Explain two pieces of evidence for genetic explanations for criminal activity

One genetic explanation for criminal activity is candidate genes. Tihoen studied 900 offenders and found abnormalities on two genes associated with violent crime (MAOA which controls dopamine and serotonin and CDH13 which is linked with substance abuse and attention deficit disorder). In the sample, individuals with the high-risk combination were 13 times more likely to have a history of violent behaviour. However, this is in its infancy and has not been replicated. This suggests that it can be an explanation for criminal behaviour.
A further piece of evidence used to suggest a link between genetics and crime are twin studies. Lang studied 13 mono-zygotic twins and 17 di-zygotic twins and found 10 MZ twins had both been in prison whereas only 2 DZ twins had both been in prison. Christiansen studied 87 MZ and 147 DZ twins: 33% concordance for MZ and 12% DZ. This study suggests there is a link between genetics and criminal activity as those sharing 100% DNA were more likely to both be in prison, however, it also shows that criminal activity is not entirely down to genes as if it were solely biological there would be an 100% concordence rate between mono-zygotic twins suggesting environmental factors must also influence criminal behaviour.