States that reduced penalties for simple possession of marijuana saw a notable decline in cases where domestic violence victims suffered serious injuries, a new study reports.

Despite a pervasive misconception stemming from the “Reefer Madness” days that cannabis use causes violent behavior, researchers at the University of Pennsylvania found that domestic assaults resulting in broken bones, laceration and other severe physical trauma dropped after marijuana was decriminalized.

Interestingly, while the research shows a reduction in serious injuries, overall incidents of domestic violence did not decline.

“When we considered assaults at all levels of seriousness, our results showed that the policy intervention had no effect on violence,” the study’s authors wrote. “However, this masked more striking and significant results concerning the most serious types of violence—decriminalization of marijuana reduced domestic assaults involving serious injury by some 20%.”

Cannabis decriminalization was also linked to a 40.7 percent reduction in severe domestic violence incidents where alcohol was a factor, as well as a 23.1 percent reduction in incidents involving a weapon.

“Within incidents with a serious injury, the number of incidents where the offender was under the influence of alcohol or used a weapon also declined significantly,” the researchers wrote.

The results “contrast with previous literature showing null effects of marijuana usage on violence or aggression,” the study says.

The research takes a closer look at how domestic violence, an issue that touches millions of Americans each year, may be impacted by changes in marijuana laws across the country. Past studies have shown alcohol to be a contributing factor in violence. As it stands to reason that marijuana might serve as a substitute for alcohol if easily accessible, the current study’s authors hypothesized that “marijuana liberalization will have the unintended effect of reducing domestic violence.”

For a “more meaningful measure of violence,” they focused on quantifying “victim injury seriousness” rather than the total number of domestic violence incidents.

Researchers looked at a number of data sets to inform their work, including what states had approved medical cannabis and marijuana decriminalization up to 2016. They also analyzed federal crime data to see how many cases of violence involving romantic partners occur each year, and pulled out certain characteristics, including the extent of injuries and whether alcohol or a weapon was involved. Ultimately, their analysis covered domestic assaults in 25 states between 2005 to 2016.

According to the study, which is listed as “preliminary” and has not been peer reviewed or published in a journal, states with legal medical access to marijuana did not see a similar decline in the severity of domestic assaults. The authors speculate it may be because access remains restricted to certain people.

Decriminalization, however, was associated with “a significant effect”—in fact, domestic violence incidents that resulted in serious injury fell by 22.5 percent after states passed laws removing the threat of jail as a punishment for cannabis possession. The total number of assaults did not significantly change; they just appeared to be less severe.

But what’s behind that drop in serious injuries?

“Despite the longstanding debate over whether marijuana contributes to violence, the medical literature suggests that marijuana is effective as a short-term sleep aid and may contribute to excessive daytime sleepiness,” the study’s authors wrote. “By making would-be assailants sleepier, marijuana consumption may make the nature of assaults less serious and injuries less severe. This is likely the simplest explanation and is certainly incomplete.”

“This substance-based mechanism is behavioral in nature, and is premised on marijuana decriminalization increasing consumption to the extent that assailants are more likely to be under the influence of marijuana at the time of assault,” the researchers wrote.

However, they added, it’s impossible to test their theory with the crime data set they used because it does not differentiate the use of marijuana from other drugs.

While the study calls for more research on the effect cannabis use has on violence, the authors do point out that “[d]omestic assaults are less likely to inflict serious injury on the victim if a weapon is not used.”

In other words, marijuana decriminalization could potentially save the lives of countless people: Every month in the U.S., 50 women are shot and killed by intimate partners.

Photo courtesy of WeedPornDaily.