neural and hormonal influences

• Neural influences include a part of the brain known as the limbic system that coordinates behaviours which satisfy motivational and emotional urges including fear and aggression. Two key structures that form part of the limbic system and believed to be linked to aggression are the amygdala and the hippocampus.
• The amygdala is part of the limbic system and evaluates the emotional importance of sensory information and prompts an appropriate response. 
• Neural explanations suggest if certain areas of the amygdala are stimulated electrically, aggression may be displayed through various behaviours. This explanation also suggests that if the amygdala was removed, an animal or person would no longer react with aggression from the same stimuli. The amygdala is also thought to be responsible for moderating testosterone levels which has been linked to aggression. If the amygdala malfunctioned either through a tumour, damage or atypical development then this could raise testosterone levels making aggression more likely.~
• The hippocampus is involved in the formation of long-term memories. These memories allow animals to compare current threats with similar past experiences to assess whether they should respond with either aggression or fear. A damaged or impaired hippocampus would prevent the nervous system from putting things into the right context and thus cause the amygdala to respond to sensory stimuli inappropriately possibly through aggression.
• Normal levels of serotonin are believed to exert a calming inhibitory effect on neuronal firing within the brain. Serotonin inhibits the amygdala which controls various emotional responses including fear and anger making their expression less likely. However low levels of this neurotransmitter remove this inhibitory effect resulting in individuals having less control over their impulsive aggressive behaviour; this theory is known as the serotonin deficiency hypothesis.
• Without serotonin, the amygdala becomes more active when stimulated by external triggers which makes aggression more likely as the person becomes more impulsive due to lowered inhibition. Serotonin is also believed to reduce aggression by inhibiting responses to emotional stimuli that may normally have led to an aggressive reaction. Low levels within the brain have therefore been linked to an increased susceptibility for impulsive, aggressive behaviour and even suicide.
• Serotonin is a neurotransmitter which has also been linked to aggression also. Neurotransmitters are chemicals that allow impulses in the brain to be transmitted from one part of the brain to another. Normal levels of serotonin are believed to exert a calming inhibitory effect on neuronal firing within the brain
• Hormones such as testosterone have also been linked to aggression as men are generally more aggressive then women and have higher levels (Dabbs 1990). Testosterone is an androgen (male hormone) and produces male characteristics one of which is believed to be status seeking domineering behaviour such as aggression. Increased levels are believed to raise levels of aggression and aggressive responses to perceived provocation.
• The link is not clear cut but when testosterone levels peak around the start of puberty, there is a peak in aggression levels in boys suggesting a correlational link. Research by Dabbs (1987) measured salivary testosterone in violent and non-violent criminals. The highest testosterone levels found in those with a history of mostly violent crimes supporting this link. However with this we still cannot be sure of cause and effect as some theories propose aggressive individuals produce more testosterone and not vice versa (not testosterone increases aggression).