Background Homicide rates in the United States are unusually high for a developed country. Despite a gradual reduction in homicide rates over the last 100 years, homicide is still one of the leading causes of pre-mature death in U.S. citizens below the age of 40. There is continual debate about the consequences of stricter gun control measures, but less discussion about other kinds of legislative changes that may affect rates of homicide adversely. Since 2005, 22 U.S. states have amended their self-defence laws, removing the “duty to retreat” principle. “Stand your ground” (SYG) laws, specifically, give individuals legal immunity for use of lethal force in any situation in which individuals perceive a threat. Critics of this legislation are concerned that weakening the punitive consequences of using lethal force may serve to escalate aggressive and violent encounters, with significant implications for public health. This study examines the impact of the first SYG law on homicide rates in Florida.

Methods An interrupted time series analysis was performed using state-level rates of homicide between 1999 and 2014. Seasonally adjusted segmented Poisson regression models were used to assess whether the onset of the SYG law was associated with a deviation from the underlying trend. Stratified analyses were conducted to examine whether effects differed by ethnicity, age, and sex. To assess the impact of simultaneous cyclical factors, we have used non-equivalent control variables: alternative outcome variables that are sensitive to similar changes in cyclical factors, but not hypothesised to be influenced by the intervention under examination.

Results The mean monthly homicide rate prior to the change in legislation was 0.49 deaths per 100,000, with an underlying trend of 0.1% decrease per month. We found an abrupt increase in monthly homicide rates of 24.7% associated with the onset of Florida’s SYG law (RR 1.25; 95% CI: 1.17–1.33, CI: ≤ 0.001). Stratified analysis found increases in homicide across all demographic groups, with notable increases in Caucasian populations and in those aged 20–34 years.

Conclusion The removal of a “duty to retreat” in Florida has been associated with a substantial increase in the homicide rates. If the reported association is causal, we estimate that the implementation of Florida’s SYG law produced an additional 20 homicides per month, or a further 2,229 homicides since the law came into effect. This suggests that the change in the law may have served to escalate the severity of harm incurred from violent altercations.