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Journal of Abnormal ChUd Psycholo~, VoL 20, No. 5, 1992 Aggression in Boys with Attention Deficit-Hyperactivity Disorder: Methylphenidate Effects on Naturalistically Observed Aggression, Response to Provocation, and Social Information Processing Debra A. Murphy, t,4 William E. Pelham, 2 and Alan R. Lang 3 High and low-aggressive boys with attention deficit-hyperactivity disorder (ADHD) were compared and the effects of methylphenidate were examined on measures from three domains of aggression: (1) directly observed verbal and nonverbal aggressive behaviors exhibited in the context of a day treatment program, (2) aggressive responding when provoked during a laboratory task, and (3) social information processing patterns exhibited on task.s" designed to tap the putative cognitive components of aggression. The high-aggressive (HA) and low-aggressive (LA) subgroups differed significantly on observational measures of aggression and on the laboratory provocation task, but the HA group showed more deviant cognitions on only one of the numerous measures of social information processing. Regarding medication effects on the direcr observation measures, methylphenidate decreased aggression for both subgroups. On the laboratory provocation task, methylphenidate had only Manuscript received in final form March 3, 1992. Portions of the data reported in this article were derived from a doctoral dissertatio_a conducted by the first author under the supervision of the third author. All of the data werc gathered in the 1985 Summer Treatment Program at the Florida State University under the direction of the second author. The authors would like to thank the staff of the summer program for their assistance. Drs. Lang and Pelham were supported in part by NIAAA grant AA06267 when the study was conducted, and Dr. Pelham was supported in part by DA 05605 and MH 48157 during preparation of this manuscript. 1Department of Psychiatry and Mental Health Science, Medical College of Wisconsin, Milwaukee, Wisconsin 53226. 2Western Psychiatric Institute and Clinic, Pittsburgh, Pennsylvania 15213. 3Florida State University, Tallahassee, Florida 32306. 4Address all correspondence, including requests for reprints, to Debra A. Murphy, Ph.D, Department of Psychiatry and Mental Health Science, Medical College of Wisconsin, 87f~1 Watertown Plank Road, Milwaukee, Wisconsin 53226. 451 0091-0627/92/1000-0451506.50/0 1992 Plenum ! ublishing Corporation

Aggression in boys with attention deficit-hyperactivity disorder: Methylphenidate effects on naturalistically observed aggression, response to provocation, and social information processing

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Journal of Abnormal ChUd Psycholo~, VoL 20, No. 5, 1992

Aggression in Boys with Attention Deficit-Hyperactivity Disorder: Methylphenidate Effects on Naturalistically Observed Aggression, Response to Provocation, and Social Information Processing

Debra A. Murphy, t,4 William E. Pelham, 2 and Alan R. Lang 3

High and low-aggressive boys with attention deficit-hyperactivity disorder (ADHD) were compared and the effects of methylphenidate were examined on measures from three domains of aggression: (1) directly observed verbal and nonverbal aggressive behaviors exhibited in the context of a day treatment program, (2) aggressive responding when provoked during a laboratory task, and (3) social information processing patterns exhibited on task.s" designed to tap the putative cognitive components of aggression. The high-aggressive (HA) and low-aggressive (LA) subgroups differed significantly on observational measures of aggression and on the laboratory provocation task, but the HA group showed more deviant cognitions on only one of the numerous measures of social information processing. Regarding medication effects on the direcr observation measures, methylphenidate decreased aggression for both subgroups. On the laboratory provocation task, methylphenidate had only

Manuscript received in final form March 3, 1992. Portions of the data reported in this article were derived from a doctoral dissertatio_a conducted by the first author under the supervision of the third author. All of the data werc gathered in the 1985 Summer Treatment Program at the Florida State University under the direction of the second author. The authors would like to thank the staff of the summer program for their assistance. Drs. Lang and Pelham were supported in part by NIAAA grant AA06267 when the study was conducted, and Dr. Pelham was supported in part by DA 05605 and MH 48157 during preparation of this manuscript.

1Department of Psychiatry and Mental Health Science, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.

