Table 3. Intraclass correlations (s.e.) for current smoking in twins and for smoking among friends, by zygosity and wave (base: twin pairs).
| Wave 1 | Wave 2 | Wave 3 | Wave 4 | |
|---|---|---|---|---|
| Current smoking in twins | ||||
| Pearson correlation | ||||
| MZ | 0.65 (0.05) | 0.58 (0.04) | 0.52 (0.05) | 0.44 (0.06) |
| DZS | 0.35 (0.05) | 0.29 (0.04) | 0.25 (0.05) | 0.24 (0.06) |
| 2(rMZ−rDZ)a | 0.61 (0.13) | 0.59 (0.12) | 0.53 (0.14) | 0.39 (0.17) |
| Tetrachoric correlation | ||||
| MZ | 0.89 (0.03) | 0.82 (0.04) | 0.75 (0.05) | 0.69 (0.06) |
| DZS | 0.59 (0.06) | 0.47 (0.06) | 0.41 (0.08) | 0.42 (0.08) |
| 2(rMZ−rDZ)a | 0.61 (0.06) | 0.71 (0.07) | 0.68 (0.09) | 0.55 (0.10) |
| Smoking among friends | ||||
| Pearson correlation | ||||
| MZ | 0.58 (0.04) | 0.45 (0.04) | 0.38 (0.04) | 0.39 (0.06) |
| DZS | 0.42 (0.03) | 0.33 (0.04) | 0.19 (0.06) | 0.11 (0.05) |
| 2(rMZ−rDZ)a | 0.33 (0.10) | 0.23 (0.11) | 0.38 (0.14) | 0.57 (0.16) |
| Tetrachoric correlation | ||||
| MZ | 0.80 (0.03) | 0.65 (0.05) | 0.57 (0.07) | 0.58 (0.06) |
| DZS | 0.62 (0.04) | 0.50 (0.06) | 0.30 (0.08) | 0.42 (0.08) |
| 2(rMZ−rDZ)a | 0.37 (0.05) | 0.30 (0.07) | 0.54 (0.11) | 0.32 (0.10) |
DZS=same-sex dizygotic or fraternal twins; MZ=monozygotic or identical twins.
a
2(rMZ−rDZ)=provides naive heritability estimates.