The intra-pair correlations for pack-years for MZ and DZ twins were 0.60 and 0.45 respectively. The amount of the total variance due to additive genetic effects was 0.29 (95% CI = 0; 0.67). Shared environmental effects accounted for 0.31 (95% CI = 0; 0.59) and unique environmental effects accounted for 0.40 (95% CI = 0.31; 0.53) of the total variance. There were no significant differences in model fit between the full ACE model and models where either shared environmental effects (p = 0.46) or additive genetic effects (p = 0.55) were removed. However, there was a significant difference in model fit when both A and C were removed compared to the full ACE model, suggesting significant effects of both genetic and shared environmental sources of variance. The heritability of subcortical volumes ranged from 0.48-0.85 and those of cortical volumes ranged from 0.03-0.79. The cortical surface area heritability estimates ranged from 0.20-0.60 and those of cortical thickness were from 0.05-0.57 (Kremen et al. 2010; Eyler et al. 2011).
