Aggression

• Neural mechanisms in aggression


  Irregularities in certain areas of the brain increase aggressive behaviour
• Limbic system


  • Circuit of structures in the brain which deals with emotions and motivations, particularly those related to survival, such as aggression
  • The most important part of the limbic system in relation to aggression is the amygdala
• Role of the amygdala and prefrontal cortex in aggression

  1. Stimulation of the amygdala leads to reactions of rage/aggression

  2. The amygdala sends input to the prefrontal cortex

  3. The prefrontal cortex tries to regulate the information sent by the amygdala

  4. The prefrontal cortex is involved in differentiating between good and bad behaviour, understanding the consequences of behaviour, and controlling aggressive urges
• Aggressive behaviour


  Caused by 1) Overstimulated amygdala leading to frequent feelings of rage and aggression and/or 2) Ineffective prefrontal cortex at controlling the impulse to behave aggressively
• Studies on the role of the amygdala in aggression

  • Kluver and Bucy (1937) - Destroying the amygdala in a dominant monkey caused it to lose its dominant position
  • Narabyashi et al (1972) - 43/51 patients whose amygdala was destroyed showed reduced aggression afterwards
  • King (1961) - Electrical stimulation of the amygdala caused a woman to become threatening and verbally aggressive
  • Wong et al (1997) - Criminals with violent tendencies have a smaller amygdala than the 'normal' population
  • Pardini et al. (2014) - Reduced amygdala volume can predict development of severe aggression
• Studies on the role of the orbitofrontal cortex (OFC) in aggression

  • Coccaro et al (2007) - Patients with psychiatric disorders featuring high aggression had reduced activity in the OFC
• Neurotransmitters in the brain causing aggression


  Neurotransmitters allow impulses to be sent from one area of the brain to another. Two important neurotransmitters in aggression are serotonin and dopamine.
• Role of serotonin


  Low levels of serotonin have been associated with aggression. Normal levels of serotonin in the orbito-frontal cortex are linked with reduced firing of neurons, and this is associated with a greater degree of behavioural self-control. Decreased serotonin may disturb this mechanism, reducing self-control and leading to an increase in impulsive behaviour – including aggression.
• Role of dopamine


  High levels of dopamine have also been associated with aggression. It may serve a reinforcing role in that people repeat aggressive acts because the release of dopamine makes it pleasurable.
• Studies on the role of neurotransmitters in aggression

  • Berman et al - Participants who had taken paroxetine (which enhances serotonin activity) were more likely to give fewer and less intense electric shocks than those who had taken the placebo
  • Denson et al - Decreased serotonin disturbs the OFC and reduces self-control, thus increasing aggression
  • Virkkunen et al (1994) - Serotonin levels were significantly lower in impulsive offenders compared to violent non-impulsive offenders
• Hormonal mechanisms in aggression

  Hormones are chemical substances that help to regulate processes in the body. They are produced by a group of glands that make up the endocrine system.
• Role of testosterone


  High levels of testosterone are associated with increased aggression due to its action on brain areas involved in controlling aggressive impulses. High levels of testosterone is linked with reduced activity in the orbito-frontal cortex, which is involved in decision-making and self-control. Testosterone also influences the activity of serotonin in the brain and can reduce serotonergic activity, which is implicated in increased aggressive behaviour.
• Studies on the role of testosterone in aggression

  • Giammanco et al (2005) - Experimental increases in testosterone are related to greater aggressive behaviour in several species of animal
  • Dolan et al (2001) - Positive correlation between testosterone levels and aggressive behaviours in a sample of 60 male offenders
  • Wagner (1979) - Castration of mice reduced aggression levels
  • Dabbs (1995) - Higher testosterone levels in rapists and violent offenders than in burglars or thieves
  • Dabbs et al (1987) - Violent criminals had higher testosterone levels than non-violent criminals
• Dual-hormone hypothesis - High levels of testosterone lead to aggression only when levels of cortisol are low. When cortisol is high, the influence on aggression is blocked.
• Biosocial model of status - Testosterone levels change rapidly during the course of the day, especially in response to social interactions related to status. A change in testosterone levels following a loss of status should affect post-competition aggression.
• Concordance rate


