Table 5. Intra-class correlations (coefficients of twin similarity) for IQ by zygosity for twins with SES.

Age	ICC (95% CI)		N
	MZ	DZ	MZ	DZ
SES index 1
2	0.91 (0.90–0.92)	0.76 (0.75–0.77)	1677	3315
3	0.95 (0.95–0.96)	0.84 (0.82–0.85)	1200	2374
4	0.89 (0.88–0.90)	0.71 (0.69–0.73)	1238	2460
7	0.68 (0.65–0.71)	0.49 (0.46–0.52)	1264	2284
9	0.75 (0.72–0.78)	0.58 (0.54–0.61)	863	1495
10	0.73 (0.69–0.76)	0.50 (0.45–0.54)	685	1197
12	0.66 (0.62–0.70)	0.42 (0.37–0.47)	777	1242
14	0.60 (0.54–0.65)	0.37 (0.31–0.43)	563	894
SES index 2
7	0.66 (0.64–0.69)	0.49 (0.46–0.52)	1614	2851
9	0.75 (0.72–0.77)	0.57 (0.53–0.60)	964	1595
10	0.74 (0.70–0.77)	0.50 (0.45–0.54)	744	1281
12	0.66 (0.62–0.69)	0.43 (0.39–0.46)	1310	2133
14	0.60 (0.55–0.64)	0.35 (0.30–0.40)	812	1201
SES index 3
9	0.76 (0.73–0.78)	0.58 (0.55–0.61)	1084	1816
10	0.73 (0.69–0.76)	0.49 (0.44–0.53)	773	1285
12	0.65 (0.60–0.69)	0.42 (0.37–0.46)	685	1072
14	0.61 (0.55–0.66)	0.33 (0.27–0.40)	510	712

ICC (95% CI) = intra-class correlation coefficient (95% confidence interval); MZ = monozygotic; DZ = dizygotic; N = number of complete cases, i.e. number of pairs in which both twins have IQ data. NB. The formal estimation of variance components, using full information maximum likelihood structural equation modelling, included data from incomplete cases.