Table 4.

Cross-trait twin similarity coefficients (ICC1.1) for reading, mathematics and g

Measures	MZ	DZall	Same individual
Twin 1 reading–twin 2 mathematics	0.46 (0.41–0.51), (n = 875)	0.37 (0.33–0.41), (n = 1548)	0.51 (0.48–0.54), (n = 2457)
Twin 1 reading–twin 2 g	0.52 (0.47–0.56), (n = 840)	0.36 (0.32–0.41), (n = 1462)	0.54 (0.51–0.56), (n = 2333)
Twin 1 mathematics–twin 2 g	0.56 (0.51–0.61), (n = 838)	0.40 (0.36–0.44), (n = 1457)	0.63 (0.60–0.65), (n = 2342)

All similarity coefficients are based on age-, sex- and download-time-corrected scores. Correlations for same individual are based on one random twin from each pair. Reversing the ordering of the pairs (e.g. twin 2 reading–twin 1 mathematics) produces the same results.

95% confidence intervals in parentheses.

Dzall, all DZ pairs; MZ, MZ pairs; n, number of twin pairs.