  The rate at which both twins in a pair show the same characteristic
• Coccaro et al (1997) found concordance rates of 50% (MZ) and 19% (DZ) for aggressive behaviour defined as direct physical assault, and 28% (MZ) and 7% (DZ) for verbal aggression
• Gottesman found concordance rates of 87% (MZ) and 72% (DZ) for aggressive and anti-social behaviour
• The large difference between the concordance rates for MZ and DZ twins in Coccarro's study indicates a genetic influence on aggressive behaviour
• Twin studies


  Assume both twins share the exact same environment, but DZ twins can be mixed genders which could result in a different upbringing
• If the aggression was entirely genetic, the concordance rates for the MZ twins would be 100%. As it isn't, it implies something else may be the cause of the aggression
• The findings suggest that whilst there is a genetic influence on aggressive behaviour, a holistic approach should be taken
• Adoption studies


  Similarities in aggressive behaviour between an adopted child and their biological parents suggest genetic influences, but similarities with adopted parents suggest environmental influences
• Rhee and Waldman (2002) conducted a meta-analysis of 51 twin and adoption studies involving 87,000 individuals and found that genetic influences accounted for 41% of the variance in aggression
• Meta-analysis


  • Large sample size which could increase generalisability of the results (high population validity)
  • The 51 studies differed significantly in how aggression was measured, which raises issues about the validity of the conclusions
• Research has suggested that aggressive people have a particular gene which predisposes them to be aggressive, such as the XYY gene combination and the MAOA gene
• Jacobs et al (1965) found that about 3% of inmates had XYY syndrome compared to only 0.1% in the general population
• More recent research (Witken et al 1970) has doubted the link between XYY syndrome and aggression, finding that XYY people were criminals but their crimes were likely to be non-violent and they had a low IQ
• Vassos et al (2014) conducted a meta-analysis and could find no evidence of an association between any single gene and aggression
• MAOA gene


  Monoamine oxidase A (MAOA) is an enzyme that 'mops-up' neurotransmitters in the brain following synaptic transmission. Variants of the gene that result in low MAOA activity are linked to highly aggressive behaviour.
• Brunner et al study


  • Studied 28 members of a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviours, and found they had abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene
• Stuart et al study


  • Studied 97 men who had been convicted of intimate partner violence, and found men with the low variant of the MAOA gene were the most violent and inflicted the worst injuries
• Mertins et al (2011) study


  • Found that people with the high-activity variant of the MAOA gene were more co-operative and less aggressive than those with the low-activity variant
• Genes are crucial influences on aggressive behaviour but they do not function in isolation. Low MAOA gene activity is only related to adult aggression when combined with traumatic life events
• Ethology


  The study of animal behaviour, which can help understand human aggression by defining it, looking at how it occurs with other behaviours, and understanding its evolutionary function
• Konrad Lorenz


  • Key ethological theorist on aggression who published a book called 'On Aggression' (1963)
• Ritualistic aggression


  Aggressive behaviours in animals that are carried out in a set order and result in very little physical damage, emphasising the adaptive nature of aggression
• Innate releasing mechanism


  A biological structure or process (e.g. in the brain) which is activated by an external stimulus, which in turn triggers a fixed action pattern
• Fixed action pattern


  A sequence of stereotyped pre-programmed behaviours triggered by an innate-releasing mechanism, which are universal and found in every individual of the species
• Tinbergen (1951) study


  • Presented male sticklebacks with wooden models and found that regardless of shape, if the model had a red spot the stickleback would aggressively display and attack it, but with no red spot there was no aggression, demonstrating innate releasing mechanisms and fixed action patterns