Also, using criminality as a measure of aggression can be questioned as not all crimes are aggressive such as shop lifting. Therefore, it lacks sample bias.
Additionally, another weakness is MZ twins get treated alike because they look the same. Evans et al argued that findings within twin studies had low validity as MZ twins get treated in the same way which could contribute to aggression.
It should also be considered that results never showed 100% concordance rates in MZ twins which means that there must be other factors that contribute to aggression such as the environment.
Therefore research was conducted on adoption and both accounts for the environment and genes. Mednick conducted research into children who were adopted and looked at the likelihood of them showing aggressive behaviour if biological or adopted parents had already. They found that children who had a biological parent who had a criminal conviction had a higher percentage of a conviction than adopted parents. This proves that it is genes that have the main influence of aggression not as much as the environment.
Supporting research was conducted by Holfling which showed similar results. They also found that biological parents who showed aggression were more likely to have a child with similar behaviour than an adopted parent. This provides support that it is biological factors that mainly influence aggression.
Another positive of these findings is that it does research within the nature/nurture debate. Adoption studies look at the environment at adopted house but also the biology of the biological parents. Therefore, these findings are very valid and useful and it’s easy to conclude that genes are the factor that impact aggression.
However, it should be noted that children who have been adopted are likely to show anti-social behaviour due to separation from family or the stress of foster homes. Therefore, it may not be completely down to genetics but more the environment the children live in.
Although research into twins highlights a genetic component, it doesn’t explain what gene could make them more susseptable to aggression.
One gene in particular that has been linked to aggression is the MAOA gene. This regulates the metabolism of the neurotransmitter serotonin of which low levels are associated with impulsive and aggressive behaviour. Therefore low levels of MAOA are linked with aggressive behaviour.
Brunner et al support this theory as they found that males with particular violent and aggressive mannerisms, and who were involved with serious crimes had low levels of MAOA.
These findings have been replicated in animal’s studies. Cases et al found that there was effect of MAOA in mice, as they disabled the gene and found that when levels of serotonin and dopamine increased, aggression increased. Suggesting a link between MAOA and aggression.
However, it is hard to determine the nature of the relationship if completed on animals due to the complexity of humans. Therefore, findings from animals cannot be generalised to humans as findings may differ.
Overall, research has real-world application. There have been suggestions that if people's genes predispose them towards aggressive behaviour, then doctors can use genetic engineering to change their genes and reduce this risk. Morley suggested that if a genetic predisposition is found early in childhood, then interventions can be used to reduce the risk in the future.
Explanations that are purely genetic have been criticised for being too deterministic. They argue that our aggression is pre-programmed, while ignoring the human characteristic of free will. If aggression is purely biologically determined, then why is everyone not aggressive?
In conclusion this approach is criticised for being very reductionist. As well as genetic factors, there are many environmental factors that influence the manifestation of aggressive behaviour. Therefore, a more holistic approach should be considered using factors from Social learning theory and how observation can increase the likelihood of aggression.