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Severity of physical aggression reported by university students: A test of the interaction between trait aggression and alcohol consumption
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Personality and Individual Differences 45 (2008) 3–9 www.elsevier.com/locate/paid
Severity of physical aggression reported by university students: A test of the interaction between trait aggression and alcohol consumption Paul F. Tremblay *, Kathryn Graham, Samantha Wells The Centre for Addiction and Mental Health, The University of Western Ontario, 100 Collip Circle, Suite 200, London, Ontario, Canada N6G 4X8 Received 19 September 2007; received in revised form 4 February 2008; accepted 13 February 2008 Available online 24 March 2008
Abstract As part of an online study of aggression, 2647 full-time undergraduate students at six Canadian universities completed the Aggression Questionnaire (Buss & Perry, 1992) and reported on whether they had been involved in incidents of physical aggression in the past 12 months at a (1) bar, nightclub or pub, (2) home or residence, and (3) party or social event. About one in four students reported an incident. Scores on the physical trait aggression scale correlated significantly with severity of aggression in the three locations. Males reported higher severity than did females in the three locations. Alcohol consumption was significantly related to severity at bars and parties but not in the home location. Support was found for an interaction between the physical trait aggression scale and drinking at the time of the incident predicting the severity of aggression for incidents reported in bars. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Trait aggression; Severity of aggression; Alcohol consumption; Physical aggression
1. Introduction Recent studies indicate that the risk of physical aggression is particularly high among college students. A survey of 47,202 college students on 74 campuses by the American College Health Association in 2004 revealed that 6.7% (4.2% females and 10.8% males) had been involved in a fight and that 4.7% (2.8% females and 7.8% males) had injured another person after drinking in the last school year (American College Health Association, 2006). A study of the one-year prevalence of aggression among young adults in Buffalo, New York revealed that, for the college sample, 19.8% of men and 12.3% of women had initiated an act of violence in the previous year, and 28.8% of men and 16.6% of women reported being the target of violence (Leonard, Quigley, & Collins, 2002). *
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A large proportion of naturally-occurring aggression involves alcohol (Barnwell, Borders, & Earleywine, 2006; Graham, Wells, & Jelley, 2002; Wells, Graham, & West, 2000). Experimental studies have found a pattern for aggression to be more severe when subjects have been drinking (see meta-analysis by Bushman, 1997). Naturalistic studies have also shown a link between severity of aggression and level of alcohol consumption (e.g., Graham, Osgood, Wells, & Stockwell, 2006; Graham & Wells, 2001; Leonard, Collins, & Quigley, 2003; Wells & Graham, 2003). It is particularly important to study college populations among whom substantial proportions report being in a fight while drinking (Quigley, Corbett, & Tedeschi, 2002). Since ‘‘there is no one-to-one relationship between alcohol use and human behavior” (Pernanen, 1981, p. 13), it is necessary to identify types of people among whom and situations in which alcohol increases aggression severity. The same proposition has been reiterated by Giancola (2003) in a special issue article on the role of individual difference
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and contextual factors contributing to the alcohol-aggression relation. A key variable thought to moderate the alcohol-aggression relation is trait aggression which refers to a stable disposition or tendency to engage in aggressive behaviors on repeated occasions and in various situations. Experimental research suggests an interaction effect of trait aggression with alcohol consumption resulting in higher levels of aggression than from the separate effects of these two variables (Bailey & Taylor, 1991; Dougherty, Bjork, Bennett, & Moeller, 1999; Giancola, 2002). This interaction may be due to alcohol lowering the already low threshold for aggression among those high on trait aggression. More specifically Giancola (2002) has suggested that alcohol may be stimulating pre-existing angry and hostile cognitions and affect in high trait aggressive individuals. Although there are an increasing number of experimental studies investigating this interaction, the present study addresses a need for research assessing this interaction in naturalistic settings. Situational factors such as provocation, the presence and behavior of third parties and level of environmental permissibility may also be involved (see review by Graham et al., 1998). Drinking settings vary considerably in terms of norms regarding socially appropriate behavior as well as in terms of the presence and levels of provoking stimuli. Aggression has been found to be more likely when drinking occurs in bars (Leonard et al., 2002; Norstro¨m, 1998; Stockwell, Lang, & Rydon, 1993) and at parties (Wells, Mihic, Tremblay, Graham, & Demers, 2008) than in other contexts. Some evidence indicates that the effect of alcohol consumption on aggression may be greater for men than for women (Bushman, 1997; Giancola et al., 2002; GusslerBurkhardt & Giancola, 2005). Additionally, several studies suggest that the increased risk of aggression in bars is higher for men than for women (Collins, Quigley, & Leonard, 2007; Graham & Wells, 2001) while drinking at a party, compared to other circumstances, poses a risk for women but not for men (Wells et al., 2008). In the present study, we investigate the relationship of naturally-occurring physical aggression reported by university students with trait aggression and sex of the student and alcohol consumption at the time of the incident, for incidents occurring in three common social contexts for student drinking (Adlaf, Demers, & Gliksman, 2005): bars, house/residence, and party/social events. The main hypothesis is that, in all three social contexts, aggression will be most severe when alcohol consumption is combined with high physical trait aggression. The second hypothesis is that alcohol consumption will be more strongly related to severity of aggression for men than for women.
