Sean Z. Kaliski, MB, BCh, PhD

At some point in their life, most people are likely to have acted on impulses or reacted to provocations. This is accepted normal human behavior. However, in some cases it is pathological and some individuals behave this way habitually-as part of a pattern of behaviors that may have begun sometime in their youth. Although there are protean manifestations of these behaviors, ranging from suicidal gestures, substance abuse, risk taking, and antisocial behaviors, a subset of individuals are also aggressive and violent.

[[{"type":"media","view_mode":"media_crop","fid":"40384","attributes":{"alt":"Â© Cartoonresource/shutterstock.com","class":"media-image","id":"media_crop_4776623887150","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4130","media_crop_rotate":"0","media_crop_scale_h":"266","media_crop_scale_w":"260","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","title":"Â© Cartoonresource/shutterstock.com","typeof":"foaf:Image"}}]]The terms “impulsivity” and “disorders of impulse control” have customarily been used interchangeably. Yet there have been contrasting definitions in the literature. “Impulsivity” has been defined as a decreased sensitivity to negative consequences, rapid unplanned reactions to stimuli (without adequate processing of information), and lack of regard for long-term consequences. “Disorders of impulse control” have been characterized as repeated failures to resist an impulse or perform an act that is harmful, with a preceding subjective sense of increasing tension (or arousal) and an experience of pleasure or gratification, ie, catharsis, while committing the act.

 In both cases, the consequences of the acting out are usually deleterious, with subsequent feelings of regret or guilt.

No studies have directly compared individuals whose impulsivity only takes the form of acting precipitously to stimuli with those who act solely because of impelling urges. In practice, there are many who possess an admixture of both aspects, such as those with borderline personality disorder who repeatedly act out their urges and can also respond explosively to stimuli. In DSM-5, an important criterion for borderline personality disorder is impulsivity, which also encompasses risk-taking activities that are exemplars of poor impulse control, such as excessive spending, promiscuity, and reckless driving. Individuals with intermittent explosive disorder, a “pure” impulse control disorder, exhibit “impulsive (or anger-based) aggressive outbursts” in response to minor provocations or stressors. Whatever the distinctions, individuals with these disorders all have in common a deficit in inhibiting damaging behavior.

In clinical practice, it may actually be difficult to differentiate between compulsions, addictions, and irresistible impulses. Almost all self-damaging behaviors, especially if they occur in the context of a psychiatric disorder, can be reframed as impulse disorders, such is the looseness of the definitions. Commonly, individuals who habitually cannot control their aggressive impulses also have other impulse control disorders, such as gambling and alcohol and substance abuse.

Forensic psychiatrists and the courts have long grappled with the difficult distinction between “impulses that cannot be resisted” and “impulses that are not resisted”-namely the “irresistible impulse” defense. In the former, there is the possibility that the person has an inherent biological propensity to act violently and therefore ought to be excused; the latter implies that the loss of control was voluntary and consequently not excusable. This applies mostly when someone responds violently following provocation. But impulses can also build up over an extended period until they demand urgent expression. Even serial killers who plan their homicides, sometimes meticulously, often report that they had to surrender to overwhelming urges.

Over 30 years ago, Linnoila and colleagues

 found that impulsive violent offenders had significantly lower cerebrospinal fluid (CSF) concentrations of the major metabolite of serotonin, 5-hydroxyindoleacetic acid (5-HIAA). Their findings have been convincingly validated.

Serotonin is an important inhibitory neurotransmitter, especially in the amygdala, anterior cingulated cortex, and dorsal-lateral prefrontal and orbitofrontal cortices. Reduced or dysregulated serotonin activity is associated with impulsivity and aggression. The possible mechanism may be the disruption of circuits between the amygdala and the medial prefrontal cortex, which results in amygdala hyperactivity and reduced prefrontal inhibition.

 Impulsive aggression presumably occurs consequent to ongoing arousal (from the amygdala) that primes negative urgency-the tendency to respond impulsively and aggressively to provocations or perceptions of threat.

Individuals who have the X-linked allele that codes for low-functioning monoamine oxidase A (MAOA-L), the most important enzyme for the metabolism of central serotonin, tend to display enhanced activation in subcortical limbic areas (especially the amygdala) and reduced prefrontal inhibition. This allele has now acquired the moniker “warrior gene” because of its consistent association with impulsive aggressive behavior.

Individuals who have the s/s allele for the serotonin transporter promoter gene also tend to exhibit patterns of impulsive violence, probably because of the reduced presynaptic re-uptake of serotonin. It may seem paradoxical that low-functioning versions of 

 and serotonin promoter genes are associated with impulsive aggression because these genes lead to increased levels of serotonin. The most likely mechanism is that increased levels of serotonin occupy serotonin 1A and serotonin 1B autoreceptors that “switch” the presynaptic neuron off and functionally cause a serotonin deficiency.