2Western Psychiatric Institute and Clinic, Pittsburgh, Pennsylvania 15213. 3Florida State University, Tallahassee, Florida 32306. 4Address all correspondence, including requests for reprints, to Debra A. Murphy, Ph.D, Department of Psychiatry and Mental Health Science, Medical College of Wisconsin, 87f~1 Watertown Plank Road, Milwaukee, Wisconsin 53226.

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0091-0627/92/1000-0451506.50/0 �9 1992 Plenum ! ublishing Corporation

452 Murphy, Pelham, and Lang

minimal effects. Significant drug effects were obtained on only two recall social information processing measures.

The primary objective of pharmacological intervention in attention deficit-hyperactivity disorder (ADHD) is to reduce inattention, impulsivity, and overactivity, and the effects of Central Nervous System stimulants on these behaviors are well documented (e.g., Gitteiman & Kanner, 1986). However, effects of psychostimulant medication on aggressive behaviors that contribute heavily to referrals for treatment of ADHD have been less fully explored (Hinshaw, 1991). Several early studies (Arnold, Kirilcek, Cor- son, & Corson, 1973; Eisenberg et al., 1963) noted general effects of psy- chostimultants in decreasing antisocial behavior, as rated by parents, teachers, and therapists, in children with characteristics of ADHD. In more recent studies with more clearly defined samples and more objective meas- ures, methylphenidate (MPH) treatment of ADHD children has been found to reduce (a) instigations of negative interactions with peers (Cun- ningham, Siegel, & Offord, 1985), (b) disruptive behaviors in a classroom setting (Pelham, Bender, Caddell, Booth, & Moorer, 1985), and (c) nega- tive social behaviors directed toward peers (Whalen et al., 1987).

Other investigations of psychostimulant effects on negative social be- haviors in ADHD boys during observations of play have pointed to the potentially critical role of subgroup differences. For example, Pelham and Bender (1982) reported on MPH effects in a play group setting and pemo- line effects on the playground. They found no overall effects of the stimu- lants on peer-directed aggression, but clear beneficial effects for the children who exhibited high levels of aggression in the unmedicated state. Other studies have also shown greater effects of medication on the most aggressive boys (Hinshaw, Henker, Whalen, Erhardt, & Dunnington, 1989).

Results from studies of stimulant effects on social behavior have de- pended in part on the contexts in which the behavior has been assessed, and a firm conclusion regarding how stimulants affect ADHD boys' nega- tive interpersonal behavior has not yet been drawn. Studies of dyadic in- teractions, for example, have not yielded results consistent with studies of group settings (cf. Cunningham et ai., 1985; Pelham & Hoza, 1987). Fur- ther, it is not clear that initiated vs. provoked aggression are similarly af- fected by stimulants (e.g., Hinshaw, Buhrmester, & Heller, 1989). In order to study some of these contextual variables, it would be worthwhile to study stimulant effects on aggression not only in natural settings, but also in labo- ratory settings in which better control over relevant parameters can be ob- tained. There is a long history of studying adult aggression and drug effects thereon in laboratory settings, but this research has rarely focused on clini- cal populations (Lang & Sibrel, 1989). Similarly, the laboratory research

Methy|phenidate and Aggression 453

that has been conducted with children has focused primarily on television's effects on children's aggression (e.g., Parke & Slaby, 1983) and not on psy- chopathology. Although several early studies (e.g., Peterson, 1971), and sev- eral recent ones (Dodge, 1980) have included laboratory analogues of aggression with children, none of the paradigms has been used to study drug effects in children and none has studied clinical samples. One par- ticularly applicable laboratory analogue is the aggression game (Eron, WaP der & Lefkowitz, 1971) in which a child's response to provocation from a presumed opponent in a game can be assessed. This paradigm is similar to analogues that have been used to study adult aggression in the laboratory (e.g., Taylor & Gammon, 1975), and drug effects thereon (e.g., Cherek, Steinberg, Kelly, & Robinson, 1986; Zeichner & Pihl, 1979). One advantage of the laboratory analogue is that it permits study of psychostimulant effects for both provoked and unprovoked aggression, which can be manipulated experimentally.

