There are two noteworthy limitations of the data analyzed here. First, the diagnoses were based on fully structured lay interviews for which no information is available either on test-retest reliability or validity. Second, estimates of onset and course were based on retrospective rather than prospective reports. A limitation of the data analysis is that many separate significance tests were computed, introducing the possibility of some false positive associations. Caution is consequently needed in interpreting results prior to independent replication.
Within the context of these limitations, DSM-IV IED was estimated to be a fairly common disorder, with lifetime prevalence of 5.4–7.3% and 12-month prevalence of 2.7–3.9% (equivalent to approximately 11.5–16.0 million lifetime cases and 5.9–8.5 million 12-month cases in the US). These prevalence estimates are somewhat higher than those found in the two previously published studies of DSM-IV IED.2, 3
The Baltimore ECA study findings suggest that prevalence would have been roughly 25% higher if we had also included cases that met research criteria for IED.1
The latter extend the definition of IED to include recurrent aggressive outbursts that do not rise to the level examined in this study (e.g., verbal aggression against others in the absence of either threats or physical aggression against people or objects). As the latter behaviors are significantly impairing and have been shown to respond to psychopharmacologic treatment,21
a rationale exists for including them in the definition of IED in DSM-V.
Although we found a number of socio-demographic correlates of IED, these associations are modest in substantive terms. As an indication of this fact, the Pearson’s contingency coefficient, a generalization of the phi coefficient for polychotomous predictors,22
is only in the range .04–.05 for the significant socio-demographic correlates of lifetime broad IED. This means that IED is very widely distributed in the population rather than concentrated in any one segment of society.
We also found that IED usually begins in childhood or adolescence, that it is quite persistent over the life course (averages of 6.2–11.8 years with attacks), that it is associated with substantial role impairment, and that it has high comorbidity with other DSM-IV mood, anxiety, and substance use disorders. Although these NCS-R results cannot legitimately be compared with the results obtained in previous studies of patient samples, it is worth noting that similar patterns have consistently been found in clinical studies using mostly older diagnostic criteria.1, 23-27
As described in the section on measures, explicit questions to exclude anger attacks due to substance use disorders and major depression were included in the CIDI and a post hoc exclusion was made for bipolar disorder. As McElroy et al.13
found that some patients with comorbid IED and bipolar disorder have anger attacks when they are not in manic or hypomanic episodes, our blanket exclusion of cases with comorbid bipolar disorder underestimated the prevalence of IED. We did not make comparable exclusions of comorbid impulse-control disorders stipulated in DSM-IV as exclusions for IED (oppositional-defiant disorder, conduct disorder, attention-deficit/hyperactivity disorder) based on the fact that DSM-IV says that an additional diagnosis of IED is warranted in the presence of “discrete episodes of failure to resist aggressive impulses.” An observation indirectly supporting this decision is that IED was reported to be much more persistent than comorbid impulse-control disorders.
DSM-IV also excludes anger attacks due to antisocial personality disorder and borderline personality disorder). The NCS-R did not include a core assessment of Axis II disorders, making it impossible to consider these exclusions. However, the Baltimore ECA study, which focused on personality disorders, found unexpectedly low proportions of respondents with IED who also met criteria for antisocial personality disorder or borderline personality disorder,2
suggesting that the failure to exclude these cases in the NCS-R might not have had a major effect on results. DSM-IV also excludes anger attacks due to non-affective psychosis (NAP), but the estimated prevalence of NAP was so low in the NCS-R that this exclusion made no meaningful difference to the results reported here.28
In evaluating the NCS-R finding that IED is significantly comorbidity with a wide range of other DSM-IV disorders it is important to recognize that the CIDI is a fully structured instrument that cannot make the subtle distinctions made in clinical interviews. This means that comorbidity is probably over-estimated in the NCS-R. Importantly, the ORs of IED with other CIDI/DSM-IV disorders are not markedly higher than those among the other disorders themselves. Nonetheless, the documentation of comorbidity between CIDI and a wide range of other disorders is consistent with the finding that undiagnosed IED is common in clinical samples.29
Although such associations are more intuitive with other impulse-control disorders and substance use disorders that with anxiety or mood disorders, evidence exists in clinical studies of an association between violent behavior and such anxiety disorders as PTSD30
while anecdotal reports link panic attacks to violent behavior.32
Clinical evidence of an association between violent behavior and depression is even stronger.33
The finding that the ORs of IED with impulse-control (3.3–3.5) and substance use (2.7–3.6) disorders were not higher than those with mood (2.8–3.2) and anxiety (2.4–3.6) disorders raises the possibility that IED may be as much related to affective instability and dysregulation as to problems with impulse control. This possibility is consistent with the observation that affective instability is a risk factor for impulsive self-injury and suicidal behavior.34
It also needs to be noted, though, that impulsivity itself is associated with neuroticism35
and is known to be a risk factor for depression,36
suggesting that the joint effects of impulsivity and affective instability on IED are likely to be complex.
