Early experiments focused specifically on the role of the hypothalamus in aggressive behaviour. Such experiments typically involved electrical and chemical stimulation of the hypothalamus of a cat placed in an otherwise stimulus free environment. Bard (1928) and Bard and Rioch (1937) who experimented with cats in 1928, were one of the first researchers to attempt to elucidate the role of the hypothalamus in aggressive behaviour. Bard found that the cats would exhibit sham responses with little or no provocation. The heightened sympathetic and somatic arousal seen in the stimulated cats was referred to as sham rage or a psuedo-affective response.
One brain structure that has been considered important in aggression is the amygdala. Research in hamsters has found that stimulation of the corticomedial amygdala increases aggression and lesioning of this area reduces aggression in hamsters (Potegal et al 1996). In humans, Amygdalectomy (surgical removal of the amygdala) reduces aggression in previously violent individuals; however, a side effect of this type of surgery is the loss of emotion, initiative and enthusiasm (Groves and Schlesinger, 1982). It is possible that the amygdala does not directly cause aggression, but is involved in processing associated emotions, such as anger.
The frontal lobes are likely candidates for a role in aggression, as they are known to be involved in many activities requiring decision making, such as social behaviour, motor functions, problem solving, judgement, impulse control and personality.
Evidence that aggression is related to frontal lobe function has been around since the 1800s. In 1848, Phineas Gage had an accident while packing gunpowder into a blasting hole while working on a railroad in Vermont, USA. The tamping iron he was using was propelled through his skull when he accidentally ignited the powder, causing it to explode. The damage to Gage's frontal lobes did not kill him, but it apparently changed his personality, making him more aggressive, obstinate, impatient and impulsive (Harlow, 1868).
The case of Phineas Gage suggests that the frontal lobes may be important in moderating aggressiveness. Nevertheless, it is impossible to find out exactly where Gage's brain was damaged, as it can only be deduced from examination of his skull and this only provides a rough estimate. Another slight problem with these explanations could be the generalisation of animal researches to humans and other studies argue that aggression is caused by socialisation, observation of media, situational factors e.g alcohol, temperature, crowding, noise, cue arousal which results in frustration.