Genetic factors (AG)

Can compare degree of similarity for aggression between sets of MZ and DZ twins

MZ twins: Share all of their genes
DZ twins: Share only 50%

Means that if MZ twins have similar aggression, it could be due to genes rather than environmental factors
 (Both types of twins experience the same environment/upbringing) 

COCCARO ET AL. 

- Specifically studied adult twin pairs
- Nearly 50% of the variance in direct aggressive behaviour could be caused by genetic factors 

MILES & CAREY

- Meta-analysis of 24 twin and adoption studies 
- Most relied on participants' self-reports of aggressive behaviour and tendencies 

Results proposed a strong genetic influence on at least 50% of the variance in aggression

Supports the idea that genetic factors play a role in at least some cases of aggression

Adoption studies can untangle links between the contribution of the environment and heredity in aggression

+ve correlation between...

1) Aggression in adopted children / aggression in biological parents = Genetic link implied

2) Aggression in adopted children / aggression in rearing family = Environmental link implied

HUTCHINGS & MEDNICK

- Study of over 14,000 adoptions in Denmark
- Significant number of adopted boys with criminal convictions had biological parents (mainly fathers) with criminal convictions
- Gives evidence for a genetic link

- Issues with sampling

+ Less convictions for violent crimes compared to the actual number of violent attacks
+ Only represent a small minority of those regularly showing aggressive behaviour  

+ Offenders labelled as violent by court convictions are not always the most serious offenders
+ One person could be labelled violent for one offence despite never committing any before

This could be why many psychologists have found little evidence of heritability for violence with sampling studies

Fails to distinguish between individuals who have a tendency for violence, and those who have only committed a crime once
  

Studies of aggression in animals are valuable in helping us explain human aggression

- E.g. Rodents can be experimented on in order to test the effects of specific genes on aggression
- Includes selective breeding/'knockout' techniques

YOUNG ET AL.

- Claim to have found a genetic mutation- nicknamed 'Fierce'
- Causes violence in mice and other effects
- Human counterpart doesn't seem to exist
- But it demonstrates that non-human studies can be important in helping us explain genetic factors in aggression

- There are methodological limitations when looking at the genetic inheritance of aggression in criminals

- Studies investigating this often fail to distinguish between violent / non-violent crime

- E.G. in Hutching & Mednick's adoption study

Makes it more difficult to specifically study the role of genetics in aggressive violent only