Men who have high CSF levels of free testosterone (which possibly mediates dominance-seeking) and low CSF levels of 5-HIAA are even more likely to be aggressive. Genes that determine the activities of the dopamine transporter protein and low levels of D2/D3 receptors in the nucleus accumbens have also been associated with impulsivity traits, possibly by interfering with the neural reward system.

Why don’t all individuals with the above genotypic profiles act impulsively and aggressively? Epigenetic factors that modulate gene expression may either potentiate or ameliorate these traits. This was investigated as part of the seminal longitudinal Dunedin Multidisciplinary Health and Development Study that followed up 1037 people born around 1973. The findings strongly suggest that boys with low MAOA activity but who had been severely maltreated before age 12 had, by their third decade, markedly the highest rates of convictions for violent offenses and dispositions toward violence.

 There were, nevertheless, some individuals who had not been maltreated and had high MAOA activity that ultimately also had violent convictions and dispositions. In other words, there are obviously a host of other factors that predispose persons to violent behavior (including the subset of impulsive aggressors).

These data have been confirmed by other studies, although generally, none of the studies seem to have differentiated between impulsive and non-impulsive violence.

 A notable exception was the large National Longitudinal Study of Adolescent to Adult Health (Add Health study) in which severe maltreatment was the sole factor that seemed to determine adult violence and the 

 genotype only seemed to contribute to a disposition toward violence.

 This suggests that the actual mechanisms by which environmental (and situational) factors interact with genotypes to produce habitually impulsively aggressive individuals are not yet fully understood.

Alcohol and substance abuse are probably the most powerful facilitators for impulsive aggression, increasing the risk almost exponentially. Alcohol abuse reduces serotonin neurotransmission, and intoxication may particularly provoke intemperate violent outbursts in individuals who have inherited dysregulated serotonergic neurotransmission.

 Findings suggest that intoxication with stimulant drugs, such as methamphetamine, promotes disinhibition via the enhancement of dopamine and epinephrine pathways, but not consistently.

There are many experiential and situational factors that are important, but it is difficult to prove their effects empirically. Apart from maltreatment, exposure to violence in the community also increases the risk for poor impulse control.

It is rare for violent individuals to seek help voluntarily. Treatment may be a consequence of an involuntary admission, ie, court mandated. In such cases, the individual may be unwilling or embarrassed to provide information; he or she will likely be angry and not very treatment-adherent-a good clinician-patient relationship will need to be established quickly. Collateral information will be useful for the initial assessment.

Impulsive aggression can occur as an isolated singular event, but usually is expressed as part of a defined disorder (

). When no other disorder accounts for the behavior, intermittent explosive disorder (IED) is diagnosed. This disorder is characterized (in DSM-5) by verbal aggression that occurs twice a week for 3 months and 3 behavioral outbursts over a 12-month period that resulted in damage or destruction of property or physical assault that resulted in injury. These phenomena need to occur independent of other mental disorders, comorbid medical conditions, or the effects of any psycho- tropic substances. To make this diagnosis confidently, the clinician has to have detailed and reliable information about the patient. The true prevalence of this disorder in the community is not known, although estimates indicate a life-time prevalence of 1% to 11%.

 An exact diagnosis of IED on the basis of aggressive outbursts is complicated, especially when there is a comorbid disorder-just over a fifth of patients may have another impulse control disorder, such as obsessive-compulsive disorder.

In most cases, treatment of the presenting psychiatric disorder should reduce the violent behavior. Alternatively, treatment of the aggressive or violent behavior should also address the problem of impulsivity. Most treatment strategies are guided by a structured assessment that identifies important dynamic risk factors and helps treatment decision making for each patient.

There is evidence that mood stabilizers, SSRIs, atypical antipsychotics, Î²-blockers (eg, propranolol), and Î±2-agonists have some efficacy in treating impulsive aggression.

 A recent meta-analysis of randomized controlled trials suggests that mood stabilizers are effective, especially carbamazepine, phenytoin, and lithium.

 Dialectical behavioral therapy–corrections modified, cognitive-behavioral therapy, group therapy, family therapy, and social skill training are also considered to be valid interventions.

 Despite the current dearth of good evidence for treatment strategies, it is probably prudent to use multimodal treatments-namely, a combination of drugs and psychotherapeutic approaches. When or if one should ever withdraw treatment remains a moot issue.

Dr Kaliski is Associate Professor/Senior Specialist for the Forensic Mental Health Service, Western Cape, department of psychiatry, University of Cape Town, South Africa. He reports no conflicts of interest concerning the subject matter of this article.

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The authors explore ways to address aggression in clinical practice and examine the potentially dangerous impulsivity-violence link across a broad range of conditions. 

Sean Z. Kaliski, MB, BCh, PhD

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Impulse Control, Impulsivity, and Violence: Clinical Implications