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A change in Florida self-defence laws which gave citizens the right to use lethal force to protect themselves in public, has been linked with the state's homicide rates rising by nearly a quarter, according to a new study published in .

The research, involving the London School of Hygiene & Tropical Medicine and led by the University of Oxford, looked at homicide rates before and after the enactment of State Bill 436, known as the Stand Your Ground law, which was signed by Governor Jeb Bush in 2005.

Before 2005, Florida's so-called 'Castle doctrine' allowed the use of lethal force in situations where individuals believed there was an imminent threat of death or serious physical harm from an intruder within their own home. The 2005 Bill extended the 'no duty to retreat' clause of the Castle doctrine, giving individuals immunity for using lethal force to defend themselves in public places, as well as on private property.

Prior to the introduction of the law, Florida, which has a population of 19.8 million people, experienced on average 82 homicides per month, of which 49 deaths per month on average resulted from firearm-related injuries. The researchers examined publicly available data showing monthly totals of homicides - in total and for firearm-related cases only - for January 1999 to December 2014.

They found the change in the law was associated with homicide rates in Florida rising by 24% over 2005-2014 (compared with 1999-2004). Rates of homicide involving just firearms went up by 31%. Elsewhere in the United States homicide rates in general have been declining since the 1990s.

Increases in homicide rates affected all the demographic groups examined, with the largest proportional rises in the 20-34 age group (31%) and among the white population (28%). A 20% increase was also found among African-Americans.

The researchers considered a number of explanations. Firstly, they examined homicide rates in four US states that had not enacted a Stand Your Ground law over the same period of time - New York, New Jersey, Ohio and Virginia. They found no significant changes with or without the involvement of guns.

Secondly, they tested whether the global recession of 2008-09 could have contributed to the observed rise in violence, examining outcomes known to be sensitive to economic shocks but unlikely to be affected by changes in self-defence laws. Suicide rates in Florida between 2005 and 2014 did not increase significantly, and the researchers concluded that the recession was not the major factor associated with the rise.

Co-author Dr from the London School of Hygiene & Tropical Medicine said: 'Stand Your Ground laws have been implemented across US states since 2005. Surprisingly, in spite of controversy surrounding guns and gun law in the US, very little research analysing its introduction has been conducted. This study highlights how Stand Your Ground is likely to be a cause of the rise in Florida homicides, and provides crucial information which may influence future decision-making that affects wellbeing in the US and abroad."

Lead author Dr David Humphreys, Associate Professor of Evidence-Based Social Intervention and Policy at the University of Oxford, said: "For some time, critics of Stand Your Ground laws have been concerned that laws extending the rights of citizens to use lethal force are likely to result in increased homicide and injury rates. Given Florida was the first state to extend the use of lethal force in this way, it is an important test case that many other states have since followed.

"Our study shows that the enactment of the law is linked with a sudden reversal in the decline in homicide rates and homicide rates have risen particularly where guns are involved. We hope these findings will inform the ongoing debates about the implications that Stand Your Ground laws may have for public safety in Florida and other US states."

The authors acknowledge limitations of the study including circumstances unique to Florida may have contributed to the findings which they could not identify. They also could not compare the impact of Stand Your Ground laws across states as such analyses are susceptible to biases.

Publication

• David K. Humphreys, Antonio Gasparrini, Douglas J. Wiebe. . JAMA Internal Medicine. DOI:10.1001/jamainternmed.2016.6811