Due to insufficient antecedent evidence regarding the population-based prevalence of IED, the present results regarding such prevalence are difficult to compare with the previous data. However, both the lifetime and 12-month prevalence of IED observed in the present survey were obviously lower than those in the United States (Coccaro et al., 2004
; Kessler et al., 2006
; Ortega et al., 2008
) or in South Africa (Fincham et al., 2009
). As mentioned above, Japanese are less likely to openly express their emotions compared with Westerners. In addition, Japanese are generally more likely than Westerners to share a stigmatization toward mental disorders (Griffiths et al., 2006
), and to suppress their emotions accordingly. This cultural reluctance to express emotion may also have affected responses to the structured interviews conducted in the present survey. Ortega et al. (2008)
also suggested that the different prevalence of IED among sub-ethnic Latino groups might reflect differences in response styles rather than any disabling psychopathology. However, we could not include a measure that would reflect the suppression of anger because the present study was conducted as a part of a widespread prevalence survey of various mental disorders including IED. Thus, the possibility of a Japanese tendency to suppress their emotions still remains speculative.
Nevertheless, it should be noted that the total amount of property damages of subjects with narrow lifetime IED and the number of cases in which they sought medical attention were rather fewer despite the fact that they had more anger attacks than those in the ‘broad only’ category. This suggested that each anger attack of subjects within the narrow criteria was relatively trivial, and that those people in such criteria might exaggeratedly regard their trivial emotional reactions as “anger attacks.” No role impairment in home and work of those with 12-month IED supports this supposition.
As for demographic factors, both a lifetime and 12-month prevalence of IED tended to be higher in men than in women. This is not surprising given that men are generally more impulsive than women. The strong association between younger age and a lifetime prevalence of IED may be somewhat affected by recall bias, since young people are more readily able to recall their behavioral problems. On the other hand, employment and a good educational background were associated with a somewhat increased risk of IED, which suggested that job-related stress among the educated classes might be one of the environmental triggers of impulsivity. These findings are consistent with those in South Africa (Fincham et al., 2009
Regarding the comorbidity of mood and anxiety disorders with IED, such disorders were shown to be associated with an increased risk of a lifetime prevalence of IED, which was consistent with the previous studies (Kessler et al., 2006
; Amara et al., 2007
; Ortega et al., 2008
; Fincham et al., 2009
). Specific phobia, which had younger age of onset than that for IED, might be a possible risk factor for the later onset of IED. On the other hand, IED seems a precursor of MDD and alcohol abuse.
The habitual use of tranquilizers, psychostimulants, and analgesics (except for medical treatment), has been statistically significantly associated with an increased risk of lifetime prevalence of IED. Those drugs are relatively common and easy to obtain. In general, patients with IED, after calming down, often regret their sudden explosive outbursts. Indeed, 52% of our subjects usually or invariably regretted their behavior after such outbursts, which might lead them to a depressive or anxious state. Therefore, they are apt to use such drugs as a natural element in the course of such stressful events. Because the habitual use of these drugs increases the risk of drug dependence, they may sometimes use the drugs for other than the primary medical purpose. On the other hand, no comorbid conditions were observed between drug abuse or dependence, and IED in our sample.
However, substance-related disorders have actually been reported as significantly associated with an increased prevalence of IED (Kessler et al., 2006
; Ortega et al., 2008
; Fincham et al., 2009
). The above-mentioned commonly used drugs that are relatively easy to obtain are likely to be the causes of bad habits from recreational use to harmful dependence, even if the use of those drugs did not reach the diagnostic threshold for substance-related disorders.
The comorbidity of any mood or anxiety disorder may mediate these interactions between substance-use and IED. The present results have demonstrated that those disorders, especially extreme anxiety, were more frequently observed or more strongly associated with IED in women than in men. It has also been reported that the association between suicide and substance-related disorders is stronger in women (Yoshimasu et al., 2008
). As mentioned above, these findings suggest that women with both IED and mood or anxiety disorders should be regarded as a particularly high-risk group for suicide. Consequently, special vigilance must be used to forestall suicide or suicidal attempts, especially because women tend to frequently use harmful substances as the means of taking their lives (Bostwick and Rundell, 1999
Furthermore, the experience of alcohol-related problems as well as their possibility has been significantly associated with a lifetime prevalence of IED. Though subjects with lifetime IED in our sample did not present the symptoms of alcohol abuse or dependence at the time when an interview was conducted, these results indicate that patients with IED may actually be prone to cause serious accidents when drinking. Alcohol-related disorders have also been associated with depression or suicidal risks (Kõlves et al., 2006
; Sher, 2008
; Watts, 2008
). Thus, the vicious interaction between drinking or substance abuse and impulsivity leading people to suicidal acts should itself be regarded as a serious suicidal risk. This hypothesis deserves close attention, especially because subjects in the present study with lifetime IED have in fact showed an especially high prevalence of suicidal ideation. In this sense, IED patients with harmful drinking habits or substance abuse might well comprise a high-risk group for suicide. Indeed, another US epidemiological study suggested that patients having both IED and MDD or drug dependence should receive rigorous assessment for their self-aggression (McCloskey et al., 2008
In summary, the present study showed that the prevalence of IED was relatively low in Japanese compared with Western populations, and that IED was associated with mood and anxiety disorders as well as suicidal ideation. In addition, common substance use and drinking-related problems were associated with an increase in the prevalence of IED. Considering the impulsivity caused by IED and its relation to suicidal ideation, those having IED as well as such mental and behavioral problems should be regarded as one of the highest suicide-risk groups, and must be given the benefit of an appropriate interventional approach specifically designed for suicide prevention.