The early age of onset of IED is an important finding with regard to comorbidity because it means that IED is temporally primary to many of the other DSM-IV disorders with which it is comorbid.37
Within-person analyses (detailed results available on request) found that this was especially true for major depression, generalized anxiety disorder, panic disorder, and substance use disorders, where the vast majority of respondent reported that their IED began at an earlier age than these other disorders. This raises the possibility that IED might be either a risk factor or a risk marker for temporally secondary comorbid disorders.38
Consistent with this possibility, a recent family study showed that the offspring of depressed adults with anger attacks have higher delinquency and aggressive behavior than the children of depressed adults without anger attacks.39
This suggests that intermittent explosive behavior might emerge quite early in subjects at risk of the subsequent onset of mood disorders. However, we are aware of no systematic research on the possibility that IED is a risk marker for temporally secondary disorders. It is interesting to note in this regard that the one published study that examined the family aggregation of IED found high inter-generational continuity of the disorder independent of comorbid conditions,37
which means that common genetic factors are unlikely to account for the comorbidity of IED with other DSM disorders.
This last observation suggests that the association of IED with the later first onset of secondary comorbid disorders is unlikely to be due to common underlying genetic risk factors or to phenotypic factors that are under strong genetic control, such as an impulsive personality style. If IED is a causal risk factor, in comparison, it might promote secondary disorders by leading to divorce, financial difficulties, and stressful life experiences that promote secondary disorders. If this last scenario is correct, then the fact that so few people obtain treatment for IED becomes even more important than otherwise because it means that an opportunity is being missed to intervene in the disorder at a point in time when it might still be possible to prevent the onset of secondary disorders.
It is noteworthy that a detailed analysis of delays in seeking treatment for IED found that the minority of people with IED who obtain professional help for their anger attacks typically wait a decade or more after onset before first treatment contact.40
Given the differences in the typical age of onset of IED compared to temporally secondary comorbid disorders,41
this means that initial treatment usually occurs only after the onset of most temporally secondary disorders and that the focus of the treatment is probably on the comorbid disorders. This interpretation is consistent with the finding that the majority of people with IED were found to receive treatment for emotional problems at some time in their life, but not for their anger. It is not clear from this result whether the low treatment of anger is due to greater reluctance to seek professional help for anger than other emotional problems or due to failure to conceptualize anger as a mental health problem. Given that so many people with IED obtain treatment for other emotional problems, a question can also be raised why treating clinicians do not include anger as a focus of their treatment or if the anger problems of their patients with IED are not recognized. We have no data in the NCS-R to adjudicate among these possibilities.
Another issue of importance for diagnosis and treatment of IED relates to the distinction between broad and narrow definitions. The stipulation in DSM-IV that the presence of only three serious lifetime episodes of aggression may be sufficient to make the diagnosis of an aggression disorder is one of the few instances in which DSM-IV does not have a temporal clustering requirement (e.g., three episodes in one year). It is noteworthy in this regard that even though the most severe form of IED in our study (narrow) is much more persistent than its less severe form (broad-only), the two did not differ significantly in most measures of functional impairment. As such, these data raise questions as to when to treat individuals with IED. Prospective treatment data will be needed to resolve this uncertainty. A related question for future research is whether successful early detection, outreach, and treatment of IED would help prevent the onset of secondary comorbid disorders. Given the age of onset distribution of IED, early detection would most reasonably take place in schools and might well be an important addition to ongoing school-based violence prevention programs.42, 43