Specification: Neural and hormonal mechanisms in aggression, including the roles of the limbic system, serotonin and testosterone. Genetic factors in aggression, including the MAOA gene.

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. 

Evaluation

Theilgaard (1984) undertook research into men with the XYY genotype and found that XYY can cause an increase in height in individuals but not an increase in aggression.  

Coccaro et al. (1997) compared monozygotic versus dizygotic twin pairs. In this study, twin pairs were examined for the concordance of criminal behaviour for both twins. In cases of monozygotic or identical twin sets, the siblings are genetically identical (100% same genes) whereas in dizygotic or fraternal twin sets, the siblings are merely genetically similar (50% same genes). The MZ twin pairs were found to have a 50% concordance whereas DZ twins were only 19%. This research supports the role of genetics in aggression as the twins that were genetically identical, i.e. 100% of the same genes, were more likely to display criminal behaviour than the twins that only shared 50% of the same genes.  

Scientists such as Godar et al. (2014) have refined selective breeding by specifically removing (knock-out) certain genes in mice. This has been done with the MAOA gene. The MAOA knockout mice showed increased aggression and also higher levels of serotonin, demonstrating the relationship between genes and neurotransmitters. When the mice were given fluoxetine to raise their serotonin levels, their behaviour returned to normal.  

Brunner (1993) undertook a famous study on the males in a large family from the Netherlands, whose members were displaying high levels of aggression. Five of the males were found to have the dysfunctional version of the MAOA gene. The females in the family were not affected by the genetic dysfunction. The MAOA gene is carried on the X chromosome, and as females have two X chromosome even if they possess the dysfunctional MAOA gene, their corresponding X chromosome is likely to have a functional MAOA gene, which has been found to be dominant. The female genotype presentation has the effect of preventing the issue of the flawed monoamine metabolism.  

Stuart et al. (2014) studied 97 men who had been involved in severe domestic abuse to their partners. They found the most violent men had the faulty MAOA gene. These men engaged in the highest level of physical and psychological aggression and inflicted the worst injuries on their partners. 

Possible Exam Questions 

1. Briefly outline and evaluate the findings of one research study into genetic factors in aggression. (4 marks) 

2. Outline and evaluate the role of genetic factors in aggression with reference to the MAOA gene. (16 marks)