Assuming that stimulant medication reduces some types of aggressive behavior in some contexts for at least some aggressive ADHD boys, an important question is whether these effects are cognitively mediated. The theoretical focus of this question is especially intriguing when one considers the increasing attention given to the role of social cognition in mediating aggressive behavior in recent years (Parke & Slaby, 1983). For example, it has been proposed that aggressive children have deficits in one or more of several stages involved in responding to social stimuli, including (a) en- coding, (b) interpretation, (c) generation and selection of appropriate be- havioral responses, and (d) verbal or motor enactment of the response (Dodge, 1986). Recent modifications of this theory suggest that social in- formation processing deficits are most prominent in the subgroup of ag- gressive children who are impulsive, hyperactive, and reactive (e.g., Dodge &Coie, 1987; Milich & Dodge, 1984). Milich and Dodge (1984) noted that the social information processing deficits found in hyperactive-aggressive boys may actually be due to attentional difficulties, impulsivity, or to a com- bination of both. Indeed, the reactive aggressive boys described by Dodge and Coie (1987) appear to be boys that might alternatively be labeled as impulsive. Indeed, quick responding and limited recall may be the "cognP tive paths that lead to attributional bias" (Dodge, 1980, p. 378). Given that these cognitive processes are among those affected positively by CNS stimuo lants (Pelham, 1986), it is reasonable to speculate that social information processing in hyperactive-aggressive boys may be affected by stimulants~ and further, that these changes in information processing may mediate the apparently salutary effects of stimulants on aggression. No studies of this possibility have been conducted.

454 Murphy, Pelham, and Lang

The present study thus had two purposes. The first was to compare subgroups of ADHD children rated as high and low aggressive on three sets of measures related to aggression: (a) directly observed verbal and non- verbal aggressive behaviors exhibited in the context of a day treatment pro- gram; (b) aggressive responding when provoked during a laboratory task; and (c) social information processing patterns exhibited by these boys on tasks designed to tap the putative cognitive components of aggression noted above. The second purpose was to assess the effects of 0.3 mg/kg MPH on these three different aspects of aggression in the high- and low-aggressive ADHD subgroups.

METHOD

Subjects and Setting

Subjects were 26 ADHD boys participating in the 1985 Summer Treatment Program at the Florida State University. Based on structured, parental interviews, as well as parent and teacher rating scales, all boys were diagnosed as attention-deficit-disordered 5 and were divided into high- aggressive (HA) and low-aggressive (LA) subgroups. Children were as- signed to HA and LA groups according to the following procedure conducted jointly by two clinical psychologists, who conducted the diagnos- tic intakes. First, boys with three or more symptoms of conduct disorder based on the structured parent interview were assigned to the HA subgroup and the remaining boys were assigned to the LA group. Second, teacher and parent ratings and history were examined to determine whether they supported the original assignment. In five cases, both of the evaluators de- cided that the bulk of the data failed to confirm maternal report and sup- ported switching the original group assignments. The resulting groups consisted of 14 HA boys and 12 LA boys. Seven of the HA boys and none of the LA boys met DSM-III-R criteria for conduct disorder. Parent ratings were obtained on the Abbreviated Conners Parent Rating Scale (Goyette, Conners, & Ulrich, 1978), and teacher ratings were also obtained on the Conners Teacher Rating Scale (CTRS; Conners, 1969). Two scales were derived from the CTRS: the Abbreviated Conners Teacher Rating Scale (ACTRS; Conners, 1969) and the IOWA Conners Teacher Rating Scale (Loney & Milich, 1982; Pelham, Milich, Murphy, & Murphy, 1989). The

5The structured interview was the one employed in the DSM III-R field trials, in which these boys were subjects. Analyses of the reliability of ADHD and conduct disorder diagnoses for the field trial are reported in Spitzer, Davis, and Barkley (1990). We are grateful to R. Milich for assisting in the diagnoses of the subjects described herein~

Methylphenidate and Aggression 455

children ranged in age from 6 years 5 months to 11 years, and none was mentally retarded or had gross neurological disorders. Rating scale scores and subject characteristics are summarized in Table I.