(one Atlantic, three Eastern, one Central, and one Western provinces) ranging in size from 2000 to 26,000 students to participate in an online study assessing alcohol consumption, physical aggression, and perceptions of alcoholrelated hypothetical conflict situations. The focus of the present article is on the self-reports of incidents of physical aggression in the previous year. A standard invitation letter requesting participation and providing a link to the password-protected study website was sent to full-time undergraduate students via e-mail at the beginning of the second academic semester (January). The website included a consent form that students were asked to read and print prior to participating and a questionnaire that required approximately 30 min to complete. Three reminder e-mail letters were sent at two-week intervals to students who had not yet participated. As an incentive to participate, respondents were entered into a draw for a chance to win one of two $1000 cash prizes. Of the 8788 valid invitations, 3423 students responded to the request. The final sample was 2647 after exclusion of 776 responses due to a large amount of missing data. The response rate (ratio between usable data sample/email invitation sample) was 30.1%. 2.2. Sample description The sample was comprised of 63.6% females and 36.4% males with 26.5% in first year, 26.0% in second year, 26.5% in third year, and 20.9% in fourth year of university study. The mean age was 21.2 years and 90.8% were between the ages of 18 and 24. In terms of experience with alcohol consumption, 94.8% had ever consumed alcohol, and 94.2% reported feeling mildly intoxicated at least once. The mean number of drinking occasions per week based on the last 12 months was 0.79 times for females and 1.12 times for males. The mean number of drinks per typical drinking occasion was 4.77 drinks (SD = 3.63, median = 4.0) for females and 5.75 drinks (SD = 4.29, median = 5.0) for males.1 These values are similar to those reported by Adlaf et al. (2005) in the 2004 Canadian Campus Survey. 2.3. Measures 2.3.1. Aggression severity Experiences of aggression were assessed in three locations in addition to one ‘other’ location category. For the bar, nightclub or pub location, subjects were asked: ‘In the past 12 months, have you been personally involved in an incident that occurred at a bar, nightclub, or pub where someone grabbed, hit, kicked, pushed, shoved or was physically aggressive towards you in some way or where you did any of these things to someone else?’
2. Method 2.1. Recruitment procedure A total of 1500 full-time undergraduate students were randomly selected from each of six Canadian universities
1
Note all responses of 16 or more drinks were scored as 16.
P.F. Tremblay et al. / Personality and Individual Differences 45 (2008) 3–9
The same question was presented for (2) home or residence with people you live with; (3) party or social event at someone’s home, residence, at the beach, wedding, etc; and (4) any other location. Due to the small number of respondents indicating aggression in other locations and the heterogeneous nature of this category, ‘‘other location” was not included in the analyses by locations. When respondents answered ‘‘yes” to aggression in any of the locations, they were asked to focus on the most severe incident (in that location) in which they had been involved during the past 12 months and asked: ‘‘How physically aggressive were you during this incident?” (0 – Not at all physically aggressive and 10 – Extremely physically aggressive). 2.3.2. Alcohol consumption For those reporting aggression, they were asked: ‘‘Were you or your main opponent drinking at the time of the incident?” The options were (1) both of us were drinking; (2) only my opponent was drinking; (3) I was the only one drinking; and (4) neither of us was drinking. Given the focus of the present study on trait aggression of the participants, we measured only whether or not the participant was drinking (i.e., options 1 and 3), regardless of whether the opponent was drinking. 2.3.3. The Aggression Questionnaire (Buss & Perry, 1992) This 29-item measure consists of four related dimensions: physical (nine items), verbal (five items), anger (seven items), and hostility (eight items), with respondents rating each item on a scale of 1 (extremely uncharacteristic of me) to 5 (extremely characteristic of me). Internal consistency values (Cronbach’s alpha) based on the current sample were .84 (physical aggression), .74 (verbal aggression), .82 (anger), .78 (hostility) and .89 (total scale). 3. Results 3.1. Trait aggression scores and prevalence of incidents Trait aggression scores by sex are presented in Table 1. Independent t-tests revealed significantly higher scores for males than for females on all scales (p < .01) except anger. Of the 2581 respondents2 who completed the section on aggressive incidents in the past year, 636 (24.6%) reported experiencing at least one incident. Significantly more males (30.2%) than females (21.5%) reported at least one incident ðv2ð1Þ ¼ 23:91; p < :01Þ. Of those reporting aggression, 75.1% reported an incident in only one location, 20.6% in two locations, 2.7% in three, and 1.6% in all four locations (bar, home/residence, party/social event, and other). The proportion of respondents reporting at least one incident varied across universities (20.1–26.9%), but these differences were not significant ðv2ð5Þ ¼ 5:02; nsÞ. 2
Sixty six respondents (2.5%) did not respond to this question.
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Table 1 Mean and (standard deviation) of trait aggression scores by sex Scale
Males M (SD)
Females M (SD)
Physical Verbal Anger Hostility Total score
20.31 14.01 14.19 18.69 67.16
15.57 13.12 14.43 17.95 61.03
(7.11) (4.09) (5.29) (6.12) (16.84)
(5.66) (3.94) (5.02) (5.99) (15.41)
Note: Independent t-test indicated that males have significantly higher scores (p < .01) than do females on all scales except anger. Male n = 952– 958; female n = 1670–1676.
