Ever since the Reefer Madness days, there have been lingering claims that smoking weed leads to violence and crime.

Meanwhile, advocates of marijuana legalization often argue that regulating cannabis will thwart the black market – and the crime that goes along with it.

But a recent study out of New Zealand suggests neither claim is true, with data that shows legalizing medical marijuana doesn’t appear to make residents any more or less safe.

“As far as policy, this means politicians should not just say legalization is something that is going to increase a lot of crimes, which is clearly not the case,” said Yu-Wei Luke Chu, an assistant professor in the School of Economics and Finance at Victoria University of Wellington, who helped conduct the study. “But it is probably also not true that it’s going to stop a lot of problems.”

A poster for the 1968 film “Maryjane” is on display at the Rose Bowl Flea Market.

There was one exception: In California, the study indicates that legalizing marijuana helped lower violent and property crimes by 20 percent.

Wilbur Townsend – a research analyst with the nonprofit Motu Economic and Public Policy Research group and coauthor of the study – believes that may be in part because statistics show that trafficking marijuana across the border from Mexico tends to involve more violence than illicit sales within or between states. So shifting the weed-loving state of California’s supply chain from cross-border traffickers to local marijuana farms may have had a bigger impact on crime than in other parts of the country.

The study suggests another reason legalizing marijuana doesn’t appear to have increased crime as some feared it would is because, at least in the short-term, the drug tends to make people less violent and aggressive. Also, Townsend and Chu said that legalizing marijuana may give police more time to focus on deterring more serious crimes.

Wilbur Townsend, courtesy of Motu Economic and Public Policy Research

To study the link between crime and marijuana legalization laws, Townsend and Chu started by gathering crime data through the FBI’s Uniform Crime Reports. They analyzed data from 1988 to 2013, giving them a baseline to look at trends before California became the first state to legalize medical marijuana in 1996.

After controlling for other relevant factors, they looked at national figures and compared data over time between states that had legalized marijuana and states that hadn’t. And they found no substantial changes – positive or negative – in either violent or property crime after the passage of medical marijuana laws.

Their working paper only factors in medical marijuana legalization, since Chu said recreational marijuana laws are still too new to yield reliable data, with the first sales starting in Colorado in 2014. But given what he knows, he said he doesn’t have any reason to suspect recreational legalization will have a more significant effect on crime.

Chu first started thinking about the impacts of medical marijuana legalization when he was earning his doctorate in economics from Michigan State University.

Yu-Wei Luke Chu, courtesy of Victoria University of Wellington.

He saw legal dispensaries start to open in the state and opted to do his dissertation on whether more people who aren’t patients start using cannabis once medical marijuana is legal. They do, he found, with his 2014 study showing an increase in adult males who weren’t medical marijuana patients being arrested and treated for marijuana after it became legal as medicine.

Next, in 2015, he examined whether medical marijuana leads to use of harder drugs – a.k.a., the gateway theory. It doesn’t, he found, instead decreasing arrests and treatment for heroin, in particular. Chu was initially surprised by those results, and went back to make sure he hadn’t made an error in his research. But multiple studies have since confirmed the same thing: Heroin and opioid use tends to go down in legal marijuana states.

In 2016, along with researching the correlation between marijuana legalization and crime states, he and a colleague studied how medical marijuana laws impact the way high school and college students spend their time. They found that part-time college students in states with medical marijuana laws on the books spend less time studying and more time watching TV. But he found no impact on how high school or full-time college students spend their time in legal weed states.

Chu and Townsend said they hope their work can help regulators make more informed decisions about marijuana policy, with New Zealand – and many others countries around the world – looking to the United States as they consider their own legalization laws.