Dopamine is another neurotransmitter, but this time high levels are associated with aggression. A study which supports this is one carried out on participants using antipsychotics. They were given the antipsychotics which lowered dopamine levels. It was found that aggression correlated with the dopamine levels and also decreased. This shows that dopamine also has a role in the levels of aggression and can regulate aggression by itself without the support of other neurotransmitters. A problem with this study however is that patients that would need to take antipsychotics would have mental problems anyway. This means that this study can’t be generalised and that the patients minds would produce unpredictable results from taking the drug. A study that contradicts this study however, was one that was done on mice. It was found that they have a reward pathway in the brain, directly linked to aggression. It was found that if a mouse committed an aggressive act that the reward pathway was stimulated. Dopamine was a positive reinforce of this pathway, but not the cause of its stimulation. This suggests a different idea from the previous study, that dopamine is a side effect of aggression, not the cause of it. This study however cannot be generalised as well as it is on mice and not humans. Also it is hard to study the brain and exactly what is happening. The researchers cannot be 100% with what they found.
Though the evidence isn’t as strong, it has been found that Testosterone has a role in aggression levels. High levels are generally associated with aggression however. One study looked at men who had committed violent crimes. It found that the ones who committed the most violent crimes had the highest levels of testosterone. This suggests that Testosterone is directly linked to violent behaviour, however like the research on dopamine it could also be a side effect rather than the main cause of aggression. This study however lacks population validity as it is only on men.
Another study found that men who act aggressively when drunk, after being tested using a swab test, had higher levels aggression. This supports the idea that aggression is linked to testosterone, but it could also be possible that alcohol raises levels of testosterone. This study like the previous one lacks population validity as it is only on men.
Though these studies support the idea that aggression is linked to Testosterone, there are lots of studies that don’t support it. Especially ones that have investigated testosterone and less aggressive people. The studies I have mentioned above both lack population validity and are based on small samples.
The biological explanation also claims there are genetic factors which control aggression. Twin studies are especially helpful in supporting this claim. Mono-zygote twins are generally used more as they have more gentic similarities than Dizygotic twins. Also twin studies investigate environmental factors as well as generally twins share the same environment. An example of a twin study is by Coccaro. It is one of the few studies which investigates the aggressive behaviour rather than the criminal convictions of twins. It found that 50% of the aggressive behaviour can be linked to genetics.
Adoption studies also are also very helpful, unlike twin studies they can distinguish the difference between genetic imposed aggression and environmentally caused aggressive behaviour. If a child is aggressive and so are their non-biological parents, but their biological parents are not aggressive, then a environmental link can be made. And vice versa for a biological link. Hutchings a medrick did a study which supports the idea of a biological link. They studies 14,000 adopted children from Denmark and found that a significant number of boys with criminal convictions had biological fathers with criminal convictions. This suggests that only boys are effected genetically. However this study doesn’t take into account the severity of the crimes and the amount of re-offences.
MAOA is a gene which has been found to be linked to aggression. No gene has been found to be linked to aggression except the one which releases the protein MAOA. MAOA regulates the levels of Serotonin, which is linked to aggressive and impulsive behaviour. A study which supports this was done on a Dutch family which were all reported of committing violent crimes. The fact that they all committed violent crimes suggests that their could either be a genetic or environmental factor playing a role. When it was investigated however it was found that they all had ineffective genes which produced the protein MAOA. It was only the males ones again however who were reported being aggressive. This suggests the idea, again like the previous study, that only men are effected by gene abnormalities associated with aggression.