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."
That result fit with other studies showing that "people's perception of their ability to act influences their perception of the world," Witt said.
For instance, she has found that when someone holds a long stick, objects seem closer - apparently because people think their ability to reach the object means it is nearby. People with broader shoulders perceive doorways as narrower than smaller people do, strong hitters see a softball as larger than poor hitters do, skilled golf putters perceive the hole as larger than poor duffers do, and out-of-reach objects look closer when people can hit them with a laser pointer.
"The perception system and the motor system evolved together," Witt said. "They share circuitry, so it makes sense that one would affect the other."
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."
(Reporting by Sharon Begley; Editing by Michele Gershberg and Paul Simao)
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