A new study from American University found that the tightened gun-control measures that went into effect in Massachusetts six years ago had little effect on the violent crime rate in the state, raising questions about enforcement of these laws.

“Gun violence remains at the forefront of the public policy debate when it comes to enacting new or strengthening existing gun legislation in the United States,” said Janice Iwama, assistant professor of justice, law, and criminology at AU, who conducted the study. “Yet the political polarization and relatively limited scholarly research on guns and gun violence make it difficult for policymakers and practitioners to enact and implement legislation that addresses the public health and safety issues associated with gun violence.”

The study, published in Justice Quarterly, used modeling and FBI data from 2006 to 2016 to examine the impact of the 2015 gun law on crimes including murder, rape, robbery and aggravated assault.

The law, enacted in the wake of the Sandy Hook, Conn., school shooting by former Massachusetts Gov. Deval Patrick, paved the way for the creation of a web portal for gun dealers to check the validity of a firearm license and track sales and transfers of firearms. It also tightened requirements for background checks on the sale of firearms and licensing procedures.

Iwama, who authored the study, noted that the entire country, including the Bay State, experienced a drop in crime since the 1990s. Still, Massachusetts had 287,000 violent crimes from 2006 to 2016, including “198,402 aggravated assaults, 70,361 robberies, 19,107 rapes, and 1,698 instances of murder or non-negligent man-slaughter.”

About 1% to 5% of adult residents in each Massachusetts county have a firearms license.

She found that a one percentage point rise in denied licenses and denied licenses due to unsuitability increased robberies by 7.3% and 8.9%, respectively, after the new law took effect. For every other type of violent crime, including rape, murder and aggravated assault, she found no statistically significant change.

Iwama suggested the issue could be caused by uneven enforcement of the laws across counties, an overall lag in enforcement and/or because residents are obtaining firearms in neighboring states with looser gun laws. She recommended that policymakers revisit the legislation to ensure it’s being property applied and enforced.