- John Lange found a much higher concordance rate for criminal tendancies for MZ twins compred to DZ twins.
- Christiansen studied 3586 twin pairs from Danish islands and found a 35% concordance rate for male MZ twins and 13% for male DZ twins, and 21% for female MZ twins and 8% for female DZ twins
- dalgaard and Kringlen found in a norwegian study found 26% concordance for MZ and 15% for DZ
- all concordance rates are low, suggesting environment does have some factor.
- MZ twins may share a more similar environemnt that DZ twins as they look similar