Figure 1.
Path diagram representing the basic twin model.
A = additive genetic influence; C = shared environmental influence; E = non-shared (unique) environmental influence. Paths a, c and e = effects of A, C and E on the trait. rMZ and rDZ = genetic or shared environmental correlations for monozygotic and dizygotic twins, respectively.
Table 1.
GCSE grade means (standard deviations).
Table 2.
Intraclass twin correlations (with 95% confidence intervals) and approximate variance component estimates for monozygotic (MZ) and same-sex dizygotic (DZss) twins, and separately for MZ and DZ males (m) and females (f), and opposite-sex (os) DZs.
Table 3.
Model fitting results for additive genetic (A), shared environmental (C) and residual (E; i.e., non-shared environment and error) components of variance, with 95% confidence intervals.