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The Biological Basis of Aggression
by Joanna Schaffhausen
One need only pick up the daily newspaper to see how serious a problem violence is in today's society. Although the incidence of violent behavior in the US has fallen significantly in the past few years, there is still about an 80% chance that a person will be the victim of a violent crime during his or her lifetime. Even more troubling is the trend of increasing violence among the very young. After each school shooting, there is a media blitz of experts searching to explain how and why troubled teens sometimes turn to violence. Much of what they say is the result of research on the psychobiology of aggression, a field that has recently experienced many breakthroughs in identifying correlates of violent behavior. Some researchers claim that we are coming closer to predicting from a brain scan or a blood test whether a person is at risk for committing an act of violence. Ethical complications aside, a closer look at the neurobiology of aggression shows why we are unlikely to find a conclusive test for potential violent behavior. While there are many biological factors associated with aggression, their predictive value remains still quite low.
Figure 1. The Human Hypothalamus
The hypothalamus and pituitary gland are important parts of the brains limbic system associated with emotional response and arousal. These structures, along with the septum and amygdala, may play a role in mediating aggression.
The first hurdle in researching aggression is how to define it. It is an easier task with animals, who tend to display stereotyped patterns of violence such as killing to gain food or territory. With humans and non-human primates, classifying aggression becomes more difficult because there is complication of intent. Punishment, for example, represents an especially gray area. Should spanking be considered an aggressive act? What about capital punishment? Indeed, almost all acts we consider aggressive have been socially sanctioned by some cultures over the years. To simplify matters, many psychologists and ethologists find it useful to classify aggressive behavior into one of three main categories: (1) predatory aggression, which refers to stalking and killing of other species, (2) social aggression, which is unprovoked aggression that is directed an members of the same species for purposes of establishing dominance, and (3) defensive aggression, which refers to attacks delivered when an animal is cornered by a threatening aggressor. There is evidence from animal studies that suggests the different types of aggression are controlled by different subsets of brain structures within the limbic system, including the amygdala, the septum, and the hypothalamus (figure 1). For example, in the rat, lesions of the lateral septum decrease social aggression but increase predatory aggression, suggesting that neural substrates for offensive and defense aggression are intertwined but separate.
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