3.2. Aggression severity as a function of sex and alcohol consumption Table 2 shows the results of analyses of variance of sex by alcohol consumption at the time of the incident on the severity scores (0 – not at all physically aggressive and 10 – extremely physically aggressive) performed separately for each location. As shown by the numbers for each cell, only a small number of males were not drinking when incidents occurred at a bar or party (n = 26 for bar and n = 13 for party) while only a small number of females (n = 20) were drinking when aggression occurred in the home. The severity scores of males were significantly higher than those of females in the bar and home locations (and close to significance in the party location; p < .06). Respondents who indicated that they were drinking at the time of the incident rated their aggression significantly higher than those not drinking in the bar and party locations but not in the home location. No significant drinking by sex interactions were found. 3.3. Aggression severity as a function of trait aggression by sex and location Table 3 presents the correlations between the trait aggression scales and severity of aggression by sex for each location. Severity was most strongly correlated with physical trait aggression for males and females in all locations except for a slightly higher correlation with the total aggression scale for females in the party location. 3.4. The interaction of trait aggression with alcohol consumption The hypothesis that drinking and trait aggression interact to determine severity of aggression was investigated in multiple regression analyses presented in Table 4. The same model was applied to the data for the three drinking locations. As in previous studies assessing the interaction of trait aggression with alcohol consumption (e.g., Giancola, 2002), the physical scale rather than the total scale from the Aggression Questionnaire was used as the trait measure because it is particularly relevant to physical aggression. Physical scale scores were centered around the grand mean and the drinking status and sex variables were dummy
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Table 2 Mean severity of aggressive behavior by sex and drinking at the time of incident for each location Location
Males
Females
Relationship with severity of aggression
Respondent drinking status
M (SD)
n
M (SD)
n
Bar Drinking Not drinking Total
5.14 (3.32) 3.42 (3.06) 4.85 (3.34)
126 26 152
3.53 (3.04) 2.49 (2.66) 3.28 (2.98)
132 41 173
Drink: F(1, 321) = 10.05, p < .01, x2 = .03 Sex: F(1, 321) = 8.55, p < .01, x2 = .02 Drink sex: F(1, 321) = 0.60, ns
Home Drinking Not drinking Total
5.35 (2.74) 4.93 (2.90) 5.12(2.82)
46 56 102
3.40 (3.17) 3.66 (2.91) 3.63 (2.93)
20 141 161
Drink: F(1, 259) = 0.03, ns Sex: F(1, 259) = 12.74, p < .01, x2 = .04 Drink sex: F(1, 259) = 0.57, ns
Party Drinking Not drinking Total
5.37 (3.03) 3.77 (2.32) 5.08 (2.97)
59 13 72
4.21 (3.30) 2.50 (3.07) 3.51 (3.29)
29 20 49
Drink: F(1, 117) = 6.65, p < .01, x2 = .04 Sex: F(1, 117) = 3.60, p < .06, x2 = .02 Drink sex: F(1, 117) = 0.07, ns
Table 3 Correlations between trait aggression scales and severity of aggression by sex for each location Measure
Bar
Trait aggression scales
Male (n = 152) .52**
Female (n = 174) .45**
Total (n = 327) .53**
Male (n = 102) .30**
.07 .16* .02 .28**
.08 .25** .11 .32**
.10 .20** .03 .33**
.03 .14 .11 .14
Physical Verbal Anger Hostility Total Note: *p < .05,
Home
Party Female (n = 161) .36** .17* .25**
Total (n = 263) .39**
Male (n = 71) .60** .25* .33**
.11 .19** .01 .26**
.11 .30**
Female (n = 49) .50** .19 .44** .48**b .52**
.03b .44**
Total (n = 120) .59** .23* .34** .19* .48**
p < .01, bz test of significance indicating significant difference between correlation coefficients at .01.
**
Table 4 Aggression severity as a function of physical trait aggression, sex and drinking in three locations Bar (N = 325) DR 1.
2.
3.
2
**
Intercept sex drinking physical
.29
Intercept sex drinking physical drinking physical
.01*
Intercept sex drinking physical drinking physical drinking sex sex physical drinking sex physical
.01
Home (N = 261)
b
SE b **
B
DR
2
**
3.32 .23 .74 .19**
.37 .33 .38 .02
.17 .04 .09 .50
3.03** .24 1.01* .10* .12*
.39 .33 .40 .05 .05
.04 .13 .25 .27
3.14** .26 .85 .13 .05 .15 .07 .14
.57 .78 .62 .08 .08 .86 .10 .11
.00
.01 .04 .11 .33 .12 .02 .12 .23
b
Party (N = 120) SE b
**
B
3.93 .92* .31 .13**
.23 .38 .43 .02
.15 .05 .36
3.94** .90* .25 .14** .02
.23 .39 .45 .03 .05
.15 .04 .37 .04
4.11** .59 .97 .17** .08 1.12 .08 .10
.25 .45 .69 .03 .08 .92 .06 .10
DR2
b
.37**
3.59** .45 .84 .19**
.51 .51 .56 .03
.07 .12 .54
3.63** .45 .81 .20** .01
.54 .51 .58 .06 .06
.07 .11 .57 .03
3.83** .27 .56 .26** .14 .08 .16 .27*
.67 1.00 .83 .07 .09 1.17 .12 .13
.04 .08 .72 .32 .01 .31 .49
.00
.03 .10 .14 .46 .12 .14 .14 .14
Note: Physical = physical aggression scale score centered; drinking = yes (1) no (0); male (1) and female (0); *p < .05;
coded (drinking = 1 and not drinking = 0; male = 1 and female = 0). The analyses were conducted in three steps as described below. The analyses were conducted in three steps. At step 1, sex, drinking, and physical trait aggression were entered
SE b
B
**
p < .01.
to inspect their independent relations with severity of aggression. The physical trait aggression scale was a significant predictor of aggression severity in all three locations. Although the previous analyses of variance that controlled only for sex (Table 2) indicated that alcohol consumption
P.F. Tremblay et al. / Personality and Individual Differences 45 (2008) 3–9
10
Females 10
9
9 Aggression Severity
Not Drinking Drinking
8 Aggression Severity
7
7 6 5 4 3
Not Drinking
8
Drinking
7 6 5 4 3 2
2
1
1
0 -1 SD
0 -1 SD
M
1 SD
M
1 SD
Trait Aggression (Physical) Centered
Trait Aggression (Physical) Centered
was significantly related to aggression severity in bar and party locations, this relationship was no longer significant in any of the locations when both sex and physical trait aggression were in the model. Similarly, the higher severity of aggression for males than for females found in the previous analyses was no longer significant in these models except in the home location where males reported significantly more severe aggression than did females. In step 2, the interaction between physical trait aggression and alcohol consumption was tested, with this interaction found to be significant in the bar location but not in the home or party locations. As illustrated in Fig. 1, aggression severity increased with higher levels of physical trait aggression more for respondents who were drinking than for those who were not. In step 3, the three–way interaction between physical trait aggression, sex and alcohol consumption was tested to determine whether the trait aggression by alcohol consumption interaction differs for men and women. A significant three-way interaction was found in the party location only and is illustrated in Fig. 2 separately for males and females. The male graph shows an interaction of physical trait aggression by drinking similar to the overall interaction found in the previous step in the bar location (i.e., Fig. 1). The pattern of females is in the opposite direction, with the effect of drinking stronger for females with low physical trait aggression than for females with high physical trait aggression. Although this three-way interaction was significant at p < .05, given that the increase in R2 was not significant from Step 2 to Step 3, this effect should be interpreted with caution. In fact, post hoc tests investigating the physical trait aggression by drinking interaction separately for males and females revealed that it was not significant in the male or the female sample.
