirical methods to parse out the effects of this action on crime in the first years after legalization."

The effects appear to be marginal—if they exist in the first place. The only exception to this rule was the rate of burglary in Washington, which declined following the legalization of pot. There were slight increases in property crimes in both states and aggravated assault in Washington immediately after legalization, but these were short-lived blips rather than permanent shifts, the study's authors found.

However, the study's authors do point out some limitations to their findings. One is that the results highlight broad, cross-state trends but may exclude changes that take place on a smaller scale. They say that they cannot rule out the possibility that legalization may have different effects within the state depending on the area and community. The analysis is also limited in the types of crime it considers—namely, serious crime. Rates of those driving while under the influence, for example, would not have been counted.

"This is but one study and legalization of marijuana is still relatively new, but by replicating our findings, policymakers can answer the question of how legalization affects crime," said co-author Dale W. Willits, assistant professor of criminal justice and criminology at Washington State University.