Texas A&M University researchers say controversial ”stand your ground” laws have increased the number of murder and manslaughter cases – rather than serve as a deterrent to crime.

The study, which looked at 23 states that have passed the castle doctrine laws, comes at a time when many critics say such self-defense statutes encourage vigilantism and escalate violence. At the center of that debate is the case of Trayvon Martin, an unarmed black teenager who was fatally shot in February in Sanford, Fla., by neighborhood watch volunteer George Zimmerman.

Zimmerman, who initially was not arrested for the shooting, claimed he shot Martin in self-defense, permissible under Florida’s Stand Your Grand law that allows people to use deadly force when they feel threatened.

In Houston, a trial is underway that has also raised questions about self-defense laws. Raul Rodriguez, 47, must convince a jury that he was acting in self-defense when he shot his neighbor to death after a noise dispute two years ago in Huffman. In a video recorded that night, he can be heard telling a police dispatcher, “I’m standing my ground here. Now these people are going to go try and kill me.”

In their study, Texas A&M associate professor Mark Hoekstra and grad student Cheng Cheng analyzed crime data from 2000 to 2009, finding that murder and manslaughter cases increased between 7 to 9 percent in those states with castle doctrine laws. The castle doctrine term is derived from the idea that people have a right to defend their home or castle using lethal force, if necessary.

The homicide rates in states that had not adopted castle doctrine laws remained steady, the study found.

The study concludes that the laws “do not appear to offer any hidden spillover benefit to society at large.”

Hoekstra said it’s difficult to determine the reason for the increase, but there are a few possible explanations. He said homicides may have increased because more citizens used deadly force in a self-defense situations, instead of being required by law to retreat or use non-lethal force during an assault or other threatening circumstance. This was likely the intended purpose of the law, he said.

Another explanation could be that the number of homicides increased because force was used in situations that otherwise would not have been violent.

Hoekstra said he and Cheng began their research last fall, before the Trayvon Martin case, and ”before everybody started caring deeply about the issue.”

He said he was surprised not to see any deterrence in crimes: “I didn’t know what to expect.”

In the wake of the widely covered Martin slaying, state Rep. Garnet Coleman, D-Houston, has vowed to reverse Texas’ self-defense law during the 2013 legislative session. In 2007 , Texas altered its own Castle Doctrine law to say that Texans no longer have a “duty to retreat,” in the home or elsewhere, before using deadly force in a self-defense situation. Coleman was one of 13 in 2007 who voted against changing the law. It had wide support in both the House and Senate.

In Florida, Gov. Rick Scott has established the Citizen Safety and Protection task force to review that state’s controversial law, which has been in place since 2005. On Tuesday, Martin’s parents, who allege Zimmerman started the confrontation, testified before the panel. They are planning to present the task force with a 300,000-signature petition asking for the law to be repealed, or reformed to exclude those who initiate confrontations, the Associated Press reports.

The Texas A&M University researchers do not take a position on the statutes, but in the study say that “any evaluation of these laws ought to weigh the benefits of increased self-defense against the increased loss of life caused by the laws.”

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