Males
10 9 Aggression Severity
Fig. 1. Aggression severity in bars as a function of the interaction between trait aggression and drinking status. Note: Aggression Severity’Drink** (Physical Trait Agg); Aggression Severity’Not Drinking = 4.15 + .22 * * ** p < .01. ing = 3.14 + .10 (Physical Trait Agg); p < .05,
8
Not Drinking
7
Drinking
6 5 4 3 2 1 0 -1 SD
M
1 SD
Trait Aggression (Physical) Centered Fig. 2. Aggression severity at parties as a function of the interaction between trait aggression, drinking status, and sex. Note: Aggression Severity’Female Drinking = 4.39 + .12 (Physical Trait Agg); Aggression Severity’Female Not Drinking = 3.83 + .26** (Physical Trait Agg); Aggression Severity’Male Drinking = 4.74 + .24** (Physical Trait Agg); Aggression Severity’Male Not Drinking = 4.10 + .10 (Physical Trait Agg); **p < .01.
4. Discussion This study revealed that about one in four students was involved in an incident of physical aggression in the twelve month period preceding data collection. Males were more likely than females to report an incident and had higher trait scores than did females on all scales except the anger scale. This pattern is similar to sex differences reported by Buss and Perry (1992). Males reported using more severe aggression than did females in all locations, and alcohol consumption was associated with more severity at bars and parties. However, in the regression models including all three predictors (sex, alcohol consumption, and physical trait aggression) only the physical scale was a significant predictor of aggression severity in all three locations, and sex remained significant in only the home/residence location. Taken together, these findings suggest that the main determinant of severity of aggression is physical trait aggression, with at least some of the relationship between
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P.F. Tremblay et al. / Personality and Individual Differences 45 (2008) 3–9
severity and alcohol consumption and sex due to males being both more likely than females to drink and to have higher trait aggression scores. The primary aim of the study was to test the interaction between trait aggression and drinking in predicting aggression severity and partial support was found. The hypothesis that trait aggression has a greater influence on the aggression of people who have consumed alcohol was supported for incidents in a bar but not in the other settings. This may reflect differences in social norms among locations. In particular, proscriptive norms regarding socially appropriate behavior may be weaker in bars where it is not unexpected for some fights to occur. Evidence suggests that loud, boisterous, and aggressive behavior is often perceived as being completely normative and acceptable in barroom settings (Graham & Wells, 2003; Tomsen, 1997). This in turn may allow for the expression of individual differences in trait aggression, especially for people who have been drinking. Some evidence, although statistically weak, was found for a three-way interaction between sex, trait aggression, and alcohol consumption but only in the party location. Among males, the influence of trait aggression was greater when the respondent had been drinking; however, among females the increase was greater among respondents who had not been drinking. This finding may reflect that aggressive experiences of men and women are very different, particularly in the social setting of a party, and thus is influenced by different processes. However, because this finding was not entirely stable (i.e., not evident when separate analyses were conducted for men and women) it requires further investigation in future research. Contrary to some experimental studies (reviewed Section 1), no evidence was found in the present study, in any of the three locations, supporting the hypothesis that alcohol consumption has a greater effect on the severity of aggression by males than by females. This finding is congruent with some studies (Dougherty et al., 1999; Wells et al., 2008). Gussler-Burkhardt and Giancola (2005) have discussed the inconsistent findings regarding sex differences in alcohol-related aggression in the experimental literature and argue that the discrepancy may be due to different experimental stimuli (i.e., Taylor aggression paradigm vs. point subtraction aggression paradigm). They add that women may have a greater threshold than men, and that in some situations, alcohol alone may not be sufficient to trigger an aggressive response. It is important, to interpret the results of the present study in relation to how the variables were operationalized and how these measures differ from those of studies in the experimental literature. In the present study, an item assessed whether the respondent consumed alcohol at the time of the incident without taking into account the volume of alcohol or the level of intoxication. It is therefore realistic to assume that there would be considerable variability in intoxication levels among respondents. Thus, in future research it would be necessary to assess not only whether the respon-
dent was drinking but also ascertain how much alcohol was consumed and how intoxicated the person became. In the present study, severity was a self-assessment of how much physical aggression was used by the respondent without taking into consideration the motive or the nature of the provocation. By contrast, in experimental studies, physical aggression is measured using an objective standardized outcome (e.g., the number and intensity of shocks) and the provocation for all participants is standardized. Thus, the heterogeneity and subjectivity of measuring aggression in real-life settings, while more natural, may also obscure some results. It is also important to note that analyses in the three locations are not directly comparable due to differences in the sample size (i.e., much smaller sample size in the party location) and therefore differences in statistical power. Overall, while taking into account these limitations, the results have important implications for understanding aggression risk factors as they highlight the role of individual differences in trait aggression after controlling for sex and alcohol consumption. The present study highlights the importance of including trait aggression measures in naturalistic research designs. Additionally, future naturalistic studies assessing the interaction between trait aggression and alcohol consumption in predicting the severity of aggression need not only ensure control for alcohol and intoxication levels but also need to take into account the social context. Acknowledgements This study was supported by a grant (MOP62747) to Paul F. Tremblay and Kathryn Graham from the Canadian Institutes of Health Research (CIHR). We thank R.C. Gardner for his suggestions related to the statistical analyses. References Adlaf, E. M., Demers, A., & Gliksman, L. (Eds.). (2005). Canadian campus survey 2004. Toronto: Centre for Addiction and Mental Health. American College Health Association (2006). National college health assessment. Journal of American College Health, 54, 201–211. Bailey, D. S., & Taylor, S. P. (1991). Effects of alcohol and aggressive disposition on human physical aggression. Journal of Research in Personality, 25, 334–342. Barnwell, S. S., Borders, A., & Earleywine, M. (2006). Alcohol-aggression expectancies and dispositional aggression moderate the relationship between alcohol consumptions and alcohol-related violence. Aggressive Behavior, 32, 517–527. Bushman, B. J. (1997). Effects of alcohol on human aggression: Validity of proposed mechanisms. In M. Galanter (Ed.). Recent developments in alcoholism (Vol. 13, pp. 227–244). New York: Plenum Press. Buss, A. H., & Perry, M. (1992). Personality processes and individual differences. The aggression questionnaire. Journal of Personality and Social Psychology, 63, 452–459. Collins, R. L., Quigley, B. M., & Leonard, K. E. (2007). Women’s physical aggression in bars: An event-based examination of precipitants and predictors of severity. Aggressive Behavior, 33, 304–313.
