Normal levels of serotonin produce a calming effect on the individual. However, low levels remove that calming effect leaving the individual less able to control their impulsive and aggressive behaviour. Serotonin usually keeps the amygdala under control so that the emotional responses are disciplined. If there is less serotonin, there will be less inhibition and so when the individual's amygdala is stimulate by external events, it becomes more active causing the person to become more aggressive. Flynn and other psychologists conducted research in the 60s to test the effect that the amygdala has on aggression. There was a study where cats had the amygdala removed. After being removed, the cats were described as tame and significantly reduced aggression. This studied showed that the amygdala did cause aggression, but when amygdalectomy was performed on humans, although the aggression was reduced, they were left with what was observed as no emotion at all. And so the amygdala is necessary for emotions and therefore cannot really be removed to prevent aggression. However, not all low serotonin sufferers are violent. Booij et al conducted a longitudinal study measuring aggression from parental and self-reports and PET scans. They found higher levels of aggression in children with low levels of serotonin. This study supports the idea that serotonin levels affect aggression.
Dopamine may influence aggression as the brain appears to see it as a reward and so whenever we do something that