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.

2: double the difference between MZ and DZSS), shared environment (c2: the MZ correlation minus h2), and unique environment including error (e2: 1 - h2 - c2). All variables except for mathematics are composites. Variance component estimates are heritability (h