The majority of research has focused upon the etiology of physical aggression, as opposed to relational/social/indirect aggression. The etiology of physical aggression is complex, with theories emanating from both social and natural sciences. While several theories explain the development of aggression, most researchers agree that aggressive behavior is multidetermined, that it begins early in one's life, and can be explained in part by the theories discussed in the following sections.
While most research on the role of genetics and the heritability of aggression has ruled out the idea of an aggressive gene (Coie & Dodge, 1998), it is currently thought that one might inherit the biological vulnerability for being aggressive. The current hypothesis is that one's biological/physiological vulnerability (i.e., genotype) may interact with environmental and psychological stressors to cause one to act in aggressive ways (i.e., phenotype).
Aspects of brain functioning have also been linked to aggressive behavior. Quay (1993) hypothesized that aggressive behavior is the result of an imbalance between the different brain systems that are responsible for controlling behavior. Quay posits that aggressive behaviors result when the reward system (REW) of the brain is dominant over the behavioral inhibition system (BIS). The REW causes a person to "go" while the BIS stops behavioral responding. Each system is controlled by different neurotransmitters; dopamine has been linked to REW, whereas ascending nora-drenergic fibers have been associated with BIS.
In addition to neurotransmitters, neuroanatomy as a whole has been linked to the development of aggressive behavior. While there is some research suggesting that different areas of the frontal lobe may contribute to emotional, impulsive, and angry outbursts, it is not clear if frontal lobe dysfunction causes violence or vice versa. Other studies have suggested that dysfunction in the left hemisphere is related to violence. Researchers in this area are also testing theories that cortical underarousal is linked to violence, mediated by sensation-seeking behaviors. Finally, sex hormones such as testosterone have also been implicated in aggressive behavior.
One of the most widely discussed theories of aggressive behavior is Albert Bandura's (1973) social learning theory. According to this theory, children develop repertoires of aggressive behavior by observing and/or imitating aggressive models (e.g., family members and television). Early research conducted by Bandura demonstrated that preschoolers imitated an adult female's aggressive actions toward an inflatable Bobo doll that was decorated as a clown.
Various social information processing models have also been advanced to explain the development and maintenance of aggressive behavior. These theories suggest that how a child processes a series of environmental cues affects whether he or she will behave in a socially appropriate or inappropriate manner. The sequential processing steps are:
1. Encoding relevant cues
2. Interpreting the cues in an accurate manner
3. Accessing potential behavioral responses to the interpreted cues
4. Evaluating and selecting potential responses
It is assumed that these steps happen quickly and at an unconscious level. Physically aggressive children are deficient in each processing step, which is thought to predispose them to react in an impulsive and aggressive manner as compared to their nonaggressive peers. Recent research also suggests that relationally aggressive children are deficient in their interpretation of social cues and in their response enactment decisions (steps 1 and 2). Research has not yet investigated whether relationally aggressive children have processing difficulties in the other three areas (steps 3 through 5).
In general, studies show that children born into disadvantaged backgrounds are likely to be aggressive later in life. More specifically, researchers find that associations between poverty and low self-esteem, difficulties in peer relationships, and overall adjustment influence later aggressive behavior. It is not clear if poverty has a direct influence on children's behavior, or if its effect is mediated by poverty's influence on a parent's emotional state and stress level.
Developmental psychology research demonstrates that witnessing violence or experiencing abuse and neglect may be related to later aggressive behavior in children. For example, a child's exposure to early abusive experiences may be related to later aggressive behavior; however, the link is not direct and may be mediated by other factors such as life stressors and television viewing. Factors such as prenatal drug exposure or contact with lead are also linked to aggressive behavior.
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