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If you were handed a can of bug spray, you might start killing a lot of insects around the house. And if you had reason to believe that the bugs were poisonous — maybe because your neighbor told you there was an infestation, or you had seen a show about creepy crawly things on the Discovery Channel — you might even become convinced that the little piece of lint under your bed was a nasty spider, curled up just waiting to bite you. Americans tend to overestimate both their risk of exposure to violent crime and their ability to judge dangerous situations with reasonable care. Unfortunately, these misperceptions are killing people, not insects.

These problems assumed special urgency with the killing last month of Trayvon Martin, the teenage boy who was shot by George Zimmerman, an armed neighborhood-watch volunteer, after buying a bag of candy and iced tea. Most of the outrage has focused on the potential racism, bias and incompetence of the parties involved. As we’ve learned more about what happened on that fatal night, it seems increasingly likely that Zimmerman’s reckless actions may have violated Florida’s lenient law on justifiable force.

(PHOTOS: Trayvon Martin’s Death Sparks National Outrage, Mourning)

But the particulars of this case obscure a larger concern. “Stand your ground” laws, which allow a person to use force as a first resort in virtually any setting in which a threat is felt, do not account for the significant limitations and distortions of human perception.

Psychologists have known for more than 40 years that possession of a weapon can increase aggression. The link has been observed — in all kinds of lab-based and real-life settings — with different weapons and behavioral outcomes. We know that people exposed to movie or television violence are more prone to violent behavior, a robust finding that has held up under decades of examination. In fact, the mere image of a weapon, independent of the plot, the bad guys and the “blood and guts” can result in aggressive behavior.

But why should this be so? Advances in neuroscience have shown how guns increase the desire to kill others, putting the lie to lobbyists’ claim that “Guns don’t kill people, people do.” It turns out that our brains are very efficiently wired to make certain cognitive links based on our experiences and memories. If there’s a cake with candles, there must be a birthday party. Once an association is formed — for example between gun and danger — experiments have shown that aggressive thoughts become more accessible even when primed by something as simple as pictures or words. These aggressive thoughts can then distort a person’s interpretation of reality and decisionmaking, as when a drunken stumble is mistaken for a hostile shove. Once hostile thoughts have been primed, the gun can almost be seen to pull the trigger. For a comprehensive review of these issues, click here.

Another perception problem has to do with our risk of violence. Many Americans are unaware that violent crime has declined to levels not since the 1960s. In major cities, the decline has been the most dramatic, with homicide rates dipping from 35.5 per 100,000 in 1991 to 11.9 per 100,000 in 2008. Yet, despite this four-decade low, polls show that a majority of people think crime has worsened.

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In addition, Americans generally overestimate the risk of violent crime from strangers and from people of different races. In 2007, only 5% of assaults, 1% of rapes and 25% of robberies were committed by a person carrying a firearm. In 2010, strangers accounted for 39% of violent crime, and less than 10% of violent crime was committed with a firearm. Approximately 90% of homicide victims are murdered by someone of their own race.

It’s easy to understand why people might be surprised by some of these data. Television characters are murdered at vastly higher rates than people in real life. But it’s essential to communicate these facts because fear and misinformation can strengthen the priming mechanism that leads from hostile thoughts to hostile actions.

And those actions cost lives. Since the Florida law was enacted, violence from “justifiable force” has tripled. In most cases, the perpetrators were never even brought to trial because the statute provides “true immunity.” If a person feels force was justified, police officers and judges have been generally willing to go along with the perception, however flawed.

The revised Florida statutes allow justifiable force when there is “reasonable fear of imminent peril or death.” It’s understandable that lawmakers would trust citizens to exercise good judgment. Unfortunately, our brains are not wired to be sufficiently “reasonable” when we are holding a firearm. A grip on a gun can loosen our grip on reality.

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