Does Strengthening Self-Defense Law Deter Crime or Escalate Violence? Evidence from Castle Doctrine

From 2000 to 2010, more than 20 states passed laws that make it easier to use lethal force in self-defense. Elements of these laws include removing the duty to retreat in places outside of one's home, adding a presumption of reasonable belief of imminent harm, and removing civil liability for those acting under the law. This paper examines whether aiding self-defense in this way deters crime or, alternatively, increases homicide. To do so, we apply a difference-in-differences research design by exploiting the within-state variation in law adoption. We find no evidence of deterrence; burglary, robbery, and aggravated assault are unaffected by the laws. On the other hand, we find that homicides are increased by around 8 percent, and that these homicides are largely classified by police as murder. This suggests that a primary consequence of strengthened self-defense law is a net increase in homicide. Finally, we present back-of-the-envelope calculations using evidence on the relative increase in reported justifiable homicide, along with assumptions about the degree and nature of underreporting, to assess whether the entire increase was legally justified.