Heritability - Animal breeding study
Selective breeding allows for particular genetic traits to be kept, therefore, in theory it is possible to breed out certain traits, such as aggression. Belyaev bred silver foxes (which are naturally aggressive), the breeding program lasted 50 years! He only bred the tamest of each generation together. They are now friendly.
The study has face value, as it was highly controlled and shows aggression can be bread out and that aggression is not due to the environment. Therefore it is influenced by genetics in humans.
However, the study was done on animals and humans have a much more developed and complex brain, so it is not possible to generalise the findings to humans.
Heritability - Twin studies
There have been studies looking at aggression in MZ twins (identical twins, sharing 100% genes) and DZ twins (non-identical twins, sharing 50% genes). If MZ twins have similar levels of aggression then it should be due to genes and not the environment. Rutter carried out a meta-analysis on twins for criminality, he found a concordance rate of 13% - 22% for DZ twins and 26% - 51% for MZ twins.
This suggests that aggression is heavily influenced by genetics, as it was higher in MZ twins, who share 100% genes compared to DZ twins who share 50% of their genes.
However, as it is not 100% it cannot be purely due to genetics.
Furthermore, MZ twins could be treated more similarly, as they look a like.
Heritability - Adoption studies
Mednick et al looked at the criminal record of 14,000 danish adoptees between 1927 and 1947. If they have more similar levels of aggression to their biological parents than their adoptive parents, then it would support the theory that aggression was genetical. Those with criminal biological fathers were more aggressive, however, those with criminal biological AND adoptive fathers were the more aggressive.
This shows aggression is linked to genetics, but it is increased further when nature and nurture are mixed together.
The Diathesis-stress model argues that the genetics model is too reductionist. This model argues that aggression is due to genetics and early learning.
The "Warrior Gene" - MAOA (1)
There hasn't been any discovery of a specific gene that causes aggression, however the gene that produces a protein called Monoamine Oxidase A (just remember it as MAOA) has been linked to aggression. MAOA regulates the metabolism of serotonin in the brain, low levels of serotonin are linked to impulsive and aggressive behaviour. Brunner et al found that in 4 generations of a dutch family. The male's were found to be very violent and involved in things such as **** and arson. They had low levels of MAOA and the defective gene responsible was later discovered.
This shows there is a link between aggression and genetics. However, the sample is biased as it only uses one family and it only looked a the males. The aggression could have also been learned, supporting the social learning theory (SLT).
The "Warrior Gene" - MAOA (2)
Moffie et al conducted a logitudinal study of 422 males in New Zealand. He looked at their history of abuse, criminal convictions and penchant for violance and MAOA. It was found that low levels of MAOA lead to violent crimes, but only in those who who sufferd abuse as a child.
However, most research into this area focuses on criminality as the operational definition of aggression. We need to be sure we are measuring aggression. The crimes usually range from first degree murder to tax evasion and copyright theft. 1 gene cannot explain all crimes. It is also hard to explain why criminal behaviour peaks in the 20s and falls in the 30s.
There is evidence that genes play a significant part in aggression. Animal studies have shown aggression can be bred out, or at least reduced throught selective breeding. Family studies have also shown that aggression can be heritable in humans too.
However, enviroment factors are important too. Genes and enviroment combined together increase the level of aggression.