Aggression: Evaluating Genetic Factors

These study notes evaluate the genetic factors implicated in aggression, which can be found here.

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.

tuart 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.

Exam Hint: AO1 marks can also be achieved by presenting research evidence as illustration and the mark scheme for genetics questions advises that genetic research can be accepted as AO1 or AO3. Depending on the AO1 marks available either presenting research evidence as an illustration (AO1) or evidence (AO3) is the key to achieving the maximum AO1 marks.

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