Florida Homicide Rate Increased After Passage of 'Stand Your Ground' Law, Study Finds

— -- The rate of homicides, especially homicides by firearms, sharply increased in Florida after the "stand your ground" law was passed, according to a new study published today by The Journal of the American Medical Association Internal Medicine.

Passed in 2005, the "stand your ground" law in Florida allows residents to use force, including deadly force, if they "reasonably believe" they are at risk of death or great bodily harm. The law specifies that people have "no duty to retreat" from their homes or vehicles if they feel threatened.

To see if they could find any measurable effects in the homicide rate after the law's passage, researchers from the University of Oxford looked at Florida homicide data at various times from 1999 to 2014. They then compared the rates with those in four control states (New Jersey, New York, Ohio and Virginia) with no "stand your ground" laws.

By examining data from a database run by the U.S. Centers for Disease Control and Prevention, researchers found that after the law was passed in Florida, homicides increased approximately 21 percent — from an average of 82 homicides per month from 1999 to October 2005 to 99 homicides per month from October 2005 to 2014.

Additionally, they found the rate of homicide by firearm went up almost 41 percent, from a mean of 49 homicides per month to 69 during those periods.

Researchers found no similar increases in the four control states.

They also examined suicide data but did not find any comparable increase in Florida suicide rates or, more specifically, suicide by firearm rates after the law's passage. The authors acknowledged that it's possible there may be multiple factors that led to an increase in the Florida homicide rate.

"Circumstances unique to Florida may have contributed to our findings, including those that we could not identify," the study's authors wrote.

There are 23 states that have implemented some form of a "stand your ground" law, according to the researchers.

Jeffrey Swanson, a professor of psychiatry and behavioral sciences at Duke University, has studied the effects of gun-related policies and said the study published today was important.

"These 'stand your ground' laws have proliferated, and for the people who favor them, the point is that it's going to make people safer," he told ABC News today. "You can stand your ground if you perceive your life is being threatened, [but] what we're seeing here empirically is exactly the opposite."

While the researchers found an increase in homicide rates after the law's passage, they did not find enough evidence to definitively find the law caused the increase in the homicide rate.

While the study had limitations, Swanson said the research of the four control states makes the study stronger.

"They look at comparable trends in states that didn't pass the law and don't see the effect," he noted.

Swanson said these kinds of studies must continue in order for policymakers to make clear and informed decisions about legislation.

"This is always the question of balancing risk and rights," he said.

The National Rifle Association, which backed Florida's "stand your ground" law, did not immediately respond to ABC News' request for comment on the study.