Neurotransmitters are chemicals in the brain which allow messages to be sent accross neurones. High or low levels of certain transmitters can therefore lead to differences in behaviour as messages being sent accross neurones are either inhibited or enhanced.
Two neurotransmitters thought to be linked to aggression are Serotonin and Dopamine
1 effect of serotonin is that it inhibits emotional responses, therefore lower levels leads to a greater likelihood of impulsive behaviours such as aggression. This has been found in both humans and animals, and higher levels are associated with more docile behaviours.
Links are not well established, but it is thought that high levels lead to an increase in aggression. Drugs which increase dopamine levels have been found to aggression levels and vice versa.
Another possible link is that rewarding stimuli such as food and sex produce more dopamine. It is thought that aggression has the same effect and that some people behave like this to receive the rewarding sensations that dopamine give.
Mann et al;
gave drugs to 35 healthy ppts which reduced levels of serotonin in their bloodstream. Aggression levels were measured via a questionnaire. It was found that aggression scores were higher after taking the drug
However this was only found with males suggesting that serotonin levels alone are not responsible. This also reduces external validity as the findings can only be applied to males. Also, the use of self report means that responses may be unreliable, due to the fact that ppts might lie and say they are less aggressive than they are in order make themselves appear better to others. (Social desirability effect)
Animal research - Popora et al;
research conducted on animals which supports serotonin link. They found that animals selectively bred for their docile temperaments developed high levels of serotonin over generations.
However it is difficult to extrapolate the findings to humans (low external validity). Also a casual link was not established, only a correlational one meaning that other factors may have reduced aggression, and in turn serotonin.
ALSO, it may not be the levels of serotonin that cause aggression but the receptors. E.g. fewer receptors in the brain will result in less serotonin being taken up, therefore making less use of the serotonin. Low levels are also linked with depression and overeating, therefore aggression is not a definite consequence suggesting that serotonin levels are not solely responsible.
It is thought that high levels of the male hormone testosterone result in aggression. During puberty, levels of testosterone increase, as does levels of aggression. This is hormone is also found in women; however there is a much lower level. The link between hormones and behaviour is supported by the fact that hormones that result in things like PMT have been used in law to explain certain behaviours.
Dabbs et al, found higher levels of testosterone in violent criminals than non violent criminals.
However as the findings are correlational, it could be argued that aggression leads to high levels of testosterone. Also, External validity is reduced as the study was only conducted on criminals in prison, and so it is not representative of the wider population.
Animal research conducted by Wanger, Beuving and Hutchinson found that levels of aggression in mice were lower after castration. When given testosterone after, levels of aggression then increased.
Like many studies, this produced a correlational link, suggesting that the cause of aggression is much more complex
Both neural and hormonal explanations of aggression can be seen as biologically determinist, meaning that neurotransmitters and hormones such as serotonin and testosterone determine the behaviour of the individual. However the explanation does not take into account how external factors may also have an effect on levels of aggression. E.g. life events.
A testable hypothesis can be formed as the variables are easily identifiable. (IV the levels of the hormone/neurotransmitter, the DV the effect it has on behaviour) This can be done by giving ppts serotonin and giving them a self report in order to see how it effects their behaviour.
The underlying assumption is that high or low levels of certain hormones and neurotransmitters have an effect on the levels aggression of the body.