Lange: studied 30 sets of twins, 10/13 MZ twins had both been to prison, 2/17 DZ twins had both been in prison
Christiansen: studied 3586 sets of twins, concordance rates of 35% in male MZ twins and 13% in male DZ twins, concordance rates of 21% in female MZ twins and 8% in female DZ twins
has to be genetic as concordance rates for MZ twins were higher