However, the research lacks ecological validity because it is based on prisoners so it is not based on people doing ‘normal’ day-to-day activities. The study has gender bias and lacks population validity, this is because the study was only done on males so cannot be generalised to women as they are unlikely to be the same, also, because it was only done on adults it does not account for aggression in children, meaning it lacks population validity.
Another biological explanation of aggression is the ‘warrior gene’. The ‘warrior gene’, (MAO-A), has been associated with increased aggression, this is because it produces monoamine oxidase, a chemical which is involved in the breakdown of neurotransmitters serotonin, dopamine and norpine. By having too little of the ‘warrior gene’ leads to the brain being flooded with too much serotonin, dopamine and norepinephrine, leading to lowered sensitivity to neurotransmitters, low sensitivity is equivalent to low activity. Research to support the ‘warrior gene’ was done by Moffat et al. (2002). They did a longitudinal study of 422 males in New Zealand, he studied their history of abuse and criminal convictions, their penchant for violence and any symptoms of anti-social personality disorder. His findings showed that there was a link between the MAO-A and the risk of being convicted of a violent crime, therefore, supporting that having the ‘warrior gene’ can be associated with aggression. However, this was only the case for participants who suffered abuse as children.
This study by Moffat et al. (2002) has gender and culture bias. The study only looks at males from New Zealand, meaning it cannot be generalised to women as they are likely to not be the same as men, also, it has culture bias because the study only looks at New Zealand so the males in that culture may be different to males in other countries and cultures, also meaning it cannot be generalised or representative to other cultures or females.
The high levels of testosterone theory is reductionist because it takes the complex explanation of aggression and says it is all because of this one hormone, testosterone, also, it does not take into account any external factors, such as previous experiences of aggression.
One issue with the ‘warrior gene’ is that it is deterministic. This is because it doesn’t take into account any external factors, just says that no matter what, this gene will make people behave or act aggressively if they have suffered abuse as a child, meaning that the behaviours are determined by this gene and not free will.