Table 4.
Results of random-intercept linear mixed models predicting IQ in adopted and reared full- and half-siblings
| Predictor variables | Model 1 | Model 2, mean (SE) | Model 3, mean (SE) |
| Full-siblings | |||
| Adopted-away vs. home-reared | 4.41 (0.75) | 3.38 (0.79) | |
| Biological parent education | 2.73 (0.56) | ||
| Rearing parent education | 1.71 (0.44) | ||
| Share of adopted | |||
| Variance bio parents | 59.89 | 60.81 | 43.51 |
| Variance individuals | 148.61 | 142.32 | 141.44 |
| ICC,* % | 28.7 | 29.9 | 23.5 |
| Half-siblings | |||
| Adopt-away vs. home-reared | 3.18 (0.34) | 2.70 (0.34) | |
| Biological parent education | 1.56 (0.19) | ||
| Rearing parent education | 1.94 (0.18) | ||
| Share of adopted | |||
| Variance bio parents | 56.15 | 57.61 | 44.15 |
| Variance individuals | 160.32 | 156.38 | 154.03 |
| ICC,* % | 25.9 | 26.9 | 22.3 |
*
The intraclass correlation (ICC) reflects how much of the total variation in IQ can be attributed to differences between sibships.