Table 1.

Means (standard deviations) and ANOVA results.

	Science test	Learning environment	Classroom environment	Peer environment
All	0.01	0.01	0.01	0.00
	(1.00)	(0.99)	(0.99)	(1.00)
	N = 2741	N = 3196	N = 3195	N = 3188
MZ	− 0.03	0.01	0.01	0.02
	(1.00)	(1.02)	(1.00)	(1.03)
	N = 1059	N = 1196	N = 1196	N = 1192
DZ	0.04	0.00	0.02	− 0.01
	(1.00)	(0.97)	(0.99)	(0.98)
	N = 1682	N = 2000	N = 1999	N = 1996
Male	0.06	0.03	− 0.01	0.07
	(1.00)	(1.02)	(1.02)	(1.00)
	N = 1144	N = 1426	N = 1426	N = 1421
Female	− 0.03	− 0.01	0.04	− 0.06
	(1.00)	(0.97)	(0.96)	(0.99)
	N = 1597	N = 1770	N = 1769	N = 1767
Sex p-value	0.016	0.205	0.229	< 0.001
Sex effect size	0.002	0.001	< 0.001	0.004
Zygosity p-value	0.162	0.633	0.818	0.273
Zygosity effect size	0.001	< 0.001	< 0.001	< 0.001

Note. N = one randomly selected member of each pair; all measures transformed; effect size expressed as eta squared. MZ = monozygotic twins; DZ = dizygotic twins. Standardised scores are presented because the measures were rank transformed to adjust for skew prior to the ANOVA.