Table 3.

Self-perceived ability (SPA): Twin correlations and ACE parameters.

SPA	Countries	School subject	MZ	DZss	A/D	C	E
9 years	UK	Math	.40 (N = 1204)	.11 (N = 1062)	.40	.00	.60
		English	.41 (N = 1202)	.16 (N = 1023)	.41	.00	.59
		Science	.36 (N = 1197)	.23 (N = 1053)	.26	.10	.64
	Germany	Math	.39 (N = 133)	.24 (N = 121)	.30	.09	.61
		German	.55 (N = 133)	.35 (N = 121)	.40	.15	.45
10 years	Canada (Québec)	Math	.40 (N = 153)	−.01a (N = 108)	.40	.00	.60
		Reading	.41 (N = 153)	.03a (N = 108)	.41	.00	.59
	Japan	Math	.25 (N = 109)	.16a (N = 77)	.18	.07	.75
11 years	Germany	Math	.42 (N = 133)	.23 (N = 121)	.38	.04	.58
		German	.41 (N = 133)	.23 (N = 121)	.36	.05	.59
	Japan	Math	.66 (N = 107)	.58 (N = 68)	.16	.50	.34
12 years	UK	Math	.49 (N = 2011)	.15 (N = 1813)	.49	.00	.51
		English	.56 (N = 2013)	.21 (N = 1814)	.56	.00	.44
		Science	.45 (N = 2004)	.26 (N = 1814)	.38	.07	.55
	Canada (Québec)	Math	.42 (N = 147)	.00a (N = 111)	.42	.00	.58
		Reading	.48 (N = 147)	.01a (N = 111)	.48	.00	.52
	Japan	Math	.69 (N = 125)	.14a (N = 62)	.69	.00	.31
	USA	Reading	.63 (N = 144)	.09a (N = 207)	.63	.00	.37
		General school	.43 (N = 148)	.17 (N = 204)	.43	.00	.57
13 years	Germany	Math	.34 (N = 133)	.03a (N = 121)	.34	.00	.66
		German	.37 (N = 133)	.26 (N = 121)	.22	.15	.63
	Japan	Math	.49 (N = 91)	.35a (N = 31)	.49	.00	.51
16 years	UK	Math	.57 (N = 811)	.28 (N = 803)	.57	.00	.43
	Japan	Math	.47 (N = 68)	.24a (N = 32)	.47	.00	.53
	Russia	Math	.46 (N = 34)	.31a (N = 34)	.46	.00	.54

Note. MZ intra-class correlations (ICCs), same-sex DZ (DZss) ICCs, and genetic (A), shared environmental (C) and non-shared environmental (E) estimates. The results were controlled for age and sex. ACE estimates were derived from ICCs using Falconer’s formula. A/D = where DZ ICC is less than half of that of MZ ICC, non-additive genetic (D) effects are implied; in these cases A and D effects cannot be disentangled.

^aDZ ICC did not reach significance.