The basic difference between men and women lies in their genotype. Most people have 23 pairs of chromosomes and on these chromosomes are our genes. One pair of chromosomes decides whether we are male (XY) or female (XX). Early psychologists investigating aggression believed the genetic cause of aggression could lie in the Y chromosome. They were particularly interested in examining individuals with a genotype of XYY. These individuals were often referred to as ‘super males’ as they possessed two male Y chromosomes.
Court-Brown (1965) discovered 314 patients that had the XYY chromosome presentation and he put forward the view that these patients should remain hospitalised given their increased likelihood of aggressive behaviour.
More recently psychological research has focused on examining individuals with the normal XY genotype. Psychologists have used selective breeding in animals and have found that aggression is a trait that can be transmitted from parent to offspring, thus supporting the genetic explanation.
Research with human subjects has focused on twin studies that have looked at the incidence of aggression displayed by monozygotic (MZ or identical) and by dizygotic (DZ or non-identical) twins. Differences in rates (concordance) of aggression between these sets of twins have indicated that aggression has a genetic element.
With the advances in genetic testing in the last fifty years, specific genes have been identified which have been shown to carry the aggression trait down to individuals. One such gene is the MAOA gene, and one variant has been named the ‘warrior gene’. The MAOA gene is responsible for the production of the protein monoamine oxidase. This protein allows the metabolising of noradrenaline, serotonin and dopamine. A dysfunction in this gene can result in these neurotransmitters not being broken down in the body. If adrenaline isn’t metabolised, then we end up with too much adrenaline. This can cause hypersensitivity in the fight or flight response and individuals may overreact to an external stimulus and perceive a threat where one does not exist. Furthermore, if dopamine is not broken down, increased or excessive levels of dopamine are also linked to aggressive behaviour. Serotonin has a calming influence, and low levels have been implicated in a reduction of control over impulsive behaviour.
Exam Hint: Achieving enough AO1 marks on a genetics question can be problematic. In this case, good AO1 marks can be achieved by explaining the link between the MAOA gene and monoamine/serotonin levels.
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