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Research Report

Guns, Testosterone, and Aggression An Experimental Test of a Mediational Hypothesis Jennifer Klinesmith, Tim Kasser, and Francis T. McAndrew

Knox College

ABSTRACT—We tested whether interacting with a gun in-

creased 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 in-

creases aggressiveness and social dominance behavior, re-

gardless 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 re-

lations between testosterone and levels of restlessness, tense-

ness, 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 de-

veloped 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 de-

pending 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 applica-

bility 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].

P S Y C H O L O G I C A L S C I E N C E

568 Volume 17—Number 7Copyright r 2006 Association for Psychological Science

McGregor (1999) that gives subjects the opportunity to anony-

mously put hot sauce in a cup of water that they believe another

person will have to drink. We hypothesized that males who in-

teracted 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 ag-

gression levels and would indeed mediate the effects of inter-

acting with a gun on later aggressive behavior.

METHOD

Subjects

Subjects were 30 male college students (age range: 18–22) who

received extra course credit or a small monetary reward for their

participation. All subjects were run during the afternoon or early

evening.

Procedure and Materials

When recruited, subjects were informed that the study would

examine taste sensitivity in males and that they would therefore

need to provide saliva for hormone analysis; subjects were asked

not to eat, drink, smoke, or brush their teeth for 1 hr prior to

testing in order to minimize impurities in the saliva samples.

When subjects arrived at the lab, a female experimenter con-

firmed that the subjects had indeed followed these instructions

before she administered consent procedures. Next, participants

provided an approximately 6-ml sample of saliva by spitting into

a cup; this saliva was used to assess baseline, or Time 1,

testosterone levels.

All subjects were then led into a room containing a television,

a chair, and a table with an object and some paper on it. For

experimental subjects, the object was a pellet gun identical in

size, shape, and feel to a Desert Eagle automatic handgun; for

control subjects, the object was the children’s game Mouse

Trapt. Subjects were told that the study was investigating

whether taste sensitivity was associated with the attention to

detail required for creating instructions concerning the object.

Subjects were therefore asked to spend 15 min handling the

object and writing a set of instructions about how to assemble

and disassemble it; a drawing of the object was also provided for

subjects to label the object’s parts. The handgun and children’s

game were similar in number and complexity of parts.

After 15 min, the experimenter reentered the room, asked the

subject to stop working on the instructions, and obtained a Time

2 saliva sample from the subject. The subject was told he would

next perform the taste-sensitivity portion of the study. He was

given a cup filled with 85 g of water and a single drop of Frank’s

Red Hot Sauce. The subject was told that the sample had been

prepared by a previous subject, was instructed to take a sip of the

sample, and was then asked to rate the taste of the sample on a

scale provided.

The experimenter left and then returned with a tray containing

a cup of 85 g of water, a nearly full bottle of Frank’s Red Hot

Sauce, and a lid. The subject was asked to prepare a sample for

the next subject by placing as much hot sauce in the water as he

wanted. He was assured that neither the person who drank it nor

the experimenter would know how much hot sauce he had put in

the water, as the lid was to be put on the cup after the hot sauce

was added. The experimenter then left the room, and the sub-

ject signaled when he was finished adding the hot sauce.

(Throughout this process, the gun or the game remained in the

room.) The cup was then removed from the room, and the ex-

perimenter weighed it again to obtain a measure of the amount of

hot sauce, in grams, the subject had added to the water. This

served as our primary measure of aggression (see Lieberman et

al., 1999).

Because of the potentially arousing nature of the experiment,

we wanted to ensure that all subjects were reasonably calm when

they left the lab. Therefore, all subjects next watched a relaxing

video of nature scenes and classical music. Given that subjects

had been deceived, we next debriefed them, emphasizing that

they should not feel badly about any aggressive behavior they

exhibited. Interestingly, several subjects were disappointed

when told that the sample of hot sauce and water they had

prepared would not actually be given to the next subject. No

subjects expressed suspicion as to the true nature of the study.

Testosterone Levels

Time 1 and Time 2 saliva samples were stored for 24 hr at room

temperature, centrifuged, and then frozen at �20 1C until the time of the assay (Erikkson & Von Der Pahlen, 2002). The

samples were then brought to room temperature, transferred to

Eppendorf tubes, centrifuged for 15 min at 3,000 rpm to remove

debris, and then assayed in duplicate using a commercially

available microwell kit for testosterone level (Salimetrics, LLC,

State College, PA). All samples were assayed in house in a single

batch using a standard radioimmunoassay (RIA) procedure

under the supervision of an experienced RIA technician; at both

Time 1 and Time 2, the duplicates were averaged to yield our

measures of testosterone level. The intra-assay coefficient of

variation for subjects was 5.3%, and the sensitivity of the assay

was less than 1.5 pg/ml from zero for men. Mean Time 1 and

Time 2 testosterone levels were 222.59 pg/ml (SD 5 97.17) and

253.92 pg/ml (SD 5 98.32), respectively. We subtracted each

subject’s Time 1 level from his Time 2 level to obtain a measure

of change in testosterone.

