Types of Aggression. Epidemiology of Aggression. Genetics of Aggression

Aggression can be defined as intentional act committed by an individual that has the potential to result in the physical or emotional harm of a person or of an object. Although aggression is typically thought of as constituting a physical assault, acts of aggression also include verbal outbursts in which the individual’s voice is used to convey anger in an uncontrolled manner. Broadly speaking, aggression can be subdivided into two types: impulsive aggression and premeditated or nonimpulsive aggression.

Aggression is impulsive when the aggressive act is unplanned or in response to an aversive stimulus, generally uncontrolled, and committed without the expectation of achieving a tangible objective. By contrast, premeditated aggression is controlled, involves planning, and is associated with an expectation of reaching a specific goal such as, for example, the intimidation of the victim.

Although acts of impulsive aggression can sometimes be criminal in nature, premeditated aggression is more likely to occur in the context of other criminal behavior. Because impulsive, rather than premeditated, aggression appears to be associated with specific biological and pharmacological response characteristics, this review focuses on impulsive aggression.

Epidemiology of Aggression. Despite the clinical importance of impulsive aggression, there is very little epidemiological data specifically regarding impulsive aggressive behavior. Existing epidemiological data exclusively refer to homicide and physical assault (i.e., aggression in general). Currently, the age-adjusted rate for homicide in the United States, based on vital statistics records, is 0.01%. Epidemiologic survey data find that approximately 25% of all males report a history of some physical fighting since 18 years of age, a rate that is twice as high as is found in adult females.

Accordingly, approximately 10-15% of the general population report that they have engaged in physical fighting as an adult. This proportion translates into at least 25.0-37.5 million individuals in the United States alone. Although we cannot know what proportion of homicides or physical assault were impulsive in nature, it is likely that a substantial proportion represent impulsive aggressive acts.

Genetics of Aggression. Data from twin, adoption, and family studies support the hypothesis that aggression is under significant, if variable, genetic influence. The differences seen in these results are due, most likely, to the fact that different studies examined aggression in different ways using differing assessment measures.

Specifically, aggression has been examined in these types of studies by using categories that more or less reflect aggression (such as delinquent behavior, antisocial behavior, criminal behavior, and violent behavior) in populations with psychiatric diagnoses or by using scales that more or less reflect the severity of aggression in the general population or in university students. In clinical populations, genetic influences appear to underlie both delinquent behavior and antisocial behavior. In general populations, heritability estimates for measures of aggression range from substantial in children to moderately substantial in adults (44-72%) to apparently nonexistent in adolescents.

Most noteworthy is the observation that the aggression measures with the greatest heritability estimates reflect anger and hostility and/or anger, impulsiveness, and irritability. These are the type of behavioral traits that appear to be associated with impulsive aggressive behaviors and that, in turn, have been shown to be strongly correlated with biological factors and with psychopharmacological responses to treatment with anti-aggressive agents.