Genetic Factors

1. Monozygotic

   Annotations:

   • Share all genes.
2. Dizygotic

   Annotations:

   • Share only 50% of genes.
3. Studies

   Annotations:

   • Compare degree of similarity for a specific trait between sets of monozygotic twins to similarity between sets of dizygotic twins.
   • If MZ twins more alike in terms of aggressive behaviour, due to genes. 
   • Studys specifically studying aggressive behaviour using adult twins found a 50% variance in direct aggressive behaviour attributed to genetic factors.

1. Study

   Annotations:

   • Positive correlation between aggressive behaviour in adopted children and aggressive behaviour in biological parents=genes.
2. Denmark

   Annotations:

   • Study of over 14,000 adoptions in Denmark found significant number of adopted boys with criminal convictions had biological parents with the same.
   • Low ecological validity as only looks in Denmark

Annotations:

• Regulates metabolism of serotonin in brain, and low levels of serotonin is associated with impulse and aggressive behaviour.

1. Brunner et al

   Annotations:

   • 1980s. Dutch family found many of its aggressive male members to be involved in serious crimes. 
   • Due to abnormally low levels of MAOA, in which a defect was later found.
2. Caspi et al

   Annotations:

   • Linked MAOA to aggressive behaviour. 500 male children. A variant associated with high MAOA, and variant with low MAOA. 
   • Low=more likely to be aggressive, but only if they were maltreated as a child. Otherwise there was no aggression.
   • Suggesting an interaction between genes and environment determines behaviour.

Annotations:

• Meta-analysis of 24 twin and adoption studies that supported link between genes and aggression. 
• Most relied on self-report, with some observation. Results=strong genetic influence accounted for 50% variance in aggression. 
• Found that age differences is important, later ages the influence of environment decreased, but genes increased.

1. Individual Differences

   Annotations:

   • Those observational ratings were more environmental influence.

Annotations:

• Many studies focus on individuals convicted of violent crimes. 2 problems wrong with this.
• 1. They only represent a small minority, as convictions of violent crimes for more than one instance is rare.
• 2.  May not be the most serious criminals, just labelled violent.

Annotations:

• Meta-analysis. Found low to moderate correlation for link between heredity and crime. 
• Thus, inconclusive evidence.

Annotations:

• Genetic engineering if we're prediposed to aggression. Thus, an awareness of limitations of these studies is important.

1. IDA-Animal Research

   Annotations:

   • Young et al-genetic mutation violence in mice. Effects include violence towards other mice. Counterpart to gene in humans.
   • Cannot generalise to humans.