An Experimental Test of a Mediational Hypothesis

We tested whether interacting with a gun increased testosterone levels and later aggressive behavior. Thirty male college students provided a saliva sample (for testosterone assay), interacted with either a gun or a children’s toy for 15 min, and then provided another saliva sample. Next, subjects added as much hot sauce as they wanted to a cup of water they believed another subject would have to drink. Males who interacted with the gun showed significantly greater increases in testosterone and added more hot sauce to the water than did those who interacted with the children’s toy. Moreover, increases in testosterone partially mediated the effects of interacting with the gun on this aggressive behavior. Substantial evidence suggests that aggression can be increased by the presence of weapons in the environment and by the hormone testosterone. Several studies show that the presence of aggressive environmental cues such as weapons can increase the accessibility of hostile, aggressive thoughts and lead to more aggressive behavior (Anderson, Benjamin, & Bartholow, 1998; Bartholow, Anderson, Carnagey, & Benjamin, 2005; Berkowitz & LePage, 1967; Bettencourt & Kernahan, 1997; Killias & Haas, 2002). Regarding testosterone, in animal species ranging from chickens to monkeys, the injection of this hormone increases aggressiveness and social dominance behavior, regardless of whether the animals are males or females (Ellis, 1986); in humans, however, the results are more mixed, with many laboratory and field studies revealing strong positive relations between testosterone and levels of restlessness, tenseness, and tendency toward violence (Archer, 1994; Campbell, Muncer, & Odber, 1997; Dabbs, Carr, Frady, & Riad, 1995; Dabbs, Jurkovic, & Frady, 1991) and other studies failing to replicate such effects (Archer, 1991; Archer, Birring, & Wu, 1998; O’Connor, Archer, Hair, & Wu, 2001; Rowe, Maughan, Worthman, Costello, & Angold, 2004). Surprisingly, we were unable to find any studies that examined whether testosterone and the presence of a weapon might work together to increase aggressive behavior. Perhaps the presence of a stimulus such as a gun triggers increases in testosterone levels, which in turn increase aggressive behavior. Such a chain of events would be predicted by the challenge hypothesis developed by Wingfield, Hegner, Dufty, and Ball (1990) to explain aggressive behavior in male pair-bonded birds. According to this hypothesis, testosterone rises in response to situational cues that represent either a threat to a male’s status or a signal that competition with other males is imminent; such increases in testosterone then facilitate whatever competitive behaviors (including potentially aggressive responses) are necessary for meeting the challenge. The challenge hypothesis has been supported by studies across a wide range of vertebrate species (Cavigelli & Pereira, 2000; Ferree, Wikelski, & Anderson, 2004; Hirschenhauser, Taborsky, Oliveira, Canario, & Oliveira, 2004; Muller & Wrangham, 2004); most studies in humans have focused on how males’ testosterone levels rise and fall depending on success or failure in competitions (Archer, 1991; Booth, Shelley, Mazur, Tharp, & Kittok, 1989; Gladue, Boechler, & McCaul, 1989; Mazur, Booth, & Dabbs, 1992; Mazur & Lamb, 1980) or in response to insults (Cohen, Nisbett, Bowdle, & Schwarz, 1996; see Archer, 2006, for a review of the applicability of the challenge hypothesis to humans). In this study, we examined whether the presence of a gun (vs. a control object) might act as a stimulus signaling competition and a threat to status; if so, according to the challenge hypothesis, it should cause increases in males’ testosterone levels, which in turn should increase their aggressive behavior. We assessed males’ testosterone levels both before and after interacting with a gun or a children’s toy; to measure aggression, we adapted a method developed by Lieberman, Solomon, Greenberg, and Address correspondence to Tim Kasser, Box K-83, Knox College, Galesburg, IL 61401, e-mail: [email protected]. PSYCHOLOGICAL SCIENCE 568 Volume 17—Number 7 Copyright r 2006 Association for Psychological Science McGregor (1999) that gives subjects the opportunity to anonymously put hot sauce in a cup of water that they believe another person will have to drink. We hypothesized that males who interacted with the gun would show both a greater increase in testosterone levels and more aggression than would males who interacted with the children’s toy. We also hypothesized that changes in testosterone levels would be correlated with aggression levels and would indeed mediate the effects of interacting with a gun on later aggressive behavior.