RESULTS

Our first hypothesis was confirmed: Subjects who interacted with

the handgun showed a greater increase in testosterone from

Time 1 to Time 2 (mean change 5 62.05 pg/ml, SD 5 48.86)

than did those who interacted with the children’s game (mean

Volume 17—Number 7 569

Jennifer Klinesmith, Tim Kasser, and Francis T. McAndrew

change 5 0.68 pg/ml, SD 5 28.57), t(28) 5 �4.20, prep 5 .99, d 5 1.53. Thus, interacting with the gun increased testosterone

levels.

Our second hypothesis was also confirmed: Subjects who in-

teracted with the gun added more hot sauce to the water (M 5

13.61 g, SD 5 8.35) than did those who interacted with the

children’s toy (M 5 4.23 g, SD 5 2.62), t(28) 5 �4.16, prep 5 .99, d 5 1.52. Thus, interacting with the gun increased ag-

gressive behavior.

Our third hypothesis was also confirmed: The amount of hot

sauce placed in the cup was positively correlated with changes

in testosterone level (r 5 .64, prep 5 .99; R 2

5 .41). Given that

all three of Baron and Kenny’s (1986) prerequisites for a me-

diational model were met, we next examined whether the size of

the association between the predictor variable (i.e., gun vs.

game) and the outcome variable (i.e., grams of hot sauce added)

diminished once the effects of the purported mediating variable

(i.e., changes in testosterone) were controlled. Indeed, the size of

the correlation between group membership (experimental vs.

control) and aggression dropped from r 5 �.62 (prep 5 .99) to pr 5 �.36 (prep 5 .91) after controlling for changes in testos- terone. Finally, to more stringently test our mediational hy-

pothesis, we computed Sobel’s (1982) test for mediation, using

the Web site developed by Preacher and Leonardelli (2001). As

before, the evidence suggested that the effect of guns on ag-

gression was significantly mediated by changes in testosterone

levels, Sobel’s test 5 2.09, prep 5 .93.

DISCUSSION

Past research shows that both testosterone and exposure to guns

are associated with aggressive behavior, but no studies, to our

knowledge, have examined how the two factors might work to-

gether. The present results demonstrated that males who inter-

acted with a gun showed a greater increase in testosterone levels

and more aggressive behavior than did males who interacted

with a children’s toy. Mediational analyses suggested that part of

the reason that guns increase aggression is that they cause in-

creases in testosterone levels.

Such findings not only are consistent with the challenge hy-

pothesis (Wingfield et al., 1990), but also provide new support

for it both by examining a new type of ‘‘challenging’’ stimulus

and by assessing later aggressive behavior. Additionally, the

results provide evidence against some interpretations suggest-

ing that experimental effects of guns on aggression are due to

subtle demand characteristics. That is, it seems unlikely that

subtle experimenter pressures could significantly increase

subjects’ testosterone levels; instead, it seems more reasonable

to believe that the presence of the gun had this effect.

Future research could explore a variety of avenues for cor-

recting some of the limitations of the present study. For example,

would females’ biological and behavioral responses to guns be

similar to males’? Perhaps they would, as other female animals

act more aggressively when injected with testosterone (Ellis,

1986). But perhaps they would not, as the types of evolutionary

challenges faced by ancestral females were different from those

faced by males, and thus females may react to guns differently

than males. Another topic worthy of further study concerns the

fact that subjects in this study only had the opportunity to ag-

gress in an anonymous, rather indirect fashion against an un-

known individual. Would the same pattern of results hold if the

aggression was directed against a particular individual, or if

there was a possibility of retribution from the victim? Finally,

would past experience with guns moderate these effects? That is,

would individuals who frequently handle guns (such as hunters

or soldiers) respond similarly to those with little or no experience

with weapons? Previous research (Bartholow et al., 2005) sug-

gests that this may be the case, but a link with testosterone has

yet to be established.

In sum, the present study replicates past research showing

that exposure to guns may increase later interpersonal aggres-

sion, but further demonstrates that, at least for males, it does so

in part by increasing testosterone levels. Such findings raise

many of the usual questions about whether the presence of guns

in modern society contributes to violent behavior. Although our

study is clearly far from definitive, its results suggest that guns

may indeed increase aggressiveness partially via changes in the

hormone testosterone.

Acknowledgments—We thank Heather Hoffmann, Janet Kirk-

ley, Glen Normile, and Neil Schmitzer-Torbert for their assistance.

This research was funded by a grant from the Richter Memorial

Scholarship Program.

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(RECEIVED 8/11/05; ACCEPTED 10/17/05; FINAL MATERIALS RECEIVED 10/31/05)

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Jennifer Klinesmith, Tim Kasser, and Francis T. McAndrew