A day in the Summer Treatment Program was divided into the fol- lowing activities: two academic classroom periods each staffed by a special education teacher and an aide, an art class, a swimming period, three pe- riods of supervised, group, outdoor recreational activities (e.g., dodge ball), and lunch. For all activities except the academic classrooms, four or five counselors supervised 10 children grouped by sex and age. The 26 children who received the drug protocol described herein were distributed across five different groups.

Procedure

Medication and Overview. All boys in this study participated in dou- ble-blind, placebo-controlled, clinical assessments of the efficacy of psy- chostimulant medication (see Pelham & Hoza, 1987, for a detailed description). Identically encapsulated MPH or placebo was administered twice daily, once with breakfast and again at midday. Drug and. placebo were varied in random order over single days. Most of the subjects in the present study were receiving two doses or types of MPH, but the data re- ported herein were gathered on days when they received 0.3 mg/kg MPH or placebo b.i.d. Medication assessments were conducted during the last 5

Table I. Subject Characteristics

High aggressive Low aggressive

Measure Mean SD Mean SD

Age (years) 8.0 (1.6) 7.9 (1.7) IQ/W1SC-R 97.1 (15.7) 101.3 (13.4) Abbreviated Conners Rating Scale

Parent 18.3 (7.0) 19.1 (5.8) Teacher 20.1 (4.4) 19.8 (4.0)

IOWA Conners Teacher Rating Scale Inattention/overactivity 11.6 (2.5) 12.5 (1.7) Oppositional/defiant 9.9 (4.1) 6.8 (3.3)

Woodcock-Johnson Achievement Test (standard scores)

Reading 90.6 (14.4) 92.3 (13.8) Math 100.9 (14.0) 94.2 (14.9) Language 92.4 (14.0) 89,4 (I 1.7)

aNote. The only variable on which the groups differ is the IOWA Conners oppositional/defiant factor [t(24) = 2.1, p < .05]. There were 14 children in the high-aggressive subgroup and 12 children in the low-aggressive subgroup.

456 Murphy, Pelham, and Lang

weeks of the 7-week program, and boys received medication for 5 to 9 days per medication condition. The nonlaboratory measures reported below are based on that number of days. The laboratory provocation measure and the three social information processing measures were administered by blind experimenters in separate sessions twice, once each on MPH and placebo. Each session lasted approximately 65 min. Order of the four tasks was randomized.

Naturalistic Observations and Frequency Counts. A token econ- omy/point system was in effect during each day of the treatment program, and frequency counts of behaviors included in the point system were used as dependent measures (see Pelham & Hoza, 1987 for a complete descrip- tion of these measures). Behavioral categories formed by grouping individ- ual categories from the point system included (a) negative verbalizations (the sum of verbal abuse, name calling/teasing, and cursing/swearing), and (b) conduct problems (the sum of lying, stealing, destruction of property~ and aggression). The median interrater reliability (correlations between in- dependent observers' records and those of the group counselors) was .85 for the individual categories that made up these higher order categories (range, .69 to .99; Pelham & Hoza, 1987). In addition, direct observations were made daily by independent observers while subjects were in recreation periods, and the percentage of time the boys engaged in negative interac- tions with peers was recorded using a modification (Pelham & Bender, 1982) of the reprogramming environmental contingencies for effective so- cial skills (RECESS) code described by Walker et al. (1978). Reliability (percentage agreement) for negative interactions was 72% (Pelham & Hoza, 1987).

