Researchers at Texas A&M University, for a study published in the Journal of Human Resources, concluded that homicides had increased by 8 percent in the more than 20 states that had passed "castle doctrine" laws, many of which include Stand Your Ground provisions. That equals 600 additional homicides every year in those states, they wrote.
At the same time, however, the researchers found no detectable decrease in burglary, robbery or aggravated assault.
"Collectively, these findings suggest that incentives do matter in one important sense: lowering the threshold for the justified use of lethal force results in more of it," the authors concluded in the report. "On the other hand, there is also a limit to the power of incentives, as criminals are apparently not deterred when victims are empowered to use lethal force to protect themselves."
As the Texas A&M researchers noted in that 2013 study, their findings aligned with those of a 2012 study published by Georgia State University on Stand Your Ground laws.
The Georgia State researchers found that homicides in states with those laws increased by 7.1 percent. In particular, they detected an increase in homicides among white males, while the effect was mostly negligible among white females and blacks.
"Whether or not these killings should be considered justified in this case is beyond the purpose of this paper," they wrote. "However, it cannot be argued that the SYG laws are saving the lives of innocent people in this scenario as these individuals would not have been killed in the first place."
So were these increases in homicides made up entirely of justified homicides allowed under the new laws? Both studies were inconclusive, but the authors seemed doubtful. The Georgia State study noted that substituting assailants for innocents didn't account for the increase. The Texas A&M analysis concluded it was "unlikely, albeit not impossible."
The two sets of researchers relied on different data and methods to analyze these trends. The Texas A&M study relied on state-level crime data, covering the years 2000 to 2010, from the FBI Uniform Crime Reports. The Georgia State analysis used the Centers for Disease Control and Prevention's Vital Statistics data.
In different ways, they attempted to control for other factors. The Texas A&M study compared their findings to states that did not adopt these laws and whether adopting states had already diverged in their homicide rates. The Georgia State analysis contrasted Stand Your Ground states with others with more restrictive laws. It also notably found that Stand Your Ground laws did not lead to a greater prevalence of guns.
The studies have faced some criticism from conservative media, which focused on the studies' chosen data sources and methodology. For more details, read the Texas A&M and Georgie State studies in full.