Violence and Aggression — The Dana Guide


by Antonio Damasio

March, 2007

sections include: emotion, regulation, aggression, and violence: common neurological connections? 

The prevalence of violence in society has motivated sociologists and biological scientists to search for the predictors and causes of this destructive human behavior. Scientists are specifically searching for biological factors that might predispose an individual to this aberrant form of behavior. The propensity for impulsive aggression appears to be associated with a lack of self-control over certain negative emotional responses and with a lack of fear or understanding of the negative consequences of behaving aggressively.

Psychological research has examined the relation between aggression and emotion. Numerous studies show that negative affect (a term that describes a mixture of emotions and moods including anger, distress, and agitation) can cause or intensify aggressive behavior. The forms of aggression that are relatively unplanned and spontaneous, termed impulsive aggression, are different from premeditated aggression. Although most neurobiological studies of aggression and violence typically do not differentiate between premeditated and impulsive aggression, the distinction is probably relevant in understanding their genetic, neurochemical, and functional neuroanatomical bases. In other words, a person who commits a violent act in the heat of passion may possibly display different neurological and chemical brain symptoms than someone who plans and commits a violent act.

Defects in the normal dispersal of serotonin in the brain have been linked to aggression and violence by a variety of methods. Researchers have found evidence that serotonin exerts inhibitory control over impulsive aggression. Measurements of the levels of a chemical known as 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid (CSF) are believed to reflect serotonin activity in the brain. For example, reduced CSF 5-HIAA has been found in aggressive psychiatric patients; impulsive, violent men; and victims of suicide by violent means. Furthermore, low levels of 5-HIAA concentration have been found to predict aggression two to three years in the future in boys with conduct disorders and repeat criminal offenders. Finally, lower CSF 5-HIAA levels have been reported in impulsive violent offenders and impulsive fire setters.

Scientific findings suggest that genetic abnormalities may contribute to serotonin function as well as to individual differences in aggressive behavior. In a study of 251 community volunteers, researchers examined the association between a specific genetic abnormality in the serotonin system and measures of aggression and anger related personality traits. Subjects having the mutation scored significantly higher on several measures of aggression, including the tendency to experience unprovoked anger, than individuals with the other form of the gene. The prefrontal cortex (PFC) is known to be a crucial brain area in emotion regulation and has been implicated in aggressive and violent behavior. Scientists have discovered abnormalities in serotonin activity in the prefrontal cortices of persons with impulsive aggression.

Imaging studies with positron-emission tomography have revealed prefrontal abnormalities in glucose metabolism in individuals prone to impulsive aggression. A study of 41 murderers found lower levels of glucose metabolism in areas of the prefrontal cortex and increased metabolic activity in the right amygdala compared with age- and sex-matched controls. An increased metabolic rate was also observed in the hippocampus, amygdala, thalamus, and midbrain in the right hemisphere in impulsive murderers, compared with both the control group and those murderers who had planned their crimes in advance.

Brain lesions in the orbital frontal cortex (OFC) and adjacent PFC regions have been shown to produce syndromes characterized by impulsivity and aggression. Researchers reported in a 1999 medical study that a 56-year-old man who sustained bilateral damage to the OFC and some damage to the left amygdala showed unpredictable, impulsive aggression and violence. The patient was described by a relative as “a quiet, rather withdrawn person who was never aggressive” before the brain damage.

Emotion Regulation, Aggression, and Violence: Common Neurological Connections?

Impulsive aggression may be the product of a failure of emotion regulation. Normal individuals are able to control negative feelings voluntarily and can also profit from restraint-producing cues in their environment, such as facial and vocal signs of anger or fear, that also serve a regulatory role. Some studies suggest that individuals predisposed to aggression and violence have an abnormality in the central circuitry responsible for these adaptive behavioral strategies—a defect in the brain regions that control emotional regulation. The evidence suggests that abnormalities in serotonin function in regions of the prefrontal cortex may be especially important. Other neurotransmitters, neuromodulators, and hormones are probably also involved.

The evidence indicates that the OFC and the structures with which it is interconnected (including other prefrontal territories, the anterior cingulate cortex, or ACC, and the amygdala) constitute the core elements of a circuit that underlies emotion regulation. The OFC, through its connections with other zones of the PFC and with the amygdala, plays a crucial role in restraining impulsive outbursts, and the ACC recruits other neural systems, including the PFC, in response to conflict. In normal individuals, activations in these brain regions that occur during anger arousal and other negative emotions restrain the impulsive expression of emotional behavior. Deficits in this circuit are believed to increase a person’s vulnerability to impulsive aggression.

Many factors influence the structure and function of this circuitry. Genetic factors clearly play a role, as revealed by the association of one particular genetic difference associated with traits of anger and aggression. However, these factors undoubtedly interact with early environmental influences. The very circuitry identified here as playing a crucial role in emotion regulation is dramatically shaped by early social influences. Biological factors alone do not determine whether a person will be aggressive. Environmental factors such as upbringing and social interactions surely also contribute to one’s propensity for aggressive or violent behavior. Novel behavioral training procedures based on experimental research that investigates how people suppress negative emotion need to be developed and then rigorously evaluated to determine whether they can be used to benefit those with a propensity for aggression. Drugs that affect the serotonin system may also be useful in counteracting aggressive tendencies. Nevertheless, the first important step is to recognize that impulsive aggression and violence, irrespective of the distal cause, reflect abnormalities in the emotion regulation circuitry of the brain. Parents who notice such tendencies in their children should see a child psychiatrist who can evaluate the causes of such problems and suggest an appropriate treatment. More research is needed to develop better and earlier treatments for those potentially devastating conditions.

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