Our brains monitor our experiences through chemical reactions that lead to memories, feelings, thoughts, and other cognitive processes; thus, brain chemistry is particularly sensitive to environmental inputs and is altered accordingly. Experiences, such as learning and social interactions, trigger emotional reactions, and the chemistry of those feelings is translated into our behavioral responses. Aggressive or violent behavior is associated with the chemistry of our emotions where behavioral responses are exaggerated, inappropriate, or out of context given the social circumstances. In these cases, the individual’s ability to properly evaluate the situation and regulate his or her emotional responses becomes impaired. Many who are violent are easily provoked, misinterpret the social interaction or stimulus, and overreact; it is as if survival mechanisms have gone awry. In other cases, internal stimulation is insufficient and only extreme behaviors can provide what is physically perceived to be adequate stimulation to the brain.
Studies of biochemical mechanisms underlying violent behavior focus on the role of central neurotransmitter systems in regulating impulse control and activity levels, or arousal of the nervous system. Neurotransmitters are chemical messengers that convey “information” in the form of an electrically charged signal from neuron to neuron, and from brain structure to brain structure. In general, neurotransmitters regulate emotion, mood, hunger, thirst, sleep, and a host of other behavioral and psychological processes. The neurotransmitters dopamine, serotonin, and norepinephrine have been strongly and consistently associated with aggressive behaviors, even in the absence of a disorder.
The dopamine system has been implicated in displays of aggressive or violent behavior. When the dopamine system is activated, novelty seeking and self-stimulation behaviors increase. When this system goes awry, however, behavior may be activated in the absence of a reward, a threat, or some other appropriate stimulus. This “approach system” can produce dangerous asocial and disruptive behavior when it is activated in the absence of an appropriate social setting or provocation. The overproduction of dopamine has been associated with psychotic behavior, and has been linked to antisocial behavior and violence. Antipsychotic drugs that decrease dopamine levels tend to decrease fighting behaviors.
An abnormally low level of serotonergic activity is another significant player in influencing impulsive aggressive behavior. Lesions that switch off areas of the brain that are dense with serotonin connections produce rage and attack behaviors. Scores of studies have found several indicators of lowered serotonin activity in studies of juveniles and adults characterized as violent or impulsive, in contrast to those who are not. Further refinements to these investigations, however, show that serotonin is more specifically responsible for regulating impulse control than aggressive behavior. The implications are that when serotonin activity levels are relatively low, the tendency or predisposition to behave in certain ways (e.g., violently) that may be related to certain personality traits (e.g., a negative or hostile mood) is less likely to be inhibited.
Norepinephrine (NE) is of particular interest due to its involvement in stress responses, emotions, attention, and arousal. It plays a primary role in the so-called fight-or-flight response by causing the release of stress hormones from the adrenal glands, and exciting the central and peripheral nervous systems. NE activates the fight-or-flight response by stimulating various brain structures, from the frontal cortex, to the limbic system (controlling emotions and survival functions), to the brainstem.
Significant changes in NE have been documented during preparation for, execution of, and recovery from activities that involve states of high arousal, including violent behavior. Drugs that increase NE activity are known to worsen violence in patients who are already agitated, and, conversely, because NE activity levels are suppressed by medications that are used in the treatment of violence, there are clear indications that NE’s role in violence is significant.
Monoamine oxidase (MAO) is an enzyme responsible for the deactivation of several neurotransmitters (e.g., dopamine, serotonin, and NE). Unusually high or low levels are believed to adversely affect social behaviors. Low MAO activity results in excessive levels of dopamine and NE, which are believed to contribute to aggression, loss of self-control, and inappropriate motivations to behave. Because MAO concentrations are particularly high in areas of the brain involved in complex thinking processes, affect and mood state, impulse control, and aggressiveness, the relationship between irregularities in its activity and possible effects on social and emotional behaviors is understandable.
For over two decades, irregularities in MAO levels have been linked with antisocial behaviors, particularly those involving psychopathy, aggression and violent behavior, sensation-seeking behavior, impulsivity, and excessive alcohol use.
Hormones are chemicals released by glands that travel to various parts of the brain and body to exert their effects. Hormones of interest can be categorized as either “sex” or “stress” hormones; they regulate sex drive, reproductive functions, aggression, territoriality, sexual differentiation, responses to environmental stimuli, and energy levels.
The most studied hormones in relation to aggression are testosterone and other male hormones, called androgens. Studies of people with a disorder caused by exposure to high levels of androgens in prenatal and early postnatal periods (congenital adrenal hyperplasia) provide evidence for testosterone’s role in aggression, in that these people are unusually aggressive. Studies have consistently found evidence for elevated testosterone levels in both male and female violent offenders relative to males and females in control groups, suggesting a role for testosterone in criminal violence and aggressive dominance.
Importantly, behaviors associated with elevated testosterone levels are substantially context dependent. In other words, high levels of testosterone are not always associated with aggression, which also depends on the person’s social circumstances and characteristics. Also, these hormones not only influence dominance and aggressive behavior, but they also increase their activity in response to behaving that way.
There is also some evidence for the role of irregularities in sex hormone levels in female antisocial behavior. High levels of testosterone have been found among violent female inmates and delinquents relative to those considered nonviolent. Also, females exposed to high levels of androgen in the prenatal and early postnatal periods have significantly higher aggression scores than those in control groups. Unusually high testosterone levels in females may contribute to the increased incidence of a masculine appearance among female offenders and may function to reinforce aggressive tendencies under certain environmental conditions. Interestingly, giving androgens to females who are not involved in criminal behavior has been clearly associated with an increase in aggression proneness.
Certain hormones are released in response to signals from the brain and glands involved in the fight-or flight mechanism. These hormones are sensitive to both psychosocial stressors and novel situations; thus, they are called stress hormones (e.g., ACTH, cortisol, prolactin). In general, studies report increased cortisol activity in individuals with unusually heightened reactions to challenging situations, and an increased incidence in conduct disordered behavior. These findings suggest that some people, as a result of predisposition or social experiences, have greater biological sensitivity to stress. On the other hand, low cortisol responses to stressful stimuli may reflect low levels of nervous system arousal, which characterizes people who are psychopathic, are aggressive, and/or have posttraumatic stress disorder. Consistent with that possibility is research showing low concentrations of cortisol in aggressive youth and violent adult offenders who lack anxiety. If biological responses to stressful stimuli do not occur, then the individual may be relatively insensitive to stress and may behave inappropriately.
Recent scientific advances have identified biochemical factors most consistently related to violence, although much additional work needs to be done to show cause and effect. Nevertheless, what is known is that a constellation of these factors interact in a fluid manner to influence behavior, and their effects are constantly changing as a function of age and developmental stage within a constantly changing environmental and social context.
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