Genetic Factors In Aggression Flashcards
What are concordance rates?
This is the percentage given to a persons likelihood of developing a characteristic, based on the prevalent of it in their family. If your father has a very high IQ what is the percentage chance of you developing a high IQ?
How do we try to separate the nature from the nurture?
By using twin studies and adoption studies to find out how much concordance is a result of inheriting genes and howof it is due to environmental factors.
Twin study for aggression
Coccaro et al studied adult male monozygotic and dizygotic twins. For direct physical aggression, the researchers found concordance rates of 50% for MZ twins and 19% for DZ twins.
Adoption study in aggression
Similarities in aggressive behaviour between an adopted child and biological parents suggest genetic influences are operating; but similarities with adopted parents suggest environmental factors. Hutchins and Mednick conducted a large scale adoption study in Denmark of over 14,000 participants. They found a significant proportion of the male adoptees who went on to have criminal convictions for violent behaviour had biological parents (especially fathers) who had criminal convictions for violent behaviour.
What is the role of the MAOA gene?
This enzyme ‘mops up’ neurotransmitters after a nerve impulse has been transmitted between neurones. It breaks down the neurotransmitter (eg serotonin) into constituent chemicals to be recycled or excreted. Production of this enzyme is determined by the MAOA gene - a dysfunction in the operation of this gene may lead to abnormal activity of the MAOA enzyme, which affects levels of serotonin.
Research evidence for the MAOA gene.
The ‘warrior gene’ is a variant of the MAOA gene that leads to low MAOA activity in the brain and is associated with aggressive behaviour. Brunner et al studied 28 male members of a Dutch family repeatedly involved in impulsivity aggressive violent criminal behaviours . These men had both abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene.
Strength of the MAOA explanation - support from animal studies.
Genetic deletion techniques allow researchers to ‘knockout’ single genes in mice; an experimental manipulated that lets them observe the effects on aggression. Godar et al showed MAOA ‘knockout’ mice have increased brain serotonin and are hyper-aggressive. When serotonin is blocked by the drug fluoxetine, the mice revert to non-aggression. These findings show MAOA must normally have some function in relation to serotonin, presumably keeping it at a ‘normal’ level by removing it from synapses.
Strength of the MAOA explanation - support from prosocial behaviour research.
The low-activity variant of the MAOA gene is associated with greater aggression so presumably the converse is true -people with the high-activity variant should be more prosocial. Mertins et al found males with the high-activity MAOA variant were more co-operative in a lab based task - they made fewer aggressive moves than participants with the low-activity variant. This finding confirms the importance of the MAOA gene in aggressive behaviour. The two predictions of the MAOA explanation are opposite sides of the same coin.
Weakness- findings depend on how aggression is measured.
Methods of measuring aggression differ significantly between studies, and include self-reports, parent and teacher reports and direct observations. In Rhee and Waldman’s meta-analysis of 51 twin and adoption studies, genetic factors had a greater influence on aggression in studies using self-reports rather than parent or teacher reports. Findings vary depending on how aggression is measured so it is difficult to ‘pin down’ the real causes of this behaviour and draw valid conclusions about the role of genetic factors.
Weaknesses genetic factors - they have been difficult to isolate.
It is difficult to separate genetic and environmental factors. Someone with an aggression-associated gene may only behave aggressively if the environmental conditions are suitable. McDermott et al showed that participants with the low-activity MAOA gene bahaved aggressively in a lab-based task, but only when they were provoked. This means that it has been proven remarkably challenging to establish exactly how influential genes are in aggressive behaviour.
Weakness of the MAOA explanation - there are multiple genetic influences.
The sizes of genetic effects are statistically significant but actually quite small - so there are probably other genes involved in aggression. Stuart et al found that intimate partner violence (IPV) in men was associated with the serotonin transporter gene (5-HTT) - the combination of this and the MAOA gene was most closely linked with IPV. Hundreds or thousands of genes interact in complex ways, casting doubt over any search for single candidate genes.
Evidence to show that the most violent domestic abusers has the low-activity MAOA gene.
Stuart et al studied 97 men from a treatment programme for domestic abusers, who had inflicted a form of aggression called intimate partner violence (IPV). Men with the low-activity MAOA gene were the most violent perpetrators of IPV - engaged in greatest psychological and physical aggression and inflicted worse injuries on partners.
Low MAOA gene activity is linked to aggression only when combined with early traumas.
Frazzetto et al found an association between antisocial aggression and the low-activity MAOA gene variant in adult males but only in those who experienced significant trauma (e.g. Physical or sexual abuse) during the first 15 years of life. Those who had not experienced trauma were not especially aggressive as adults even if they possessed the low-activity MAOA gene variant. These findings together are strong evidence of a gene-environment interaction.
