Table 1.

Means, Standard Deviations, Sample Size, and Analysis of Variance (ANOVA) Results by Sex and Zygosity (Zyg) for the Five Aspects of Mathematics

																ANOVA
	All			MZ			DZ			Boys			Girls			Sex		Zyg		Sex × Zyg
Measure	M	SD	n	M	SD	n	M	SD	n	M	SD	n	M	SD	n	p	η2	p	η2	p	η2
Applic	0.70	0.20	1,748	0.70	0.20	643	0.70	0.20	1,105	0.72	0.21	751	0.69	0.19	997	.000	.007	.714	.000	.144	.002
Underst	0.78	0.18	1,748	0.78	0.16	643	0.78	0.18	1,105	0.80	0.18	751	0.77	0.17	997	.004	.005	.917	.000	.163	.002
Comput	0.84	0.18	1,748	0.84	0.18	643	0.84	0.19	1,105	0.84	0.18	751	0.83	0.18	997	.111	.001	.706	.000	.017	.005
Interp	0.84	0.17	1,748	0.84	0.17	643	0.85	0.17	1,105	0.85	0.18	751	0.84	0.17	997	.057	.002	.529	.001	.270	.002
Non-Num	0.72	0.17	1,748	0.71	0.17	643	0.72	0.16	1,105	0.72	0.17	751	0.71	0.16	997	.072	.002	.728	.000	.088	.003

Note. Descriptive statistics are reported on the whole sample after the medical exclusions, N = 2,574. The analysis was first conducted for the same-sex and opposite-sex DZ twins separately, with very similar results for the two groups. Number of individuals without missing data = 1,748. Eta-squared indicates the proportion of the total variance that is attributed to an effect (calculated as the ratio of the effect variance to the total variance). All distributions are unimodal and positively skewed. In the ANOVA analysis, zygosity had three levels: MZ, DZ same-sex, and DZ opposite-sex. No significant effect of zygosity was found. The sex effect was significant for the Mathematical Application and Understanding Number categories, favoring boys. However, sex explained less than 1% of the variance in both cases. MZ = monozygotic twins; DZ = dizygotic twins; Applic = Mathematical Application category; Underst = Understanding Number category; Comput = Computation and Knowledge category; Interp = Mathematical Interpretation category; Non-Num = Non-Numerical Processes category.