Table 2. Intraclass correlations by zygosity, genetic model parameter estimates, and nested model statistics of each variable controlling for age.
| Intraclass Correlations | ACE Models | Nested Models | ||||||
| Variable | MZ | DZ | a2 | c2 | e2 | Best model | Δχ2(Δdf = 1) | p |
| Word reading | .90 | .54 | .73(.54,.92) | .18(−.02,.38) | .09(.08,.11) | AE | 1.31 | .25 |
| Receptive vocabulary | .68 | .62 | .11(−.06,.29) | .57(.38,.75) | .32(.28,.36) | CE | 0.87 | .35 |
| Phonological memory | .75 | .51 | .50(.28,.71) | .25(.03,.46) | .26(.22,.29) | ACE | – | – |
| Tone awareness | .55 | .29 | .52(.23,.82) | .02(−.26,.29) | .46(.40,.52) | AE | 0.01 | .93 |
| Syllable and rhyme awareness | .71 | .67 | .08(−.08,.23) | .63(.47,.80) | .29(.25,.33) | CE | 0.48 | .49 |
| Rapid automatized naming | .58 | .37 | .42(.15,.70) | .15(−.11,.41) | .43(.37,.48) | AE | 0.60 | .44 |
| Morphological awareness | .64 | .42 | .44(.19, .69) | .20(−.04,.44) | .36(.31,.41) | AE | 1.20 | .27 |
| Orthographic skills | .55 | .45 | .20(−.05,.45) | .35(.11,.58) | .45(.40,.51) | CE | 1.36 | .24 |
Note. Number of twin pairs ranged from 210 to 228 for MZ and from 80 to 84 for DZ. All ACE models had a satisfactory goodness of fit indicated by a nonsignificant χ2change between the saturated and the ACE models (Δχ2 ranged from 0.73 to 7.25, Δdf = 6, ps>.05). MZ = Monozygotic twins; DZ = Dizygotic twins; a2 = additive genetic variance; c2 = shared environment variance; e2 = nonshared environment variance; Δχ2 and Δdf are the differences between the ACE and the nested models.
(95% confidence intervals in parentheses).