This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.

Contribute Thoughts | Search Serendip for Other Papers | Serendip Home Page

Biology 202
1998 First Web Reports
On Serendip

Violent Behavior and the Brain - Do we know it all?

Ingrid Katz

The DMZ, which runs through Central Vietnam is a place where undisputed violence was unleashed for over a decade in the late sixties and early seventies. During the year that I worked in Vietnam, I often returned to this spot to watch the American Veterans, who would stand at the top of Marble Mountain, withered, with vacant stares out towards the stretch of beach known only to the Americans as China. It was mystifying to consider the sharp contrast that must have seemed all too apparent to them as the sounds of war loomed in their memories and the calm ocean stretched out before them now so serenely.

It was then that I first started to consider what causes man to become so enraged as to commit atrocities of the mind, body and soul. Violence - pure and simple, is intrinsic to humanity. It almost goes unnoticed as a way of life in many communities. Drive through North Philadelphia on a spring afternoon and witness what appears to be life disappearing, receding under the concrete and graffiti. Look closer and witness the bullet holes in the walls of homes and cars. Still, there are other communities, such as North Brooklyn, where the rate of crime has been diminishing for the past twenty-five years. It seems, in fact, that violent crime does not behave as predicted. Social scientists have begun to suggest that, in fact, violent crime needs to be viewed much like an infectious disease. Simultaneously, neurobiologists have developed intricate research models and techniques to examine whether or not there are biological triggers that cause individuals to act violently.

What if violence were to be truly considered an epidemic - one with BOTH neurological and sociological explanations? Being a public health practitioner and activist, I am inclined to champion the cause of social scientists and the role that society, economy, and politics play in the make-up of a violent individual. What is the environment that allows an epidemic of violence to overtake a community and how does it initiate in the individual?

Epidemiologists consider an epidemic to reach its "tipping point" when it turns into a public health crisis. As Malcolm Gladwell notes in his article, "The Tipping Point", "Every epidemic has its tipping point, and to fight an epidemic you need to understand what that point is." What then, is the tipping point for an individual who is motivated to act violently? Gladwell notes that recently, social scientists have started to apply the theory of epidemics to human behavior, specifically violent behavior. Research by Thomas Shelling at Harvard University, has pointed to such sociological factors affecting violent behavior as "the white flight" from urban areas. Likewise, Jonathan Crane, a sociologist from the University of Illinois has noted in a 1991 study in the American Journal of Sociology, that a lack of role models in the community can lead youth to carry on lives involving violent crime. His study shows that as the percentage of high-status workers falls just 2.2 percentage points in a community, the dropout rates more than double. Using a public health model as a framework, this dropout rate can be considered the tipping point of an epidemic where youth are caught in a cycle of violence.

Further work has been done on the sociological factors affecting men who abuse their wives. A study published in the Clinical and Consulting Psychology has found that one type of violent man who beats his wife or partner is not treatable with any known therapies. The study examines the pscho-social factors that cause men to act violently. Researchers have found that men who fear abandonment and have witnessed a high level of abuse in their family are often prone to be more violent. Still, other studies on the topic have found that men who have suffered head-injury are six times more likely to abuse their partner than non-aggressive men.

Given the myriad environmental factors that contribute to the manifestation of violence in an individual, it is difficult to assess which factors are most pertinent. Thus, it is also essential to consider the role that biological processes play in violent human behavior. Despite criticism by certain special interest groups, research on the biological basis of violence has continued, producing evidence that there may be a genetic component in violent behavior. Although it is certain that genetics alone cannot explain a pattern of violence in any individual, it may certainly play a contributory role in the manifestation of violent behavior. Several studies suggest that reduced levels of the central nervous system (CNS) neurotransmitter serotonin may be associated with aggressive behavior, which, in turn can lead to violent behavior. Further studies have shown that suicidal individuals may have reduced serotonin levels in the brain. These studies suggest that serotonin may play a part in impulsive aggressive behavior.

