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. 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."