AQA A-Level Psychology Genetic explanations of aggressionWatch
1. Discuss genetic factors involved in aggressive behaviour.
Genes can also be used to explain aggressive tendencies in humans. To do this, researchers have used twin studies and adoption studies. With regards to twin studies, researchers have looked at MZ twins and DZ twins. The former share 100% of their DNA whereas the latter only share 50% of their DNA. If the concordance rates in MZ twins is higher than that of DZ twins with regards to aggression, then this suggests a genetic basis. Research that has been carried out using twin studies have mainly focused on criminal behaviours. However, one of the few studies that have looked at aggression in twins used adult twin pairs and it found that around 50% of the variance in aggression could be attributed to genetic factors. However, this explanation using twin studies doesn't really take into account other factors that could influence aggression such as the environment and so researchers also use adoption studies.
When looking at adoption studies and aggression, if an adopted child and their biological parents have aggressive tendencies, this would imply a genetic basis. On the other hand, if the child and its rearing family show aggressive tendencies then this would suggest an environmental basis. A study was carried out on 14,000 Danish children and it was found that, out of those who had criminal convictions, they also had parents (especially fathers) with criminal convictions too and so this provided evidence for the strong genetic basis in aggression.
Miles and Carey carried out a meta-analysis on 24 studies and found that 50% of the variance in aggression could be attributed to genetic factors. In addition to this, it was also found that there were age related differences between aggression and the influence of genes and the environment in that as you got older, the influence of genetics increased and the influence of the environment decreased.
There has been a lot of research conducted into the influence of the MAOA enzyme and aggression. Researchers identified a gene responsible for the secretion of MAOA which is an enzyme responsible for the catabolism of serotonin in the brain. A study was carried out on the male individuals in a Dutch family who were extremely violent and had committed crimes such as rape and arson. It was later found that they had abnormally low levels of MAOA and a defect in the gene responsible for its secretion was later found. Researchers have also discovered variants of the MAOA enzyme, MAOA-H for high levels of MAOA and MAOA-L for lower levels of MAOA. Caspi et al. carried out a study on children with MAOA-L and children with MAOA-H. It was found that the children with MAOA-L were extremely more likely to develop anti-social and aggressive behaviour as they grew older but only if they had been maltreated in youth. Children with MAOA-H were less likely to exhibit such behaviours even if they had been maltreated as children. Furthermore, the MAOA-L variant has been found to be more common in populations with a history of violence. For example, in such countries, around 2/3 of the population have the MAOA-L variant compared to western populations whereby only 1/3 have the MAOA-L variant. Moreover, McDermott also found that individuals with the MAOA-L variant were more likely to exhibit violent behaviour when provoked.
There have been problems with sampling with regards to the studies carried out in this area. Firstly, these studies look at individuals who have been convicted of carrying out violent crimes. However, they only represent a small population of all the individuals who carry out violent behaviours but aren't convicted and so, they are only a small percentage of all of those who carry out aggressive behaviours. Secondly, contrary to what the majority believe, offenders who have been branded as violent from a court conviction may not necessarily be the most serious, persistent offenders. For example, an individual convicted of murder will be labelled as violent even though they have had a lifetime free of violence. This, in conjunction with the former point can explain why researchers have found little to no heritability of violence.
In addition to this, there are problems with using just genetics as an explanation. Firstly, usually more than one gene is involved in a behaviour. There are also other non-genetic factors such as the environment which can influence aggression and genetics and the environment can also interact with each other to cause aggression as shown in Caspi's study with MAOA-L and children being maltreated in youth.
Furthermore, assessing the influence of genetics on aggression is also a problem as research tend to use self-report of parental reports. In the 24 studies that Miles and Carey looked at in their meta-analysis, it was found that there was a strong genetic basis but this was because the self-report techniques were used. However, when observational techniques were used, there was a strong environmental basis and so these inconsistencies make it difficult to draw strong conclusions as to the influence of genes on aggression.
With regards to MAOA, there has been research evidence for its role in aggression in that studies have found that individuals with MAOA-L, in conjunction with the CDH13 gene were more aggressive and had extremely violent tendencies than those who had normal levels of MAOA. In addition to this, MAOA can also be used to explain gender differences in aggression. The gene responsible for the secretion of MAOA is located on the X chromosome and so males who inherit the X-linked chromosome from the mother will express that gene because they have no normal allele. Females however have 2 X chromosomes so they will have an allele that is normal and dominant and so they won't display the phenotype of aggression.
(Total 16 marks)