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Genetic Factors in Aggression The Genetic Link in Aggression What have twin and adoption studies shown? Concordance rate McGuffin & Gottesman (1985) study of concordance rates for aggressive and anti-social behaviour in MZ and DZ twins 100% 80% 87% 60% 72% 40% 20% MZs DZs The Genetic Link in Aggression What have twin and adoption studies shown? Hutchings & Mednick (1973) reviewed over 14,000 adoptions in Denmark Number of convictions for violent offences amongst biological parents (particularly fathers) They found a positive correlation between the two variables shown Number of convictions for violent offences amongst adopted sons The Genetic Link in Aggression Genes have been linked to brain chemistry and increased aggression MAOA is an enzyme that facilitates the breakdown of excess neurochemicals, such as noradrenaline What does this means for those who have a MAOA deficiency? They are likely to have excess levels of noradrenaline The Genetic Link in Aggression Genes have been linked to brain chemistry and increased aggression noradrenaline Too much noradrenaline in the synapses is associated with increased aggression There is evidence to suggest that MAOA deficiency is genetically inherited: The Genetic Link in Aggression Genes have been linked to brain chemistry and increased aggression Four generations of males in a Dutch family inherited a defect on their x-chromosome. The MAOA gene lays in the vicinity of the defect so it is thought that the men’s MAOA is affected – for genetic reasons, not enough of it is released.The men with the defective gene (and not all men in the family inherited it) showed aggressive, sometimes violent behaviour including arson and attempted rape. Bruner tested the urine of the men in the family: those with the defective gene had higher levels of chemicals that result from neurotransmitters, which indicates that the men’s MOAO is deficient The Genetic Link in Aggression Genes have been linked to brain chemistry and increased aggression Further support for the gene hypothesis comes from 110 men who showed an association between abnormalities in the MAOA gene and aggressiveness and impulse control (Manuck et al, 2000)