Fig. 2.

Overall behavior genetic model for DZ twins. In the six latent variables at the top of the figure, A = additive genetic variance, C = shared environmental variance, and E = nonshared environmental variance or measurement error. In the 18 latent variables at the bottom, the first two letters are either SA (specific additive genetic variance), SC (specific shared environmental variance), or SE (specific nonshared environmental variance or measurement error); the third letter is either C (cognitive ability), E (education), or A (age at first birth).