Adoption - studies

Mednick et al (1975) 

From 14,000 adoptions in Denmark, it was found that a significant number of adopted boys with criminal convictions had biological parents with criminal convictions. 

Miles and Carey (1997) 

Procedure
- Meta-analysis of 24 twin adoption studies.
- Concerned with genetic basis of aggression.

Findings
- A strong genetic influence accounted for as much as 50% variance in aggression.
- Both genes and family environment were influential.
- At later ages, influence of rearing environment decreased and influence of genes increased.

Brennan and Mednick (1993) 

A series of adoption studies in which the criminal history of an adopted male was compared with criminal history of both biological and adoptive fathers, found that genetic influences were significant in cases of property crime but not in violent crime. 


Adoption studies have shown the highest rates of criminal violence in adopted children occur when both biological and adoptive parents have a history of violent crime - clear evidence of a gene-environment interaction.