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Biology 202
2000 First Web Report
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Can the Source of Aggression be found in the Brain?

Irma Iskandar

In Holland, male members of a certain family were found to be prone to violent outbursts; one male, criticized by his employer, attempted to run him over with a car - another raped his sister and was sent to a mental hospital - a third coerced his sister into undressing by threatening her with a knife. Such men display retarded motor development, difficulties in task planning, and awkward sexual behavior. (1). Recently, researchers claimed to have found the basis of such aggressive behavior to genetic sources - specifically, a deficiency in the MAOA gene of these males (2).

In Finland, studies were conducted on males who also displayed uncontrollable behavior, and the findings demonstrated that the men possessed a neurotransmitter substance deficiency, particularly in the messenger serotonin (3). This lack of serotonin has been linked to aggressive behavior: some violent prone individuals did not effectively break down these substances

All around the globe, people have attempted to find an organic, genetic basis for aggressive behavior. Several hormones and neurotransmitters, such as testosterone and seretonin, have been implicated in the "aggression quest", as well as specific localities of the human brain. My paper will serve to suggest that although many findings have shown impressive results regarding possible biological causes of violent behavior, we still do not have sufficient means to understand the neuroanatomical or biochemical basis of aggression.

Neurotransmitters have been suggested as a significant cause of aggressive behavior. Hans Brunner, a geneticist at the University Hospital in Nijmegen, has found that the violent male members of the Dutch family mentioned earlier in this paper, lacked a gene that produces monoamine oxidase-a (MAOA) (4). MAOA is an enzyme that breaks down significant transmitters in the brain. If the MAOA does not break down these transmitters - specifically, serotonin - then buildup of serotonin will occur and could cause a person to act violently (3).

However, it is interesting how contradictory the finding is to several other claims. Studies have shown that serotonin, in normal levels, exerts a calming, inhibitory effect on neuronal firing (5) and that, in low levels of serotonin, aggressive and impulse behaviors increase (6).This contradicts the previous claim that a built-up, or high levels of serotonin caused by the MAOA mutation, causes violent behavior.

Furthermore, the claim regarding the specificity of serotonin to aggression can be analogous to a small fish in a big pond. In the New York Times, in the span of three years, articles on human behavior and health, have claimed that underactive serotonin circuits are also the cause of migraines (July 24, 1996, section C, p. 8), extreme shyness (May 18, 1999, section C, p. 1), obsessive-compulsive disorder (February 16, 1997, section 13CN, p. 3), anxiety and pessimism (November 29, 1996, section A, p. 1), and "restless leg" syndrome (night cramps) (April 10, 1996, section C, p.10) (7). We are therefore far from understanding how or why serotonin explains aggression. What further complicates the issue is that there are at least sixteen different types of serotonin receptors, which react in distinct ways to serotonin. To what extent, and to whether neurons prefer serotonin or not, is still unknown. In short, we still are a long way from discovering how serotonin plays into aggression.

What is the neuroanatomy of aggressive behavior? Scientists have linked at least 38 different parts of the brain to various behaviors considered aggressive (5).. Research has mainly concentrated on the limbic area of the brain, which houses the amygdala, an important area in controlling emotions such as fear and anger (8). For example, amygdalectomy cases reduce violent behavior in individuals, but with the side affect of loss of emotion. Temporal lobe epilepsy, which involves the amygdala, can involve aggressive behavior. However, again, problems arise regarding the specificity of aggression and the brain. It has been argued that serotonin levels affect aggressive behavior. Serotonic neurons, which reside in the brainstem, project their axons into many and functionally diverse regions of the brain, including the amygdala, hypothalamus, hippocampus, cerebellum, and temporal and prefrontal regions of the cerebral cortex (7). It would be surprising, given this wide spectrum, if abnormalities of the serotonin system affected aggression in a specific way. Furthermore, several humans suffering from a variety of head injuries have displayed aggressive behavior, but never neatly, never in a localized manner which allows for scientists to define a physical pathway of aggression (9).

What, then, can one conclude about theories seeking to localize and empirically explain behavior such as violence and aggression? This paper serves to suggest that despite the recurring belief in our neurobiology class that "brain = behavior", it is currently too difficult to prove this claim using aggression as an example. Although behavior might in principle be imagined to result from interconnected assemblages of input/output boxes derived from the permeability of the neuron membrane - still, sophisticated measures are necessary in order to take a step further to fully explain behavior. It has been proven that people are still far away from understanding the role of neurotransmitters in affecting aggression. Furthermore, many areas of the brain have been implicated in affecting emotions including violence, but the results and claims have been numerous and vague.

Perhaps it is only a matter of time to prove that brain and the organic substances in the brain equal behavior. Perhaps with more sophisticated knowledge of the mechanism of the brain and nervous system, one could pinpoint to exact organic causes of behavior, see and truly understand the "organization of neurons", if not the particular properties of neurons themselves. But until then, instead of dismissing aggressive/violent behavior with a biological explanation, I think I will continue to say to those who display these tendencies - "Is that how your momma raised you back home??"

WWW Sources

1) "Aggression in Mice and Men?" Seif, Isabelle, et. al. Letters to the editor. Science, Vol. 270, Issue 5325. (Oct. 20, 1995). 362-364.

2)Angier, Natalie. "Gene tie to male violence is studied". New York Times. 22 Oct. 1993, late edition, A21. Scientists may have identified a small genetic defect that appears to predispose some men towards aggression.

3) Is There a Genetic Variable that Affects How Violent Individuals Behave? The Attempted Determination of the Aggression Gene . Student written paper regarding violent behavior and biological plausabilities.

4)Goldberg, Jeff. "The bad seed: amid controversy, scientists hunt for the "aggression" gene. Omni v. 17 (Feb. 1995): 16. A look at current interests in explaining violence and crime by biological means.

5) Cases, Oliver. "Aggressive behavior and altered amounts of brain serotonin and norepinephrine in mice lacking MAOA". Science v. 268 (June 23 1995): 1763-6. Research interests with the MAOA gene responsible for serotonin levels.

6)Hormones in Context: Testosterone and Aggression. Debunks stereotype about testosterone that it is the cause of aggressive behavior in males and this in turn is equated with violence. Results from research with mice also explained.

7)Serotonin and Impulse Aggression: Not So Fast. Offers a skeptical analysis of attempts to define serotonin as a main factor explaining human behavior. By Joel Wallman.

8)Aggression, Comprehensive study of approaches towards the study of aggression, the influence of neurotransmitters, hormones, and neuropsychology, complete with a plethora of diagrams.

9) Angier, Natalie. Woman: An Intimate Geography. Anchor Books: New York. 1999. A tour of the female anatomy, physiology, and hormones/brain interactions with body and behavior.

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