The twins study looked into the ‘criminal gene’ and the fact that if one twin possessed this gene then the other one should as well.
Early studies show a higher degree of correlation between monozygotic twins (identical twins) rather than dizygotic twins (fraternal twins).
Monozygotic twins (MZ) - Identical Twins
Dizygotic twins (DZ) - fraternal twins
Christiansens study (1977)
3586 twin pairs were studied in denmark
52% concordance rate with the identical (MZ) twins
22% concordance rate with the fraternal (DZ) twins.
The twin studies were in fact a flawed study due to the small sample size and the poor methods which included no proper DNA testing on the twins meaning there was no way of being 100% certain weather or not twins were identical or not.