TABLE 1.
Intraclass Correlations for the Measures of IQ, Self-Perceived Abilities, and Achievement
| Cross-sectional analysis | Longitudinal analysis | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Monozygotic twins | Dizygotic twins | Monozygotic twins | Dizygotic twins | ||||||
| Measure | ICC | n | ICC | n | ICC | n | ICC | n | |
| IQ | |||||||||
| Puzzle | .64 (.60–.67) | 1,150 | .49 (.44–.53) | 1,000 | — | — | — | — | |
| Vocabulary (age 9) | .59 (.55–.62) | 1,195 | .47 (.42–.52) | 1,044 | — | — | — | — | |
| Shapes | .61 (.58–.65) | 1,184 | .49 (.44–.54) | 1,033 | — | — | — | — | |
| General Knowledge | .61 (.57–.64) | 1,181 | .53 (.49–.58) | 1,029 | — | — | — | — | |
| Conceptual Grouping | — | — | — | — | 34 (.30–.38) | 1,800 | .28 (.24–.33) | 1,639 | |
| Similarities | — | — | — | — | .52 (.49–.55) | 1,788 | .40 (.36–.44) | 1,627 | |
| Vocabulary (age 7) | — | — | — | — | .62 (.59–.65) | 1,792 | .49 (.46–.53) | 1,631 | |
| Picture Completion | — | — | — | — | .51 (.47–.54) | 1,802 | .44 (.40–.48) | 1,635 | |
| Self-Perceived Ability | |||||||||
| English | .37 (.32–.42) | 1,163 | .16 (.10–.22) | 994 | .38 (.33–.43) | 1,182 | .13 (.07–.19) | 1,021 | |
| Mathematics | .39 (.34–.43) | 1,170 | .10 (.04–.16) | 1,014 | .39 (.34–.44) | 1,181 | .12 (.06–.18) | 1,019 | |
| Science | .34 (.29–.39) | 1,163 | .23 (.17–.29) | 1,008 | .30 (.25–.35) | 1,178 | .21 (.15–.27) | 1,021 | |
| Achievement | |||||||||
| English | .80 (.77–.82) | 781 | .46 (.40–.52) | 709 | .81 (.78–.83) | 936 | .55 (.50–.60) | 822 | |
| Mathematics | .76 (.73–.79) | 779 | .45 (.40–.51) | 707 | .76 (.74–.79) | 931 | .48 (.43–.53) | 815 | |
| Science | .78 (.75–.80) | 771 | .47 (.41–.53) | 693 | .76 (.73–.79) | 923 | .57 (.52–.61) | 801 | |
Note. The cross-sectional analysis included measures of IQ, self-perceived abilities, and achievement obtained at age 9. The longitudinal analysis included IQ at age 7, self-perceived abilities at age 9, and achievement at age 10. All intraclass correlation coefficients (ICCs) were significant at the .001 level. The number of twin pairs (n) is higher for the longitudinal than for the cross-sectional analyses because IQ scores were available for more children at age 7 than at age 9, and because achievement data were available for more children at age 10 than at age 9. All pairs in which at least 1 twin had missing data were excluded from the calculations reported in this table. Overall, the cross-sectional sample included 2,287 pairs of twins, and the longitudinal sample included 3,785 pairs of twins.