From a new study from Notre Dame:

In five experiments, subjects were shown multiple images of people on a computer screen and determined whether the person was holding a gun or a neutral object such as a soda can or cell phone. Subjects did this while holding either a toy gun or a neutral object such as a foam ball.

The researchers varied the situation in each experiment — such as having the people in the images sometimes wear ski masks, changing the race of the person in the image or changing the reaction subjects were to have when they perceived the person in the image to hold a gun. Regardless of the situation the observers found themselves in, the study showed that responding with a gun biased observers to report “gun present” more than did responding with a ball. Thus, by virtue of affording the subject the opportunity to use a gun, he or she was more likely to classify objects in a scene as a gun and, as a result, to engage in threat-induced behavior, such as raising a firearm to shoot. . . .

“One reason we supposed that wielding a firearm might influence object categorization stems from previous research in this area, which argues that people perceive the spatial properties of their surrounding environment in terms of their ability to perform an intended action,” Brockmole says.

This may help explain the common refrain of “I saw him reaching for a gun!” when police (who are always armed) shoot unarmed people.