Florida, the state in which Trayvon Martin was killed, was the first to enact so called “Stand Your Ground” laws in 2005, and a host of states have followed since.
These laws are an extension of the Castle doctrine, a set of principles that almost all U.S. jurisdictions have embraced for years. The Castle doctrine comes from English Common law that gives citizens the ability to defend their property against aggressors and not be held criminally responsible for the harm they inflict in the course of doing so. The origin of the doctrine comes from British Common law, in which a person's house was considered his or her castle, and represented an inviolable legal space.
Stand Your Ground laws work by eliminating the normal duty for potential victims to retreat when threatened. In the course of defending against an attacker, a citizen may injure and in some cases kill the aggressor without being subject to laws against homicide. These types of killings fall in the category of “justifiable homicides."
But look at 2005. In that year, the number of justified homicides by private citizens fell from 219 to 192, but in 2006 it was 238, which was a massive jump. Obviously, Florida cannot be responsible for this jump, because they're just one state, but interestingly, the number of justifiable homicides has risen ever since 2005 (possibly as other states have followed suit?), reaching 278 in 2010. You might think that perhaps a rise in crime, not a change in the legal framework for self-defense, is responsible for this spike after 2005, but violent crime has remained relatively flat, providing a nice control.
Obviously there is no way to know the impact Florida's decision had on the number of justifiable homicides, and I'm suspicious that the DOJ might have changed its reporting metrics in 2005, because I had to look at two different websites to find this data. One thing, though, is clear, which is that starting in about 2003, the number of justifiable homicides committed in response to serious aggression had been going up (I can't find any post-2005 data about this).
The overall picture is complex. Justifiable homicides have been going up since 2005 and Florida may have started a trend in which more and more people are killed by private citizens (since it's more acceptable). A fair inference is that excessive vigilantism could be a cost of providing more protections to people taking justice into their own hands. However, evidence also suggests that even before Florida enacted its “Stand Your Ground” law, vigilantes were responding more and more to the most serious threats against them.
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