Biological explanations of criminality

Biological explanations for criminality suggest that criminality comes human biology. Genetics, XXY chromosome abnormalities, and twin studies have been used as biological explanations to criminality. 
Adoption studies - Mednick (1984) 
Adoption studies provide strong evidence for the biological basis of criminality. They look at relatives, siblings and twins that are adopted at a young age. This means that they share genes but not the same environment as their parents and grandparents. If an adopted child and their biological father/mother are both criminals, there is strong evidence that genes for criminality have been passed down to them by their criminal parent. 
Aim: To see if there is a genetic link to criminality
Procedure: He studied many adopted children, looked into their arrest records compared to their biological fathers 
Results: He found that even though their biological fathers did not raise them, many were also criminals, same as their fathers 
Conclusion: Genes may cause criminality, these fathers may have passed on their criminals genes causing their children to be criminal 
Twin studies - Christiansen (1977) 
Identical twins share all of their genes in common and non-identical twins have half of their genes in common. If both identical twins are criminal this suggests a very strong link to criminality. 
Aim: To see identical twins would both become criminal 
Procedure: He looked into the arrest records of many pairs of twins in Denmark. He studied identical and non-identical twins 
Results: He found that if one identical twin was criminal, the other had a 52% chance of being criminal as well, whereas for non-identical twins it was 22% 
Conclusion: This could be evidence that there is a genetic basis to crime, shows a shared criminal tendency. The decrease to 22% can be explained by that the twins don’t share all of their genes. 
Chromosome abnormalities - Theilgaard (1984) 
A key study in this syllabus, you need to know it's aim, procedure, results/findings, conclusion and evaluation extremely well. Click here to see Theilgaard's study in depth.