Response to Provocation Task. This task was a modification of one used by Taylor (1967) and Taylor and Gammon (1975) to measure the effects of alcohol on provoked aggression in adults. In our~version subjects were led to believe that they were competing with another subject in a reaction time game in which they and their opponent could punish each other with bursts of noxious white noise when they or the opponent won a trial. Outcomes for the task were actually preprogrammed, and there was no opponent. The white noise was supplied over headphones with a range of possible intensities from 113 db to 41 db. All aversive bursts of noise were 2 sec in duration, including an initial 0.5-sec "ramping" (gradual in- crease to peak intensity) to guard against auditory distress. The subject was told to press one of the 10 buttons on his task box at the beginning of each trial to choose which of the 10 intensities of noise he wanted delivered to his opponent if the opponent should lose the race to press the correct button. The boy was told that his opponent would also choose the level of white noise to be delivered to the subject on those trials when the opponent

Methylphenidate and Aggression 457

won. During the first session each subject determined an unpleasantness threshold for the white noise. Each subject's threshold was designated as 10, and the other nine buttons were set in 10% decrements of the threshold (i.e., 9 = 90%, etc.).

Provocation was defined as the level of white noise intensity that the subject thought his opponent had assigned to him on trials when the subject thought he had lost. The low, medium, and high-provocation conditions~ which were randomly interspersed throughout the 12 lost trials, each con- sisted of four trials in which the feedback setting averaged 2.0, 5.0, and 9.0, respectively. On 13 other trials the subject was told he had won. Ag- gression was defined as the averaged level of noise the subject set for his opponent to receive in response to each of the three levels of provocation.

Social Information Processing A: Hypothetical Story Attribution Task. (Dodge, 1980). Four stories were presented in which the subject imagined a peer in a situation in which the subject experiences a negative outcome. After each story, the subject was asked four questions to assess (1) attri- butions concerning the peer's intentions, (2) hypothetical behavioral re- sponses (a measure of response decision bias), and (3 and 4) expectations regarding future interactions with the peer (two questions). The first two questions were scored as hostile (3), neutral (2), or benign/benevolent (1), and summed over stories to form dependent variables. Similarly, expecta- tion of future prosocial and aggressive behavior was rated as either yes (1) or no (2), and summed across stories. Percentage agreement for two inde- pendent raters' scoring ranged from 85% to 100%.

Social Information Processing B: Recall Task (Dodge and Frame, 1982). Nine audiotaped interviews were presented in which an unfamiliar boy described behaviors (three each of positive, neutral, and negative/hos- tile statements) that he had displayed toward children in his class. After- wards, the subject was asked to recall the statements. The five dependent variables (averaged across the nine interviews) were recall proportions of (a) total items, (b) last items, and (c) statement type.

Social Information Processing C: Detective Decision Task (Dodge and Newmann, 1981). Six ambiguous, hypothetical stories were presented ic~ which a peer may have committed a hostile act toward the subject. After each story, the subject listened to audiotaped clues about the unknown peer who may have committed this hostile act. Each clue contained one piece each of condemning and exonerating evidence. The subject could choose the number of clues to hear before judging the peer's innocence or guilt~ The four dependent variables (averaged across the six stories) included the number of clues chosen, the number of judgments of guilt, the proportion of clues recalled, and the proportion of recalled clues that were positive.

458 Murphy, Pelham, and Lang

RESULTS

The point system frequency counts and daily observations conducted using the RECESS code were each analyzed in a 2 (Drug: Methylpheni- date, Placebo) x 2 (Aggressive Subgroup: High, Low) ANOVA with re- peated measures on the first factor. Significant effects for drug were found for the daily frequency counts from the token economy/point system for conduct problems, F(1, 24) = 9.11, p < .01 (PL M = .89; MPH M = .37), and for negative verbalizations, F(1, 24) = 20.53, p < .(h')l (PL M = 2.34; MPH M =1.28), as well as for RECESS negative peer interactions F(1, 24) = 6.27, p < .05 (PL M = 2.67; MPH M = 1.62). Significant aggressive subgroup effects were found for conduct problems, F(I, 24) = 6.00, p < .05 (HA M = .85; LA M = .38), and for negative verbalizations, F(I, 24) = 10.67, p < .01 (HA M = 2.8; LA M = .65), but only a trend (p < .10) for RECESS negative interactions. No significant interactions were ob- tained.

