Heritability and unique environmental influences for white matter volume, mean fractional anisotropy in pure white matter, white matter surface area and fractional anisotropy in fiber bundles at ages 9 and 12.

Values in brackets are 95% confidence intervals. Common environmental influences did not play a role in explaining variation in fiber bundles (based on both the Akaike criterion and Chi-square differences): only the results from the model incorporating genetic and unique environmental influences are displayed here. FA = fractional anisotropy, L = left, R = right, ILF = inferior longitudinal fasciculus, IFO = inferior fronto-occipital fasciculus.