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Personality and Individual Differences 45 (2008) 3–9 www.elsevier.com/locate/paid
Severity of physical aggression reported by university students: A test of the interaction between trait aggression and alcohol consumption Paul F. Tremblay *, Kathryn Graham, Samantha Wells The Centre for Addiction and Mental Health, The University of Western Ontario, 100 Collip Circle, Suite 200, London, Ontario, Canada N6G 4X8 Received 19 September 2007; received in revised form 4 February 2008; accepted 13 February 2008 Available online 24 March 2008
Abstract As part of an online study of aggression, 2647 full-time undergraduate students at six Canadian universities completed the Aggression Questionnaire (Buss & Perry, 1992) and reported on whether they had been involved in incidents of physical aggression in the past 12 months at a (1) bar, nightclub or pub, (2) home or residence, and (3) party or social event. About one in four students reported an incident. Scores on the physical trait aggression scale correlated significantly with severity of aggression in the three locations. Males reported higher severity than did females in the three locations. Alcohol consumption was significantly related to severity at bars and parties but not in the home location. Support was found for an interaction between the physical trait aggression scale and drinking at the time of the incident predicting the severity of aggression for incidents reported in bars. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Trait aggression; Severity of aggression; Alcohol consumption; Physical aggression
1. Introduction Recent studies indicate that the risk of physical aggression is particularly high among college students. A survey of 47,202 college students on 74 campuses by the American College Health Association in 2004 revealed that 6.7% (4.2% females and 10.8% males) had been involved in a fight and that 4.7% (2.8% females and 7.8% males) had injured another person after drinking in the last school year (American College Health Association, 2006). A study of the one-year prevalence of aggression among young adults in Buffalo, New York revealed that, for the college sample, 19.8% of men and 12.3% of women had initiated an act of violence in the previous year, and 28.8% of men and 16.6% of women reported being the target of violence (Leonard, Quigley, & Collins, 2002). *
Corresponding author. Fax: +1 519 858 5199. E-mail address: [email protected] (P.F. Tremblay).
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A large proportion of naturally-occurring aggression involves alcohol (Barnwell, Borders, & Earleywine, 2006; Graham, Wells, & Jelley, 2002; Wells, Graham, & West, 2000). Experimental studies have found a pattern for aggression to be more severe when subjects have been drinking (see meta-analysis by Bushman, 1997). Naturalistic studies have also shown a link between severity of aggression and level of alcohol consumption (e.g., Graham, Osgood, Wells, & Stockwell, 2006; Graham & Wells, 2001; Leonard, Collins, & Quigley, 2003; Wells & Graham, 2003). It is particularly important to study college populations among whom substantial proportions report being in a fight while drinking (Quigley, Corbett, & Tedeschi, 2002). Since ‘‘there is no one-to-one relationship between alcohol use and human behavior” (Pernanen, 1981, p. 13), it is necessary to identify types of people among whom and situations in which alcohol increases aggression severity. The same proposition has been reiterated by Giancola (2003) in a special issue article on the role of individual difference
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and contextual factors contributing to the alcohol-aggression relation. A key variable thought to moderate the alcohol-aggression relation is trait aggression which refers to a stable disposition or tendency to engage in aggressive behaviors on repeated occasions and in various situations. Experimental research suggests an interaction effect of trait aggression with alcohol consumption resulting in higher levels of aggression than from the separate effects of these two variables (Bailey & Taylor, 1991; Dougherty, Bjork, Bennett, & Moeller, 1999; Giancola, 2002). This interaction may be due to alcohol lowering the already low threshold for aggression among those high on trait aggression. More specifically Giancola (2002) has suggested that alcohol may be stimulating pre-existing angry and hostile cognitions and affect in high trait aggressive individuals. Although there are an increasing number of experimental studies investigating this interaction, the present study addresses a need for research assessing this interaction in naturalistic settings. Situational factors such as provocation, the presence and behavior of third parties and level of environmental permissibility may also be involved (see review by Graham et al., 1998). Drinking settings vary considerably in terms of norms regarding socially appropriate behavior as well as in terms of the presence and levels of provoking stimuli. Aggression has been found to be more likely when drinking occurs in bars (Leonard et al., 2002; Norstro¨m, 1998; Stockwell, Lang, & Rydon, 1993) and at parties (Wells, Mihic, Tremblay, Graham, & Demers, 2008) than in other contexts. Some evidence indicates that the effect of alcohol consumption on aggression may be greater for men than for women (Bushman, 1997; Giancola et al., 2002; GusslerBurkhardt & Giancola, 2005). Additionally, several studies suggest that the increased risk of aggression in bars is higher for men than for women (Collins, Quigley, & Leonard, 2007; Graham & Wells, 2001) while drinking at a party, compared to other circumstances, poses a risk for women but not for men (Wells et al., 2008). In the present study, we investigate the relationship of naturally-occurring physical aggression reported by university students with trait aggression and sex of the student and alcohol consumption at the time of the incident, for incidents occurring in three common social contexts for student drinking (Adlaf, Demers, & Gliksman, 2005): bars, house/residence, and party/social events. The main hypothesis is that, in all three social contexts, aggression will be most severe when alcohol consumption is combined with high physical trait aggression. The second hypothesis is that alcohol consumption will be more strongly related to severity of aggression for men than for women.
