Genetic factors in aggression

As this evidence is not clear cut, psychologists also look at evidence form adoption studies. These would help to establish a link by looking at someone who is adopted and comparing their aggressive tendencies with that of their biological parents and their adopted parents. This tells us whether the aggression is a result of genetics or the environment (nature/nurture). An example of an adoption study would be that of Hutchings and Mednick who did a study of 14000 adoptions in Denmark, they found that a significant number of adopted boys with criminal convictions had biological parents with criminal convictions, providing evidence for a genetic factor. This study however looks at criminal convictions; criminal convictions vary and do not always mean acts of aggression were carried out, therefore to generalise these findings to whether aggression is genetic would be wrong. However other studies have looked more at aggression and show that there is a definite genetic factor contributing to aggression. These studies are good evidence of a genetic factor because it clearly shows the link between the adopted ‘child’ and biological parents.    

An alternative route of research has looked to identify a single gene that might influence aggression, for example, 47 XYY karotype; the gene that was put forward by Sandberg, this gene is an added Y sex chromosome that is found in certain males. This extra chromosome, it was suggested, was the aggression gene that could be inherited through genetics. Research later carried out by Court-Brown led him to conclude that patients with the XXY would be best hospitalised due to the increased likelihood of aggressive behaviour. This explanation for aggression is widely discredited due to the fact that it ignores female aggression, it suggests that aggression is only in males with the extra Y chromosome and therefore doesn’t account for aggression in females or males without the extra chromosome. More promising was the research into MAOA mutation.  It describes aggression as being a mutation of a single gene. Brunner et al found that a mutation in a gene in the males of a Dutch family who all showed abnormal aggressive behaviour. MAOA is involved in metabolising serotonin and aggression has been linked to low levels of serotonin. This theory is supported by a lot of research linking aggression and serotonin but is this not more a hormonal factor rather than a genetic factor? On the other hand it has been shown that this mutation can be inherited (in genetics) which is the cause of maybe lower serotonin levels.

Looking at the idea that genetic factors play a role in aggression, on the whole we can identify a major difficulty: the way in which levels of aggressive behaviour is measured or assessed. Many of the studies involve self-report and questionnaires as opposed to observation, this means that individual differences can affect findings. It also creates difficulties in comparing pieces of research as methods are different and so are measures of assessment. This makes research on aggression being genetic rather unreliable and it cannot be used to fully support any theory.