Table 1.

Means, standard deviations and ANOVA results by sex and zygosity.

																							ANOVA
Measures	All		MZ		DZ		Female		Male		MZm		DZm		MZf		DZf		DZo		DZss		Zyg.		Sex		Zyg. ∗ Sex		Tot.
	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	M	SD	p	η²	p	η²	p	η²	R2
Number Sense accuracy	N = 2258		N = 836		N = 1422		N = 1315		N = 943		N = 317		N = 626		N = 519		N = 796		N = 689		N = 733
	.03	.98	.00	.99	.04	.98	.05	.95	.00	1.0	− .07	1.1	.04	1.0	.04	.93	.05	.97	.04	.99	.05	.98	.17	.00	.16	.00	.29	.00	.000
Weber Fraction	N = 2214		N = 817		N = 1397		N = 1298		N = 916		N = 301		N = 615		N = 516		N = 782		N = 677		N = 720
	− .10	.85	− .07	.85	− .11	.84	− .10	.83	− .10	.87	− .08	.86	− .10	.88	− .07	.85	− .12	.82	− .10	.85	− .12	.83	.34	.00	.95	.00	.79	.00	.001

Number Sense accuracy = accuracy scores on Dot Task (squared transformation); Weber Fraction = Weber Fraction score (square root transformed); M = mean; SD = standard deviation; MZ = monozygotic twins; DZ = dizygotic twins; MZm = monozygotic males; MZf = monozygotic females; DZo = dizygotic opposite sex; DZss = dizygotic same sex; p = p-value associated with the effect size of sex, zygosity and the interaction of the two on the means of all groups; η² = magnitude of the effect of sex, zygosity and the interaction of the two on the means of all groups; R² = proportion of variance explained by sex and zygosity; N = number of twins: one randomly selected from each pair. Scores outside +/− 3 standard deviations have been removed. The standardized means and standard deviations show that both variables are not normally distributed. Accuracy scores on the Number Sense Task are less skewed compared to the Weber Fraction scores.