Aggression Paper 3

• Limbic system


  Connects the brain stem to the cortex. Made up of the cingulate gyrus, septal area, hypothalamus, fornix, amygdala and parts of the hippocampus and thalamus. Controls a range of emotional behaviours including aggression.
• Amygdala


  • Most important structure in the limbic system associated with aggression
  • Responsible for quickly evaluating the emotional importance of sensory information and prompting an appropriate response, including aggression
  • If it malfunctions due to a tumour, damage or atypical development, aggressive behaviour may be more likely
• Pardini et al (2014)


  • Participants who had smaller amygdalas than normal showed higher levels of aggression and violence
• Ervin (1970)


  • Case study of a female patient's behaviour following electrical stimulation of her amygdala. She exhibited facial grimacing, became very angry and flung herself at the wall.
• Kluver and Bucy (1937)


  • Destroying the amygdala in aggressive monkeys, resulted in less aggressive behaviour
• Gospic et al (2010)


  • Brain scans showed heightened activity in the Amygdala when participants rejected offers of money because they felt it was unfair (an aggressive reaction to social provocation). Taking a benzodiazepine halved the number of rejections and decreased the amygdala activity.
• The amygdala does not operate in isolation. The orbitofrontal cortex (not part of the limbic system) regulates the emotional responses driven by the amygdala and damage to the orbitofrontal cortex results in impulsivity and loss of control.
• Coccaro et al (2007)


  • In patients with psychiatric disorders that prominently feature aggression, activity in the OFC is reduced, disrupting its impulse control function and thus leading to increased aggression.
• Raine et al (1997)


  • Investigated brain activity in 41 murderers using PET scans, and as well as abnormal amygdala activity, found reduced glucose metabolism in the orbitofrontal cortex, suggesting this brain area is less active than in normal controls.
• The regulation of aggression is complex involving the amygdala, the OFC and the connection between them. The limbic system has a role in producing an aggressive response, but it does not operate in isolation.
• Serotonin


  A neurotransmitter involved in the communication of impulses between neurons. Normal levels are associated with greater behavioural control as it typically inhibits the firing of the amygdala. Normal levels in the OFC are linked with reduced firing of neurons and greater behavioural self control. Low levels are associated with low behavioural control, impulsivity and aggression.
• Virkkunen et al (1994)


  • Compared levels of a waste product of serotonin in the cerebrospinal fluid of violent impulsive and violent non-impulsive offenders. The levels were significantly lower in the impulsive offenders and they also suffered from more sleep irregularities.
• Raleigh (1991)


  • Gave different diets to vervet monkeys. Monkeys whose diet was high in tryptophan (increases serotonin levels) showed decreased levels of aggression. Those whose diet was low in tryptophan exhibited increased aggressive behaviour.
• Passamonti (2012)


  • Manipulated the tryptophan level in healthy participants' diets. On low serotonin days communication between the prefrontal cortex and the limbic system was weaker, particularly in those whose questionnaire responses showed existing tendencies to behave aggressively.
• High levels of serotonin have also been associated with aggression, suggesting the role of serotonin in aggression is complicated.
• Testosterone


  An androgen (a male sex hormone) secreted by the testes in males and to a lesser degree, the ovaries in females. Males have about 8 times more testosterone than females. Typically, the higher the levels of testosterone, the higher the level of aggression.
• Wagner (1979)


  • Castrated male mice and found that aggression was reduced. He later injected the mice with testosterone which re-established their aggression.
• Giammanco et al (2005)


  • Increasing testosterone caused greater aggressive behaviour in several species and that reducing testosterone had the opposite effect.
• Dabbs (1987)


  • Measured testosterone in the saliva of 692 adult male prisoners and found higher levels in violent offenders than in non-violent offenders.
• Carre's (2011) dual-hormone hypothesis claims that high levels of testosterone lead to aggressive behaviour only when levels of cortisol are low. When cortisol is high, testosterone's influence on aggression is blocked.
• Testosterone doesn't necessarily increase aggression but increases status-seeking behaviour in animal species, which is not intended to cause actual harm.
• MAOA gene


  Responsible for regulating the enzyme MAOA (monoamine oxidase A) which breaks down the neurotransmitters serotonin, noradrenaline and dopamine. A dysfunction in the MAOA gene results in aggressive behaviour.
• Brunner (1993)


  • Investigated the case of a Dutch family whose male members had been particularly aggressive over many generations. Males in this family were found to have a rare mutated version of the MAOA gene.
• McDermott (2009)


  • Demonstrated in controlled experimental conditions how those with the MAOA-L gene were more likely to force someone to eat hot chilli sauce, despite having to pay to punish, than someone with the MAOA-H gene.
• Tiihonen et al (2015)


  • Studied prisoners in Finland and found that severely violent prisoners had the MAOA-L gene in combination with the CDH13 gene. There was no substantial evidence for either of these genes in non-violent offenders.
• Caspi (2002)


  • Studied 500 male children and found support for the role of the MAOA –L variant in aggression, but only if they had been maltreated as children.
• Lagerspetz (1979)


  • Carried out a selective breeding study on mice. After 19 generations, rates of aggression were 10 times higher than in controls.
• Coccaro (1997)


  • Used a questionnaire to measure hostility in male participants (182 MZ and 118 DZ twin pairs). They found concordance rates of 50% for MZ twins and 19% for DZ twins, suggesting genetic factors do play a part in aggressive behaviour.
• Genetic explanations of aggression support biological determinism, which has serious implications for our legal system where a person is seen to be responsible for their actions.
• Genetic explanations are also reductionist as they explain aggression by looking at genetic factors only, ignoring the many other complex factors that are likely to be involved in human aggression.
• Ethological explanation of aggression

  Aggression is an adaptive instinct which has evolved to ensure only the strongest and fittest males pass on their genes, to disperse members of a species more widely, and to help maintain hierarchies in socially organised animals.
• Innate releasing mechanism (IRM)

  A built-in structure in the brain that when triggered by a sign stimuli (environmental trigger or threat) causes a series of fixed action patterns to occur.
• Fixed action patterns


  A sequence of stereotyped pre-programmed behaviours triggered by an innate releasing mechanism, such as exposing teeth or claws, facial expressions, etc.
• Lorenz studied imprinting in greylag geese
• Aggression (ethological explanation)


  An adaptive instinct which has evolved for several reasons, all of which aid survival
• Reasons aggression has evolved

  • To ensure only the strongest and fittest males pass on their genes
  • To disperse members of a species more widely so territorial resources are exhausted less quickly and disease has less impact
  • To help maintain hierarchies in socially organised animals
• Innate releasing mechanism (IRM)


  A built-in structure in the brain that when triggered by a sign stimulus (environmental trigger or threat) causes a series of fixed action patterns to occur
• Fixed action patterns


  A sequence of stereotyped pre-programmed behaviours triggered by an innate releasing mechanism
• Ritualistic signals


  Behaviours designed to deter another male from entering an animal's territory and potentially taking away access to females
• Animals rarely engage in actual fighting as they do not want to kill the other animal as this would cause the species to die out