In the last year, 15 states have enacted laws that expand the right of self-defense, allowing crime victims to use deadly force in situations that might formerly have subjected them to prosecution for murder.

Supporters call them “stand your ground” laws. Opponents call them “shoot first” laws.

Thanks to this sort of law, a prostitute in Port Richey, Fla., who killed her 72-year-old client with his own gun rather than flee was not charged last month. Similarly, the police in Clearwater, Fla., did not arrest a man who shot a neighbor in early June after a shouting match over putting out garbage, though the authorities say they are still reviewing the evidence.

The first of the new laws took effect in Florida in October, and cases under it are now reaching prosecutors and juries there. The other laws, mostly in Southern and Midwestern states, were enacted this year, according to the National Rifle Association, which has enthusiastically promoted them.

Florida does not keep comprehensive records on the impact of its new law, but prosecutors and defense lawyers there agree that fewer people who claim self-defense are being charged or convicted.