(one Atlantic, three Eastern, one Central, and one Western provinces) ranging in size from 2000 to 26,000 students to participate in an online study assessing alcohol consumption, physical aggression, and perceptions of alcoholrelated hypothetical conflict situations. The focus of the present article is on the self-reports of incidents of physical aggression in the previous year. A standard invitation letter requesting participation and providing a link to the password-protected study website was sent to full-time undergraduate students via e-mail at the beginning of the second academic semester (January). The website included a consent form that students were asked to read and print prior to participating and a questionnaire that required approximately 30 min to complete. Three reminder e-mail letters were sent at two-week intervals to students who had not yet participated. As an incentive to participate, respondents were entered into a draw for a chance to win one of two $1000 cash prizes. Of the 8788 valid invitations, 3423 students responded to the request. The final sample was 2647 after exclusion of 776 responses due to a large amount of missing data. The response rate (ratio between usable data sample/email invitation sample) was 30.1%. 2.2. Sample description The sample was comprised of 63.6% females and 36.4% males with 26.5% in first year, 26.0% in second year, 26.5% in third year, and 20.9% in fourth year of university study. The mean age was 21.2 years and 90.8% were between the ages of 18 and 24. In terms of experience with alcohol consumption, 94.8% had ever consumed alcohol, and 94.2% reported feeling mildly intoxicated at least once. The mean number of drinking occasions per week based on the last 12 months was 0.79 times for females and 1.12 times for males. The mean number of drinks per typical drinking occasion was 4.77 drinks (SD = 3.63, median = 4.0) for females and 5.75 drinks (SD = 4.29, median = 5.0) for males.1 These values are similar to those reported by Adlaf et al. (2005) in the 2004 Canadian Campus Survey. 2.3. Measures 2.3.1. Aggression severity Experiences of aggression were assessed in three locations in addition to one ‘other’ location category. For the bar, nightclub or pub location, subjects were asked: ‘In the past 12 months, have you been personally involved in an incident that occurred at a bar, nightclub, or pub where someone grabbed, hit, kicked, pushed, shoved or was physically aggressive towards you in some way or where you did any of these things to someone else?’
2. Method 2.1. Recruitment procedure A total of 1500 full-time undergraduate students were randomly selected from each of six Canadian universities
1
Note all responses of 16 or more drinks were scored as 16.
P.F. Tremblay et al. / Personality and Individual Differences 45 (2008) 3–9
The same question was presented for (2) home or residence with people you live with; (3) party or social event at someone’s home, residence, at the beach, wedding, etc; and (4) any other location. Due to the small number of respondents indicating aggression in other locations and the heterogeneous nature of this category, ‘‘other location” was not included in the analyses by locations. When respondents answered ‘‘yes” to aggression in any of the locations, they were asked to focus on the most severe incident (in that location) in which they had been involved during the past 12 months and asked: ‘‘How physically aggressive were you during this incident?” (0 – Not at all physically aggressive and 10 – Extremely physically aggressive). 2.3.2. Alcohol consumption For those reporting aggression, they were asked: ‘‘Were you or your main opponent drinking at the time of the incident?” The options were (1) both of us were drinking; (2) only my opponent was drinking; (3) I was the only one drinking; and (4) neither of us was drinking. Given the focus of the present study on trait aggression of the participants, we measured only whether or not the participant was drinking (i.e., options 1 and 3), regardless of whether the opponent was drinking. 2.3.3. The Aggression Questionnaire (Buss & Perry, 1992) This 29-item measure consists of four related dimensions: physical (nine items), verbal (five items), anger (seven items), and hostility (eight items), with respondents rating each item on a scale of 1 (extremely uncharacteristic of me) to 5 (extremely characteristic of me). Internal consistency values (Cronbach’s alpha) based on the current sample were .84 (physical aggression), .74 (verbal aggression), .82 (anger), .78 (hostility) and .89 (total scale). 3. Results 3.1. Trait aggression scores and prevalence of incidents Trait aggression scores by sex are presented in Table 1. Independent t-tests revealed significantly higher scores for males than for females on all scales (p < .01) except anger. Of the 2581 respondents2 who completed the section on aggressive incidents in the past year, 636 (24.6%) reported experiencing at least one incident. Significantly more males (30.2%) than females (21.5%) reported at least one incident ðv2ð1Þ ¼ 23:91; p < :01Þ. Of those reporting aggression, 75.1% reported an incident in only one location, 20.6% in two locations, 2.7% in three, and 1.6% in all four locations (bar, home/residence, party/social event, and other). The proportion of respondents reporting at least one incident varied across universities (20.1–26.9%), but these differences were not significant ðv2ð5Þ ¼ 5:02; nsÞ. 2
Sixty six respondents (2.5%) did not respond to this question.
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Table 1 Mean and (standard deviation) of trait aggression scores by sex Scale
Males M (SD)
Females M (SD)
Physical Verbal Anger Hostility Total score
20.31 14.01 14.19 18.69 67.16
15.57 13.12 14.43 17.95 61.03
(7.11) (4.09) (5.29) (6.12) (16.84)
(5.66) (3.94) (5.02) (5.99) (15.41)
Note: Independent t-test indicated that males have significantly higher scores (p < .01) than do females on all scales except anger. Male n = 952– 958; female n = 1670–1676.
