Neural + hormonal mechanisms

• Aggression is a form of physical or verbal behaviour with the intent to harm. It includes: hostile - intention to hurt another person and instrumental - to achieve a goal.
• The areas of the brain that are assosciated with aggression is known as the "limbic system." It coordinates behaviours that satisfy motivational and emotional urges such as aggression and fear.
• Two key parts of the limbic system are the amygdala and the hippocampus.
• Amygdalae :

  • 2 almond shaped groups of nuclei that are located within the temporal lobes of the brain in complex vertebrates
  • Perform a primary role in the processing of memory, decision making and emotional reactions
  • Quickly evaluates sensory information and prompts an appropriate response
  • Stimulation of this area in animals results in an aggressive response (snarling and posture)
  • If removed, the animal no longer responds violently to things that would normally produce rage
• Hippocampus:

  • Located in the limbic system
  • Plays an important role in the consolidation of information from STM to LTM and in spatial navigation.
  • Allows the animal to compare a current threat to a past experience
  • Impairment of this area may cause the amygdala to respond inappropriately to sensory stimuli which results in aggressive behaviour
• Diagram:

  
• The following chemicals have been assosciated with aggression :

  • serotonin
  • testosterone
• Serotonin:

  Normal levels of serotonin have been assosciated with calming and inhibitory effects whilst lower levels of serotonin in the amygdala result in less inhibition which makes aggression more likely.

  One study reported that consuming drugs which decrease serotonin levels leads to an increase in aggressive behaviour in men but not in women.
• Testosterone:

  Produces male characteristics including aggression

  Men have higher levels of testosterone and this results in higher levels of physical aggression.

  Reduction of testosterone levels in animals leads to reduction in aggression and increasing it leads to aggressive behaviour returning.
• A03

  Supporting Evidence for the role of testosterone in aggression

  • Dabs et al
  • Found that criminals with higher levels of testosterone had committed violent crimes, whereas those with lower levels had only committed non violent crimes
  • The study suggests that there may be a link between levels of testosterone and aggressive behaviour
• A03

  Supporting evidence for the role of serotonin in aggression

  • Rosado et al
  • Used dogs of varied breeds who were referred to Spanish vets for aggressive behaviour towards humans
  • 80 referred dogs were compared to 19 controls who were not aggressive towards humans
  • Found lower units of serotonin in referred dogs (278 units) compared to 387 in the control group
  • However there is mixed evidence in humans (remember this is an animal study and therefore may not be generalisable to humans)
• A03

  Supporting Evidence for the role of amygdala in aggression

  • Pardini et al
  • Conducted a longitudinal study from childhood to adulthood on 56 males with varying histories of aggression
  • An MRI scan was used to measure the volume of the amygdalae
  • A relationship was found between the amygdala volume and the levels of aggression/violence
  • The study suggests that the amygdala plays a large role in evaluating sensory information and a lower volume makes aggression more likely
• A03

  Supporting evidence for the hippocampus in aggression

  • Raine et al
  • Studied two groups of criminals, both convicted (caught ) and unconvicted ( evaded law )
  • The first group ( caught violent ciminals ) were thought to be more impulsive and the second group were more cold and calculating.
  • The hippocampi in the left and right hemispheres were different in size in the convicted violent criminals.
  • This may impair the ability of the hippocampi and the amygdala to work together, leading to incorrect processing of emotions and inappropriate responses
• A03-

  Gender differences

  • hormonal mechanisms can accoutn for gender difference in aggression
  • the link between testosterone and aggression may explain why males are generally more aggressive than females as males produce much higher concentrations of testosterone than females, and this may therefore lead to higher levels of aggression.
  • this link also explains why aggression is highest in male adolescents as levels of testosterone are highest at this stage
• A03

  A criticism of these links between hormones and neurotransmitters and aggression is that they can be described as reductionist. The complexity of human behaviour means that biological explanations are insufficient on their own to explain the many different aspects of human aggression. For e.g. SLT.