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As the investigation continues into last month's fatal shooting of Trayvon Martin, the unarmed black teenager killed by a neighborhood watch volunteer in Florida, psychology researchers can point to one aspect of the tragedy: how easy it is to "see" that someone is holding a gun when he is not.

In the latest research, scientists found that simply holding a gun, as George Zimmerman was when he confronted Martin in a gated community in Sanford, Florida, has an effect.

"The mere act of holding a gun makes it more likely that you will perceive an object as a gun," said James Brockmole, associate professor of psychology at the University of Notre Dame and co-author of an upcoming paper on that phenomenon.

"Does that mean that if the neighborhood watch captain had not been armed he would have perceived the situation differently? It's impossible to say that about any specific situation, but our research shows it is certainly a possibility."...

Recognizing objects is not a simple matter of vision, said Brockmole. Instead, emotions, beliefs, and expectations can all affect the ability to accurately identify objects, numerous lab experiments have found.

Perhaps the most notorious real-world example of that was the 1999 killing Amidou Diallo in New York City: the 23-year-old Guinean immigrant was shot 41 times by police officers who perceived him as brandishing a gun. Standing in a dark doorway, Diallo was in fact showing them his wallet. In that case, said Brockmole, "racial stereotypes and beliefs about criminality may have caused the police to see a gun" that wasn't there.

But something else was at play in the Diallo case: the police were holding guns. That caused Brockmole and a colleague, assistant professor of psychology Jessica Witt of Purdue University, to ask whether "the mere act of wielding firearms have biased the officers to misperceive Diallo's actions," as they put it in a paper accepted for publication in the Journal of Experimental Psychology: Human Perception and Performance.

Brockmole's team ran five experiments with 220 participants. In each, the volunteers held either a foam ball or a firearm (a Wii gun - a type of gun used in videogames - or a disabled carbon-dioxide-powered BB pistol) while images flashed by on a monitor.

Each image, lasting less than a second, showed a person holding a gun or an innocuous object such as a shoe, soda can, or cell phone. The person in the image was either black or white, bare-headed or wearing a black ski mask. In one variation, the participant did not hold a gun, but one was conspicuously placed in the lab.

The mere presence of a gun nearby did not influence how likely people were to mistake a shoe or other innocent object for a gun. But holding a gun was, the scientists report in the upcoming paper.

"We got a substantial effect," said Witt. "Holding a firearm makes you more likely to see innocuous objects as guns."...

Whether that effect - holding a gun making someone more likely to "see" a gun - played any role in Sanford is impossible to say. But the proliferation of right-to-carry and concealed-carry gun laws makes that mistake more likely, say scientists. It might not even be necessary to have the gun in one's hand.

"It's all about intention," Witt said. "If you can feel the weapon on your hip and intend to use it, my prediction is that the perceptual bias would be just as great. Based on our other research, the anticipation of using an object is just as powerful an influence on perception."

Added Brockmole, "They say that when you hold a hammer, everything looks like a nail. That doesn't seem so harmless when you think about what happens when a person holds a gun."