Genes are seen as the bodies blueprint, determining our appearance. E.g. eye colour and hair colour. However they can also be seen as determining our personality traits such as aggression.Therefore these traits can be genetically inherited as genes are passed down from one generation to the next.
This is the basis for selective breeding amongst animals, whereby certain characteristics are either bred-in or bred out by careful selection of parents who posess the required genetic make up.
Though an individual gene for aggression has not been discovered, there is a gene responsible for producing the protein MAOA, which regulates the levels of serotonin, and low levels of serotonin have been found to have an effect on levels of aggression.
Evidence for this has been sought through family, twin and adoption studies. The thought behind these studies is that the closer the genetic link to someone who has aggressive behaviour, the more likely they are to be aggressive. This means that concordance rate between MZ twins should be higher than between DZ twins. Also, concordance rates will be higher between children and their biological parents than children and their adoptive parents.
Brunner et al found that male members of one dutch family were particularly violent aggressive criminals, and all were found to have a defect on the MAOA producing gene. This therefore supports a hereditory/genetic link
However - the link is only correlational, suggesting that other unknown elements may be the cause of their behaviour. Also, this was only found in male family members and doesnt take into account the behaviour of the females of the family who, according to the study, should also have aggressive behaviour due to genetic links.
Twin Studies - Coccaro et al - conducted a study into male twins. On comparing MZ and DZ twins they found that genes acocunted for approx. 50% of aggressive behaviour. Therefore this also supports the genetic link.
However - this study does not take into account external factors which may effect the behaviour of offspring. For example, MZ twins will have shared the same environment both before birth and in the upbringing, and so this may be the cause of the similarity in aggressive behaviour. Also, only males were studied, and so results may not ben applicable to females thereby reducing external validity. Furthermore, according to the theory MZ concordance rates should be 100% if the link is genetic, so this therefore reduces effectiveness
The genetic theory of aggression is a biological theory that explains how people may have inherited aggression through their genes, therefore producing a nature rather than nurture explanation. So someone with aggressive behaviour will be very likely to pass this on to their offspring. However it is difficult to establish genetic contributions to aggression because there may be many other reasons for aggression. For example other genes or social learning.
A testable hypothesis could be formed by comparing the behaviour of adopted and non adopted children in order to determine to what extent external factors may have an effect.
According to the theory it should be possible to determine who has/will have aggressive behaviour from looking at the levels of aggression in their family members, and therefore it should be able to be dealt with quickly.