A 2 (Aggressive Subgroup: High, Low x 2 (Drug: Methylphenidate, Placebo) x 3 (Level of Provocation: Low, Medium, or High) ANOVA was conducted on the boys' responses to provocation (mean noise level assigned to the opponent). 6 There were no significant main effects, but there were trends for three interactions, the highest order of which was the three-way interaction of Aggressive Subgroup x Drug x the linear component of Provocation, F(1, 17) = 4.07, p = .06. In order to investigate the effects of both subgroup and drug, the simple effects of aggressive subgroup were evaluated at each level of provocation and drug. Results in the placebo conditions revealed that HA boys had greater responses to provocation (M = 4.94; SD = 3.03) than LA boys (M = 2.61; SD = 1.2) only at the high, F (1, 17) = 4.80, p < .05 level of provocation. There were no differences between the aggressive subgroups under any level of provocation in the MPH condition. MPH did not affect aggression for the HA boys (F < 1), but increased it for the LA boys at the highest level of provocation, F(I~ 17) = 5.93, p < .05.

Following the model proposed by Milich and Dodge (1984), the social information processing measures were analyzed in three sets of analyses, according to the stage of processing measured: attributional bias, response decision bias, and encoding. The attributional bias variables were analyzed in a 2 (Aggressive Subgroup) x 2 (Drug) MANOVA. A significant main effect of aggressive subgroup was revealed, F(4, 21) = 3.20, p < .05, but

6Seven children from the original sample of 26 did not serve as subjects in this task. Although all subjects completed the threshold setting procedure, the six youngest boys and one older boy declined to participate.

Methyiphenldate and Aggression 459

followup univariate tests were nonsignificant. A separate 2 (Aggressive Sub- group) x 2 (Drug) ANOVA on the response decision bias measure revealed no significant effects. A 2 (Aggressive Subgroup) x 2 (Drug) MANOVA conducted on the eight encoding variables (five from the recall task and three from the detective decision task) revealed no significant effects of aggressive subgroup or the interaction but a significant main effect of drug~ F(8, 17) = 3.38,p < .01. Foilowup univariate ANOVAs revealed that more clues from the detective decision task were recalled on the MPH day (M = .17) than on the placebo day [M = .11; F(1, 24) = 6.12; p < .05], and a higher proportion of statements recalled were positive on MPH (M = .38) vs. placebo [M = .26; F(1, 24) = 7.39; p < .05].

The correlation matrix including all of the dependent variables was computed in order to examine relationships among the measures (because of the large number of correlations, alpha was set at .01). There were ex- pected correlations among related dependent measures on the social in- formation processing tasks (e.g., between different recall measures from the detective decision tasks), but only one significant correlation between the social information processing measures and the behavioral measures on the laboratory task, and that correlation was in the wrong direction. Response to provocation, (averaged over low, medium, and high levels) was positively correlated with observed negative peer interactions (r = .63) and with daily frequency counts of negative verbalizations (r = .68), and these two behavioral measures were positively correlated with each other (r = .45). Finally, frequency counts of negative verbalizations and conduct problems were positively correlated (r = .56).

DISCUSSION

Regarding HA-LA subgroup differences, there were consistent difo ferences between high- and low-aggressive subgroups on placebo days ira the daily frequency counts, the RECESS observations, and the high-provo- cation conditions of the laboratory provocation task, with HA boys showing more aggression than LA boys. In contrast, there were no LA-HA differ~ ences on the 14 social information processing measures. The behavioral measures of aggression were moderately correlated but the cognitive meas- ures were not related to other measures. Regarding drug effects, MPH pro~ duced decreases in the daily frequency count/point system categories of conduct problems and negative verbalizations, as well as in the observec~ negative interactions with peers on the RECESS code observations. These drug effects did not interact with aggressive subgroup. On the laboratory provocation task, MPH did not affect aggressive responding for the HA

460 Murphy, Pelham, and Lang

ADHD boys but it increased response to provocation in the low-aggressive subgroup at the high level of provocation. Twelve of the 14 social infor- mation processing variables were unaffected by MPH, which improved re- call on two measures.

