Describes and evaluations the genetic explanation of aggression. Including; MAOA gene, XYY, Twin and adoption studies as well as an overall evaluation of the biological approach.

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Sandberg= some males have been identified as XYY males (having an extra Y chromosome)
Analysis of 314 hospitalised XYY patients= more likely to be a result of aggressive behaviour.
Thematic apperception testing= XY, XYY, XXY men were tested. XYY were more likely to give
aggressive answers.
XYY can explain only attempted to explain aggression in men but not in women.
Twin Studies
87% concordance rate for aggressive behaviour in MZ twins and 72% in DZ twins. However, even DZ
shows high concordance rate so it's difficult to draw conclusions.
Difficult to separate genetic and environmental factors. If twins were raised in the same
environment, their aggression could have been learnt through nurture rather than nature
Never 100% concordance rate for MZ twins so there must be another factor that determines
Adoption studies
14,000 pps were studied and it was found that significant number of adopted boys with criminal
convictions had biological parents with criminal convictions. The highest rates of criminal violence in
adopted children occurred when biological parents have a history of crime and when the adoptees
home is disrupted (eg: adopted parent has alcoholism).
Large sample= representative
Strong support: adopted so reduces the likelihood of learning aggression from their bio
parents. However, aggression could have been because of disruption they experienced
during the adoption process (not nature).

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geneBreaks down neurotransmitters in the brain (serotonin, dopamine, nor adrenaline)
Low levels= more aggression (high dopamine ect) goes against the assumption low levels of
serotonin= aggression.
Dutch family: Males were aggressive, attempted rape ect. It was identified that they had abnormally
low levels of the MAOA gene, compared to the non-violent members of the family.
Goes against the assumption low levels of serotonin= aggression.
Dutch family: Males were aggressive, attempted rape ect.…read more


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