genetic factors in aggression

• An issue with this evidence is that it is correlational rather than experimental.

  This means that the evidence found cannot establish cause and effect so the conclusions drawn aren’t completely valid. Behaving in a violent way could trigger the change in genetic combinations rather than the other way round.

  Therefore, the conclusions drawn from the study may be hard to use to prove genetic combinations produce violent behaviour. 
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  There is research support for the MAOA gene is implicated in severe violent behaviours.

  Tiihonen 2015 studied finnish prisoners, revealing that the MAOA low-activity genotype in combination with another gene was associated with extremely violent behaviour. There was no substantial evidence for either of these genes among non-violent offenders, indicating that this combination of genes was specific for violent offending only.

  However, critics argue that although these genes may make it harder for some people to control violent urges, they don’t predetermine violent behaviour.

   
•  There are issues in establishing genetic contributions to aggressive behaviour

  One problem is that more than one gene can contribute to a behaviour. This makes it hard to fully establish what is causing the behaviour if genes are interacting to produce the behaviour. Also, there may be other influences such as environmental factors that manifest aggressive behaviours. These factors may interact with each other (genetic factors may affect which environmental factors have an influence).

  So there are difficulties to come to a valid conclusion about what exactly influences aggression.

   
• Theres difficulties in individuals tested and trying to draw conclusions.

  Convictions of violent crime are small compared to the number of violent attacks that dont get a conviction. So, theyre representing a small minority of those regularly involved in aggressive behaviour. Also, offenders classed as ‘violent’ from a court conviction aren't necessarily the most serious, persistent offenders. Someone convicted as a murder, would be designated as violet even if the rest of their lifetime free of crime. This may then explain why so many studies have found little/no evidence of heritability.
• Trying to determine the role of genetic factors on aggression is basically the nature nurture debate. A variety of techniques are used including twin and adoption studies. Monozygotic twins share all of their genes, dizygotic only share 50%. If MZ twins are more alike in terms of aggressive behaviour, then this should be due to genes rather the environment as MZ as both share the same environment, but MZ are closer in genes. One study, aggressive behaviour using adult twin pairs and found that nearly 50% of the variance in direct aggressive behaviour could be due to genetic behaviours.
• . Adoption studies can help with contributions of the environment and heredity on aggression. If there is a positive correlation is found between aggressive behaviour in adopted kids, and aggressive behaviour in the biological parents, this suggests a genetic effect. If the correlation is between the kid and the adopted parents in terms of aggression, an environmental effect is implied. A study of over 14,000 adoptions in Denmark found a significant number of adopted boys with criminal convictions had biological parents with convictions. This provides evidence for a genetic effect.
• Miles and Carey carried out a meta analysis of 24 twin and adoption studies demonstrated a genetic effect. Results showed genetic basis accounted for 50% of the variance in aggression. Several variables, including age of participant and assessment method for aggression moderated the genetic influence suggesting that although genetic factors play a significant part in the development of aggression behaviours, the influence of other factors affects their expression.
• Although no individual gene for aggression has been identified, a gene responsible for producing monoamine oxidase A (MAOA) has been associated with aggressive behaviour. It regulates the serotonin in the brain, and low levels of serotonin have been associated with aggression. In the 1980s a study of a Dutch family found many of its male members behaved in a particularly violent and aggressive manner, and a large proportion has been involved in serious crimes such as arson. These men were found to have abnormally low levels of MAOA and a deficit in this gene was later identified.
• A second study linked MAOA to aggressive behaviour involved 500 male children. Researchers discovered a variant of this gene related to high levels: MAOA-H and low levels MAOA-L. Those with the low variant were likely to grow up to exhibit anti social behaviour, but only if mistreated as children. Those with the gene and the MAOA-H who were maltreated did not. MAOA-L is much more frequent in populations with a history of warfare, 2/3rds of population has this version. Only 1/3rd of people in western pop has this version of the gene. Therefore called the ‘warrior gene’.