Experimental examination of serotonin synapses in the CNS reveal that in persons prone to suicidal behavior, the serotonin system appears to be hypofunctional. This hypofunctional state of the serotonin system then causes the disinhibition of excitatory systems mediated by other neurotransmitters such as norepinephrine or dopamine. This ultimately results in an increased irritability and a reduced ability to control impulsive - aggressive behavior. Further evidence for a genetic basis of violent behavior comes from observation of DNA polymorphisms (variants) in genes involved in serotonin synthesis and function.

Additional research published in Discover Magazine on genetic research has revealed that a specific stretch of DNA along a chromosome, if mutated, may produce violent behavior. The study of a family in which ten men over five generations displayed violent behavior has led geneticists to form specific causal links. It was noted that all violent family members were male, and the trait seemed to be passed from mother to son, suggesting that the trait for aggressive behavior may be carried one the single X chromosome of males. These same researchers found that all living males who were violent had the same genetic marker and so did some of the women in the family, who would be carriers of the defect. The non-violent men in the family were not found to have the genetic marker. Researchers suspect one gene that codes for the enzyme monoamine oxidase A (MAOA) may be malfunctioning.

Further research on the biological causes of violent behavior has pointed less to a genetic link and more to brain damage that has occurred to the fetus or postnatal infant. As Adrian Raine notes in the Journal of Biological Psychiatry, murderers' brains show a significantly lower rate of glucose uptake than a healthy brain. The study, conducted by Raine and colleagues at USC and the University of California at Irvine, used positron emission tomography to scan the brains of 41 murderers who had pleaded not guilty due to reasons of insanity. This was followed by a scan of 41 brains of control subjects matched for known mental disorders, as well as for age and gender. The study showed that the murderers, on average, showed significantly lower rates of glucose uptake in three areas of the brain -- the prefrontal cortex, the corpus callosum and the posterior cortex. Their rates were four, 18 and four percentage points lower, respectively, than the rates measured in control subjects performing the same tasks. As noted by Raine, "Poor functioning of these limbic area helps explain why violent offenders fail to learn from experience and are less able to regulate their emotions." This research demonstrates a link between brain damage and criminal violence, eliminating the possibility that mental disorders alone predispose criminals to violence.

Upon assessment of both biological and sociological data, it seems apparent that neither alone directly affects aggressive behavior. Rather, one can adopt a bidirectional causational model of the nervous system, with output affecting input and input affecting output. In this manner, one can perhaps integrate the role that both environment and biology play in relation to human behavior- specifically violent behavior. If one simply adopts a unidirectional model instead, one focuses only on the physiological response of a given neuron firing that then triggers a response in certain individuals who feel an urge to beat, kill or mame others. In other words, the neuron fires and the individual reacts. A bidirectional model, however, takes into account the stimuli that this individual has been receiving throughout his or her life. Stimuli of abuse and/or trauma could account for possible "inputs" that might cause this individual to react on suppressed desires of violence.

This model is certainly the more wholistic one when assessing an individual's desire to commit a violent act. Still, it leaves open the question of just how essential the brain is during the process of whether or not to act violently. In other words, how much of a role does the input play vs. what effect is simply base on the mental awareness and capacity of the individual? Certainly, this is a contentious issue - one that is often presented during trials of man or women accused of murder. Can this person be held responsible for his or her actions? If so, how much can be attributed to ongoing inputs of violence that one has experienced him or herself?

If, indeed, one adopts the bidirectional model, one is left then with the question of where the self truly lies. If the self, or the "I box" exerts an overwhelming influence, then the bidirectional model may be less than a useful one. If, however, one asserts that the "I" is not the initiator but rather the processing center of information (much like the CNS is when receiving stimuli form sensory neurons), then, in fact, the bidirectional model may be a useful took in examining people's behavior. (((YADA YADA SOME CONCLUSION WRAP UP HERE)

WWW Sources

"Non-linear crime epidemics have a 'tipping point'"

"Calculated abuse is the hardest to treat"

"Inside the heart of marital violence"

"the biology of aggression"

"A violence in the blood"

"Studies address link between brain damage and criminal behavior

"Neurobiology and Behavior." Biology 202 - Ingrid Katz