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.

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