Aggressive Subgroup Differences

The finding of differences on independent measures of aggression for the high- and low-aggressive subgroups is consistent with other research that has provided evidence for the validity of HA and LA subgroups in ADHD in a variety of domains (Atkins, Pelham, & Licht, 1989; Hinshaw, 1987; Lahey et al., 1988; Loney, 1987; Milich & Landau, 1984). These re- suits add to the few studies that have employed direct observations in peer group settings, showing convergence between clinical diagnosis and direct observations of verbal and nonverbal aggression toward peers.

The group differences on the laboratory provocation task represent the first extension of research on aggressive clinical groups to a laboratory analogue of aggression (see Peterson, 1971, for an early study with a non- clinical sample and similar results). At the high level of provocation, rela- tively more aggressive boys responded with more peer-directed aggression in this task than less aggressive boys, a result that we replicated in a sub- sequent investigation with a modified version of the task (Pelham et al., 1991). HA ADHD boys apparently have low "triggers" and respond to provocation with aggression. The relatively high correlations between re- sponse to provocation on the laboratory tasks and the observational/fre- quency count measures of aggression---clear evidence for concurrent validity of the provocation task--suggests that the laboratory task may be useful in the study of individual differences in aggression in ADHD sam- ples. Further, this provocation task finding is consistent with sociometric findings showing that HA ADHD boys are rated by peers as more likely than LA boys to start to fight over nothing (Pelham & Bender, 1982). This result also highlights the need for intervention strategies for HA ADHD boys that include a focus on learning to deal with provocations in ways other than aggression (e.g., Hinshaw, Henker, & Whalen, 1984).

In contrast to the results on the observational and laboratory meas- ures of aggression, there were no differences on social information proc- essing measures as a function of aggressive subgrout>--a finding not inconsistent with previous research. Despite a fair number of reports that aggressive children differ from normal children on these information proc- essing measures, the between-group differences identified have often been small and difficult to replicate when the same measures have been used

Methyiphenidate and Aggression 461

in different studies (cf. Dodge, 1980; Dodge & Frame, 1982; Dodge & New- man, 1981). Recent investigations of this theory have focused on the reac- tive or impulsive subgroups of aggressive boys (Crick & Dodge, 1989), who would appear to be similar to our HA boys. We thus expected to have a clear test of attributional biases in our subgroups. The absence of significant differences on these cognitive measures in the presence of subgroup dif- ferences on the observational measures and the laboratory tasks, as well as the complete absence of correlations between cognitive and behavioral measures, cast doubt on the robustness of the social information processing deficits in an ADHD sample (see also Milich & Dodge, 1984). Our HA and LA subgroups clearly differed in aggressive behavior, and the fact that they failed to differ in the cognitive measures and the lack of significant correlations suggest that the cognitive variables assessed did not mediate the boys' aggressive behavior.

The reasons for our having failed to find subgroup differences on these measures are not readily apparent. One possibility is that the social information processing deficits that have been proposed for aggressive chil- dren are not specific to aggression. Such deficits in ADHD children may result from common underlying mechanisms such as inattention and/or im- pulsivity, and may be unrelated to concurrent aggressiveness (Pelham & Milich, 1984). Indeed, Milich and Dodge's (1984) findings that HA boys differed from normal controls but not from LA or other psychiatric controls on these measures support this explanation. Indeed, processing deficits in reactive aggressive children but not in proactive aggressive children have recently been reported (Crick & Dodge, 1989; Dodge &Coie, 1987), im- plying that processing deficits are not specific to aggression in general. If impulsivity were associated with reactive aggression, these recent results would be consistent with those reported by Milich and Dodge, as well as o u r o w n .

Methylphenidate Effects

Turning from between-group differences in initial levels of aggression to the second purpose of the study---assessment of MPH effects on aggres~ sion-qt should first be noted that the positive effects of MPH on the daily frequency counts and observational measures of aggression taken in the summer program are consistent with both early and more recent reports showing that stimulant medication decreases aggressive behavior in ADHD children (Hinshaw, 1991). Further, the finding that MPH affected both HA and LA subgroups is also consistent with other reports (e.g., Barkley, Mc- Murray, Edelbrock, & Robbins, 1989; Hinshaw et al., 1989; Klorman et al,

462 Murphy, I)elhtlm, .rid l.ilng

1988). Our data add to previous studies in one important respect. Previous reports have typically combined verbal and physical aggression into a single category for analysis (e.g., Hinshaw et al., 1989), and it has thus not been clear that MPH reduced both verbal aggression and more severe conduct problems. Our data show clear MPH effects on both negative verbalizations (name calling/teasing, swearing, and verbal abuse toward adults) and con- duct problems (physical aggression, destruction of property, stealing, and lying) (see also Hinshaw, Heller, & McHale, 1992).

