A new study has found a 7 to 9 percent increase in homicides in states that adopt ‘stand your ground’ law. The study by Texas A&M economist Mark Hoekstra also also found the laws have "zero deterrence" effect.

The abstract from "Does Strengthening Self-Defense Law Deter Crime or Escalate Violence? Evidence from Castle Doctrine" is below:

From 2000 to 2010, more than 20 states passed castle doctrine and stand-your-ground laws. These laws expand the legal justification for the use of lethal force in self-defense, thereby lowering the expected cost of using lethal force and increasing the expected cost of committing violent crime. This paper exploits the within-state variation in self-defense law to examine their effect on homicides and violent crime. Results indicate the laws do not deter burglary, robbery, or aggravated assault. In contrast, they lead to a statistically significant 8 percent net increase in the number of reported murders and non-negligent manslaughters.

George Zimmerman, the man who shot and killed Florida teen Trayvon Martin is using the ‘stand your ground law’ as part of his defense.

Stanford law professor John Donohue interview told NPR self defense pleas are common in states with ‘stand your ground’ laws.

"I’ve been hearing from defense lawyers around the country that if they happen to have a criminal defendant in a stand your ground jurisdiction, pretty much no matter what happens, you can say, ‘Well, I shot the guy, but I felt threatened and had a reasonable basis for fearing injury to myself,’" Donohue told NPR.

You can download a PDF of the study from the Texas A&M website.