Police in Washington and Colorado appear to be closing more cases since the legalization of marijuana in both states, according to an analysis of FBI crime clearance data by researchers at Washington State University.
To do the study, the researchers examined monthly FBI crime data from 2010 to 2015. They performed a statistical analysis looking at the trends in clearances for different crimes before and after marijuana was legalized. Similar data isn’t available at the local level.
A clearance rate is the ratio of crimes ending with an arrest, or cases “closed,” with the total number of reported crimes recorded by police. The higher the rate, the more cases are cleared, and researchers and police agencies use the rate as a measure of performance.
There were no immediate changes in clearance rates after the laws passed, but Washington’s clearance rate for motor vehicle thefts jumped 5 percent.
Longer term, the trend in property crime clearance rates jumped in Colorado, then continued to climb, where Washington’s trend largely levels out to the prior trend over time.
Violent crime clearance rates were declining in both states before legalization. Post-legalization, that decline stabilized in Colorado and started to reverse itself in Washington. There’s no similar shift in the country as a whole.
The differences are more pronounced for specific crimes, with the most striking jumps in clearances for burglary and motor vehicle theft.
Furthermore, there appeared to be no decline in clearance rates for either states after legalization. David Makin, a professor at WSU’s Department of Criminal Justice and Criminology and one of the study’s authors, was quick to note that the relationship found in the research shouldn’t be interpreted as causal.
“It’s just saying there’s a relationship between legalization and what we see as visual and statistical improvements in clearances for a range of crime,” he said.
Figuring out what that jump in closed cases means, he said, will require more research.
Vancouver Police Department spokeswoman Kim Kapp said the department’s crime analysts looked at the research, and said the department doesn’t have the data to perform a similar analysis.
The analysts, as did WSU’s researchers, noted that it’s difficult to parse out the effects of staffing, policy or other internal factors and how they relate to clearance rates.
It’s one of several limitations the researchers noted for the study.
There’s little research on clearance rates as a function of law enforcement agency resources, or for clearance rates in nonlethal crimes. Furthermore, not all agencies report their clearance data to the FBI. Washington’s data were less complete than Colorado’s, covering about 60 percent of the state’s population to the Colorado data’s 80 percent.
Not every clearance reported in the data set used is connected with a case from that same month. But considering about 88 percent of clearances are within a month of a crime’s reporting, any error is likely small, they wrote.
It’s also possible there was some other shift in late 2012, when both states passed their legal marijuana laws. There appeared to have been no major public policy changes in the two states at the time, the researchers found, but some other combination of factors might explain changes in clearances.
Makin said the team plans to add more clearance data by working directly with agencies and increasing their time scales.
Also, they plan to check with other states that have legalized recreational marijuana since 2012.
“We might see that it didn’t have such a substantial impact. But that leads to more important research,” Makin said.
Any large public policy change has unintended, or unseen, consequences, he said, explaining that there’s a lot more work to be done before researchers, or policymakers, can responsibly make conclusions about legalization’s effects.
“Think about how long we have spent studying alcohol. We’re still studying it,” he said. “And yet we somehow think in a span of a few short years we can answer everything about the impact of marijuana on public safety and public health.”