In contrast to the frequency counts and observational measures, which subsumed many types of verbal and nonverbal aggressive behavior, the provocation task provided a measure of response to direct provocation. Per- haps surprisingly, MPH had no ' effect on the HA boys but provoked rela- tively more aggression as provocation increased for the LA boys. The absence of an MPH effect on the HA boys is consistent with a replication of this study, which also failed to find MPH effects (Pelham et al., 1991). However, the adverse MPH effect on the LA boys was not replicated, sug- gesting caution in interpreting those results, especially given that only bene- ficial effects were obtained on the nonlaboratory measures of aggression.

Thus, there was a discrepancy between the MPH effects obtained on the response to provocation task and the clear effects obtained on the RE- CESS observational measures and daily frequency counts from the day treatment program--especially for the HA boys. What are likely explana- tions of these differences? One possibility may relate to differences in the type of aggression assessed by the measures. The point system categories did not differentiate among accidental, intentional, self-initiated, and reac- tive aggression, and all of these were summed together with the other con- duct problems noted above to yield the conduct problem measure of aggression. The negative verbalization category similarly included numerous types of negative verbal behaviors. In contrast, the laboratory task meas- ured only intentional response to provocation. A 0.3-mg/kg dose of MPH may be sufficient to improve impulsively initiated or unintentional physical or verbal aggression, but a higher dose may be necessary to decrease in- tentional and provoked aggression (cf. Hinshaw et al., 1989). Indeed, a number of recent studies have shown that MPH effects on disruptive be- havior and aggression increase in a linear fashion up to 0.6 mg/kg (e.g., Barkley et al., 1989; Hinshaw et al., 1989), and it is certainly reasonable to speculate that this dose-response effect might interact with type of ag- gression. Whatever the basis of the discrepant results, one conclusion is dear. The laboratory aggression task does not provide information regard- ing MPH effects on aggression that generalize to ecologically valid meas- ures in a more natural setting, and its utility as a laboratory measure of medication response is therefore questionable.

Methylphenidate and Aggression 463

On the social information processing measures there was a significant MPH effect on just two of eight encoding variables---proportion of clues recalled and ratio of positive to negative clues recalled--and no medication effects for the four attributional bias measures or the response decision bias measure. The drug effect on the number of clues recalled is similar to that reported in other studies of stimulant effects on recall (e.g., Wein- gartner et al., 1980) and is not necessarily specific to social information processing. The fact that clear MPH effects were obtained on the behav- ioral measures of aggression at the same time that these social information processing variables showed minimal MPH effects indicates that (a) MPH probably does not influence social information processing as it relates to aggression, and (b) MPH-induced improvement in aggression is apparently independent of changes in these social-information-processing measures.

In closing, several limitations of this study warrant mention. First, the label "aggression" commonly describes a collection of behaviors that in- cludes physical aggressive acts and negative verbalizations, and these can be directed either toward adults, peers, animals, or objects. These acts can be either provoked or unprovoked, and the nature and operationalization of aggression can vary along other dimensions as well. That subgroup and MPH effects varied as a function of dependent measure and that the cor- relations among our behavioral measures were significant but modest both highlight the multidimensional nature of aggression in children with exter- nalizing disorders. Future studies might investigate different procedures for subgrouping high- and low-aggressive children, and then evaluate drug ef- fects on separate, carefully defined facets of aggression. A second limitation is that we did not address the question of stimulant effects on aggression in non-ADHD aggressive boys or normal controls, and such research is needed. Finally, the number of subjects was small relative to the number of dependent measures, and replication of the results is therefore critical.

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