3.2. Aggression severity as a function of sex and alcohol consumption Table 2 shows the results of analyses of variance of sex by alcohol consumption at the time of the incident on the severity scores (0 – not at all physically aggressive and 10 – extremely physically aggressive) performed separately for each location. As shown by the numbers for each cell, only a small number of males were not drinking when incidents occurred at a bar or party (n = 26 for bar and n = 13 for party) while only a small number of females (n = 20) were drinking when aggression occurred in the home. The severity scores of males were significantly higher than those of females in the bar and home locations (and close to significance in the party location; p < .06). Respondents who indicated that they were drinking at the time of the incident rated their aggression significantly higher than those not drinking in the bar and party locations but not in the home location. No significant drinking by sex interactions were found. 3.3. Aggression severity as a function of trait aggression by sex and location Table 3 presents the correlations between the trait aggression scales and severity of aggression by sex for each location. Severity was most strongly correlated with physical trait aggression for males and females in all locations except for a slightly higher correlation with the total aggression scale for females in the party location. 3.4. The interaction of trait aggression with alcohol consumption The hypothesis that drinking and trait aggression interact to determine severity of aggression was investigated in multiple regression analyses presented in Table 4. The same model was applied to the data for the three drinking locations. As in previous studies assessing the interaction of trait aggression with alcohol consumption (e.g., Giancola, 2002), the physical scale rather than the total scale from the Aggression Questionnaire was used as the trait measure because it is particularly relevant to physical aggression. Physical scale scores were centered around the grand mean and the drinking status and sex variables were dummy
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Table 2 Mean severity of aggressive behavior by sex and drinking at the time of incident for each location Location
Males
Females
Relationship with severity of aggression
Respondent drinking status
M (SD)
n
M (SD)
n
Bar Drinking Not drinking Total
5.14 (3.32) 3.42 (3.06) 4.85 (3.34)
126 26 152
3.53 (3.04) 2.49 (2.66) 3.28 (2.98)
132 41 173
Drink: F(1, 321) = 10.05, p < .01, x2 = .03 Sex: F(1, 321) = 8.55, p < .01, x2 = .02 Drink sex: F(1, 321) = 0.60, ns
Home Drinking Not drinking Total
5.35 (2.74) 4.93 (2.90) 5.12(2.82)
46 56 102
3.40 (3.17) 3.66 (2.91) 3.63 (2.93)
20 141 161
Drink: F(1, 259) = 0.03, ns Sex: F(1, 259) = 12.74, p < .01, x2 = .04 Drink sex: F(1, 259) = 0.57, ns
Party Drinking Not drinking Total
5.37 (3.03) 3.77 (2.32) 5.08 (2.97)
59 13 72
4.21 (3.30) 2.50 (3.07) 3.51 (3.29)
29 20 49
Drink: F(1, 117) = 6.65, p < .01, x2 = .04 Sex: F(1, 117) = 3.60, p < .06, x2 = .02 Drink sex: F(1, 117) = 0.07, ns
Table 3 Correlations between trait aggression scales and severity of aggression by sex for each location Measure
Bar
Trait aggression scales
Male (n = 152) .52**
Female (n = 174) .45**
Total (n = 327) .53**
Male (n = 102) .30**
.07 .16* .02 .28**
.08 .25** .11 .32**
.10 .20** .03 .33**
.03 .14 .11 .14
Physical Verbal Anger Hostility Total Note: *p < .05,
Home
Party Female (n = 161) .36** .17* .25**
Total (n = 263) .39**
Male (n = 71) .60** .25* .33**
.11 .19** .01 .26**
.11 .30**
Female (n = 49) .50** .19 .44** .48**b .52**
.03b .44**
Total (n = 120) .59** .23* .34** .19* .48**
p < .01, bz test of significance indicating significant difference between correlation coefficients at .01.
**
Table 4 Aggression severity as a function of physical trait aggression, sex and drinking in three locations Bar (N = 325) DR 1.
2.
3.
2
**
Intercept sex drinking physical
.29
Intercept sex drinking physical drinking physical
.01*
Intercept sex drinking physical drinking physical drinking sex sex physical drinking sex physical
.01
Home (N = 261)
b
SE b **
B
DR
2
**
3.32 .23 .74 .19**
.37 .33 .38 .02
.17 .04 .09 .50
3.03** .24 1.01* .10* .12*
.39 .33 .40 .05 .05
.04 .13 .25 .27
3.14** .26 .85 .13 .05 .15 .07 .14
.57 .78 .62 .08 .08 .86 .10 .11
.00
.01 .04 .11 .33 .12 .02 .12 .23
b
Party (N = 120) SE b
**
B
3.93 .92* .31 .13**
.23 .38 .43 .02
.15 .05 .36
3.94** .90* .25 .14** .02
.23 .39 .45 .03 .05
.15 .04 .37 .04
4.11** .59 .97 .17** .08 1.12 .08 .10
.25 .45 .69 .03 .08 .92 .06 .10
DR2
b
.37**
3.59** .45 .84 .19**
.51 .51 .56 .03
.07 .12 .54
3.63** .45 .81 .20** .01
.54 .51 .58 .06 .06
.07 .11 .57 .03
3.83** .27 .56 .26** .14 .08 .16 .27*
.67 1.00 .83 .07 .09 1.17 .12 .13
.04 .08 .72 .32 .01 .31 .49
.00
.03 .10 .14 .46 .12 .14 .14 .14
Note: Physical = physical aggression scale score centered; drinking = yes (1) no (0); male (1) and female (0); *p < .05;
coded (drinking = 1 and not drinking = 0; male = 1 and female = 0). The analyses were conducted in three steps as described below. The analyses were conducted in three steps. At step 1, sex, drinking, and physical trait aggression were entered
SE b
B
**
p < .01.
to inspect their independent relations with severity of aggression. The physical trait aggression scale was a significant predictor of aggression severity in all three locations. Although the previous analyses of variance that controlled only for sex (Table 2) indicated that alcohol consumption
P.F. Tremblay et al. / Personality and Individual Differences 45 (2008) 3–9
10
Females 10
9
9 Aggression Severity
Not Drinking Drinking
8 Aggression Severity
7
7 6 5 4 3
Not Drinking
8
Drinking
7 6 5 4 3 2
2
1
1
0 -1 SD
0 -1 SD
M
1 SD
M
1 SD
Trait Aggression (Physical) Centered
Trait Aggression (Physical) Centered
was significantly related to aggression severity in bar and party locations, this relationship was no longer significant in any of the locations when both sex and physical trait aggression were in the model. Similarly, the higher severity of aggression for males than for females found in the previous analyses was no longer significant in these models except in the home location where males reported significantly more severe aggression than did females. In step 2, the interaction between physical trait aggression and alcohol consumption was tested, with this interaction found to be significant in the bar location but not in the home or party locations. As illustrated in Fig. 1, aggression severity increased with higher levels of physical trait aggression more for respondents who were drinking than for those who were not. In step 3, the three–way interaction between physical trait aggression, sex and alcohol consumption was tested to determine whether the trait aggression by alcohol consumption interaction differs for men and women. A significant three-way interaction was found in the party location only and is illustrated in Fig. 2 separately for males and females. The male graph shows an interaction of physical trait aggression by drinking similar to the overall interaction found in the previous step in the bar location (i.e., Fig. 1). The pattern of females is in the opposite direction, with the effect of drinking stronger for females with low physical trait aggression than for females with high physical trait aggression. Although this three-way interaction was significant at p < .05, given that the increase in R2 was not significant from Step 2 to Step 3, this effect should be interpreted with caution. In fact, post hoc tests investigating the physical trait aggression by drinking interaction separately for males and females revealed that it was not significant in the male or the female sample.
