Neural and Hormonal Mechanisms

One of the best examples of how brain injury can influence aggressive behaviour is the case of Phineas Gage.

Working on a railway in 1848, he had an accident in which a tamping iron went up through his face, behind his left eye and out through the top of his head.

He survived the accident, but his personality was changed, including a huge increase in aggression.

Normal aggressive behaviour is not dependent on separate brain structures, but interaction of a system of structures.

Organised hierarchically and moderated by the pre-frontal cortex:

Amygdala

Hypothalamus

Periaqueductal grey

Kluver-Bucy syndrome - taming effect found in rhesus monkeys by removing part of the temporal lobes and therefore destroying the amygdala.

Narabyashi et al (1972) - 43/51 patients whose amygdala was destroyed through psychosurgery  showed reduced aggression afterwards.

Mark & Ervin (1970) - case study of female patient behaviour following electrical stimulation of amygdala.

She exhibited facial grimacing, became very angry and flung herself at the wall.

Ashford (1980) - temporal lobe epileptics often become aggressive, attacking furniture and people.

Wong et al (1997) - criminals with violent tendencies have reduced size amygdala

Van Elst et al (2000) - aggressive patients with temporal lobe epilepsy, amygdala had lost 20% of its volume.

But Muller et al (2003) - showed 6 male psychopaths and 6 male controls a series of positive and negative pictures whilst in MR scanner. Found increased activity in the amygdala. 

The exact role of the amygdala in aggression is unclear, but it is certainly a significant one.

Regulates the emotional responses driven by the amygdala. Damage to prefrontal cortex results in impulsivity, immaturity and loss of control.

Anderson et al (1999) - damage during infancy related to aggressive behaviour as adults.

Case studies comparing early onset damage with adult onset damage to frontal lobes. Early onset patients also performed poorly on tests of moral and pro-social reasoning.

Raine et al (1997) - investigated brain activity of 41 murderers using PET scans. Found reduced glucose metabolism in prefrontal cortex, suggesting this brain area is less active than in normal controls.

Volkow et al (1995) - found violent psychiatric patients had reduced cerebral blood flow to prefrontal cortex.

The neurotransmitter serotonin influences aggressive behaviour.

In research with vervet monkeys, reducing serotonin levels resulted in increased aggressive behaviour, whereas increasing serotonin decreased the aggressive occurrences.

Drugs to raise serotonin levels, such as trytophan have been given to juvenile delinquents and unpredictable institutionalised patients.

Beeman (1947) - castrated male mice and found that aggressiveness reduced.

He later injected the mice with testosterone  which re-established their aggressiveness.

Castration has since been used as a method for making domestic and farm animals more manageable.

Testosterone is also clearly related to aggression in humans.

Dabbs et al (1995) - measured testosterone  in saliva of 692 adult male prisoners. Found higher levels in rapists and violent offenders than in burglars and thieves.

Dabbs et al (1996) - looked at 12 fraternities in 2 universities. Members of fraternities with highest levels of testosterone were described as boisterous and macho, those with lowest were attentive and helpful.

The same effects of testosterone are also found in women.

Dabbs et al (1988) - female prisoners. Testosterone highest in cases of unprovoked violence but lowest where violence was defensive (eg. In domestic abuse cases)