“Guns not only permit violence, they can stimulate it as well. The finger pulls the trigger, but the trigger may also be pulling the finger.”
—Leonard Berkowitz, Emeritus Professor of Psychology, University of Wisconsin
In 1967, Leonard Berkowitz and Anthony LePage conducted a fascinating study. First, participants were angered by a person pretending to be another participant (called a confederate). Next, participants were seated at a table that had a shotgun and a revolver on it—or, in the control condition, badminton racquets and shuttlecocks. The items on the table were described as part of another experiment that the researcher had supposedly forgotten to put away. The participant was supposed to decide what level of electric shock to deliver to the confederate who had angered them, and the electric shocks were used to measure aggression. The experimenter told participants to ignore the items on the table, but apparently they could not. Participants who saw the guns were more aggressive than were participants who saw the sports items. This effect was dubbed the “weapons effect.”
The weapons effect occurs outside of the lab too. In one field experiment, a confederate driving a pickup truck purposely remained stalled at a traffic light for 12 seconds to see whether the motorists trapped behind him would honk their horns (the measure of aggression). The truck contained either a .303-calibre military rifle in a gun rack mounted to the rear window, or no rifle. The results showed that motorists were more likely to honk their horns if the confederate was driving a truck with a gun visible in the rear window than if the confederate was driving the same truck but with no gun. What is amazing about this study is that you would have to be pretty stupid to honk your horn at a driver with a military rifle in his truck—if you were thinking, that is! But people were not thinking—they just naturally honked their horns after seeing the gun. The mere presence of a weapon automatically triggered aggression.
Research also shows that drivers with guns in their cars more likely to drive aggressively. A nationally representative sample of over 2,000 American drivers found that those who had a gun in the car were significantly more likely to make obscene gestures at other motorists (23% vs. 16%), aggressively follow another vehicle too closely (14% vs. 8%), or both (6.3% vs. 2.8%), even after controlling for many other factors related to aggressive driving (e.g., gender, age, urbanization, census region, driving frequency).
Human beings can identify potentially dangerous, threatening stimuli such as spiders and snakes very quickly. This makes sense from an evolutionary perspective because some spiders and snakes are poisonous, and our ancient ancestors who could identify them quickly were more likely to avoid them and live to pass on their genes. Recent research shows that people can identify guns as quickly as they can identify spiders and snakes.,, These findings are very interesting because guns are modern threats and cannot be explained using evolutionary principles. Yet guns are a far more dangerous to people today than spiders or snakes. Poisonous spiders (e.g., Black Widows, Brown Recluses) kill about 6 Americans each year. Poisonous snakes (e.g., rattlesnakes) kill about 5 Americans each year. In comparison, guns kill about 31,000 Americans each year.
Several studies have replicated the weapons effect. A review of 56 published studies confirmed that the mere sight of weapons increases aggression in both angry and nonangry individuals. Perhaps the weapons effect occurs because weapons are closely linked to aggression in our brains.
 Turner, C. W., Layton, J. F., & Simons, L. S. (1975). Naturalistic studies of aggressive behavior: Aggressive stimuli, victim visibility, and horn honking. Journal of Personality and Social Psychology, 31, 1098–1107.
 Blanchette, I. (2006). Snakes, spiders, guns, and syringes: How specific are evolutionary constraints on the detection of threatening stimuli? The Quarterly Journal of Experimental Psychology, 59(8), 1484–1504.
 Carlson, M., Marcus-Newhall, A., & Miller, N. (1990). Effects of situational aggression cues: A quantitative review. Journal of Personality and Social Psychology, 58, 622–633.