Males
10 9 Aggression Severity
Fig. 1. Aggression severity in bars as a function of the interaction between trait aggression and drinking status. Note: Aggression Severity’Drink** (Physical Trait Agg); Aggression Severity’Not Drinking = 4.15 + .22 * * ** p < .01. ing = 3.14 + .10 (Physical Trait Agg); p < .05,
8
Not Drinking
7
Drinking
6 5 4 3 2 1 0 -1 SD
M
1 SD
Trait Aggression (Physical) Centered Fig. 2. Aggression severity at parties as a function of the interaction between trait aggression, drinking status, and sex. Note: Aggression Severity’Female Drinking = 4.39 + .12 (Physical Trait Agg); Aggression Severity’Female Not Drinking = 3.83 + .26** (Physical Trait Agg); Aggression Severity’Male Drinking = 4.74 + .24** (Physical Trait Agg); Aggression Severity’Male Not Drinking = 4.10 + .10 (Physical Trait Agg); **p < .01.
4. Discussion This study revealed that about one in four students was involved in an incident of physical aggression in the twelve month period preceding data collection. Males were more likely than females to report an incident and had higher trait scores than did females on all scales except the anger scale. This pattern is similar to sex differences reported by Buss and Perry (1992). Males reported using more severe aggression than did females in all locations, and alcohol consumption was associated with more severity at bars and parties. However, in the regression models including all three predictors (sex, alcohol consumption, and physical trait aggression) only the physical scale was a significant predictor of aggression severity in all three locations, and sex remained significant in only the home/residence location. Taken together, these findings suggest that the main determinant of severity of aggression is physical trait aggression, with at least some of the relationship between
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severity and alcohol consumption and sex due to males being both more likely than females to drink and to have higher trait aggression scores. The primary aim of the study was to test the interaction between trait aggression and drinking in predicting aggression severity and partial support was found. The hypothesis that trait aggression has a greater influence on the aggression of people who have consumed alcohol was supported for incidents in a bar but not in the other settings. This may reflect differences in social norms among locations. In particular, proscriptive norms regarding socially appropriate behavior may be weaker in bars where it is not unexpected for some fights to occur. Evidence suggests that loud, boisterous, and aggressive behavior is often perceived as being completely normative and acceptable in barroom settings (Graham & Wells, 2003; Tomsen, 1997). This in turn may allow for the expression of individual differences in trait aggression, especially for people who have been drinking. Some evidence, although statistically weak, was found for a three-way interaction between sex, trait aggression, and alcohol consumption but only in the party location. Among males, the influence of trait aggression was greater when the respondent had been drinking; however, among females the increase was greater among respondents who had not been drinking. This finding may reflect that aggressive experiences of men and women are very different, particularly in the social setting of a party, and thus is influenced by different processes. However, because this finding was not entirely stable (i.e., not evident when separate analyses were conducted for men and women) it requires further investigation in future research. Contrary to some experimental studies (reviewed Section 1), no evidence was found in the present study, in any of the three locations, supporting the hypothesis that alcohol consumption has a greater effect on the severity of aggression by males than by females. This finding is congruent with some studies (Dougherty et al., 1999; Wells et al., 2008). Gussler-Burkhardt and Giancola (2005) have discussed the inconsistent findings regarding sex differences in alcohol-related aggression in the experimental literature and argue that the discrepancy may be due to different experimental stimuli (i.e., Taylor aggression paradigm vs. point subtraction aggression paradigm). They add that women may have a greater threshold than men, and that in some situations, alcohol alone may not be sufficient to trigger an aggressive response. It is important, to interpret the results of the present study in relation to how the variables were operationalized and how these measures differ from those of studies in the experimental literature. In the present study, an item assessed whether the respondent consumed alcohol at the time of the incident without taking into account the volume of alcohol or the level of intoxication. It is therefore realistic to assume that there would be considerable variability in intoxication levels among respondents. Thus, in future research it would be necessary to assess not only whether the respon-
dent was drinking but also ascertain how much alcohol was consumed and how intoxicated the person became. In the present study, severity was a self-assessment of how much physical aggression was used by the respondent without taking into consideration the motive or the nature of the provocation. By contrast, in experimental studies, physical aggression is measured using an objective standardized outcome (e.g., the number and intensity of shocks) and the provocation for all participants is standardized. Thus, the heterogeneity and subjectivity of measuring aggression in real-life settings, while more natural, may also obscure some results. It is also important to note that analyses in the three locations are not directly comparable due to differences in the sample size (i.e., much smaller sample size in the party location) and therefore differences in statistical power. Overall, while taking into account these limitations, the results have important implications for understanding aggression risk factors as they highlight the role of individual differences in trait aggression after controlling for sex and alcohol consumption. The present study highlights the importance of including trait aggression measures in naturalistic research designs. Additionally, future naturalistic studies assessing the interaction between trait aggression and alcohol consumption in predicting the severity of aggression need not only ensure control for alcohol and intoxication levels but also need to take into account the social context. Acknowledgements This study was supported by a grant (MOP62747) to Paul F. Tremblay and Kathryn Graham from the Canadian Institutes of Health Research (CIHR). We thank R.C. Gardner for his suggestions related to the statistical analyses. References Adlaf, E. M., Demers, A., & Gliksman, L. (Eds.). (2005). Canadian campus survey 2004. Toronto: Centre for Addiction and Mental Health. American College Health Association (2006). National college health assessment. Journal of American College Health, 54, 201–211. Bailey, D. S., & Taylor, S. P. (1991). Effects of alcohol and aggressive disposition on human physical aggression. Journal of Research in Personality, 25, 334–342. Barnwell, S. S., Borders, A., & Earleywine, M. (2006). Alcohol-aggression expectancies and dispositional aggression moderate the relationship between alcohol consumptions and alcohol-related violence. Aggressive Behavior, 32, 517–527. Bushman, B. J. (1997). Effects of alcohol on human aggression: Validity of proposed mechanisms. In M. Galanter (Ed.). Recent developments in alcoholism (Vol. 13, pp. 227–244). New York: Plenum Press. Buss, A. H., & Perry, M. (1992). Personality processes and individual differences. The aggression questionnaire. Journal of Personality and Social Psychology, 63, 452–459. Collins, R. L., Quigley, B. M., & Leonard, K. E. (2007). Women’s physical aggression in bars: An event-based examination of precipitants and predictors of severity. Aggressive Behavior, 33, 304–313.
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