Table 1.
Mean Age (Standard Deviation) and Sample Size by Country, Zygosity and sex at the End of Kindergarten, and Grade 1
| Characteristic | Australia | United States | Scandinavia |
|---|---|---|---|
| End of kindergarten | |||
| Age (in months) | 72.4 (4.2) | 75.1 (3.7) | 80.8 (3.6) |
| Total sample (pairs) | 191 | 483 | 138 |
| MZ pairs | 113 | 224 | 65 |
| DZ pairs | 78 | 259 | 73 |
| MZ girls | 55 | 127 | 33 |
| MZ boys | 58 | 97 | 32 |
| DZ girls | 37 | 115 | 36 |
| DZ boys | 41 | 144 | 37 |
| End of Grade 1 | |||
| Age (in months) | 83.5 (4.3) | 88.8 (3.9) | 92.3 (3.5) |
| Total sample (pairs) | 160 | 405 | 125 |
| MZ pairs | 98 | 185 | 59 |
| DZ pairs | 62 | 220 | 66 |
| MZ girls | 48 | 107 | 29 |
| MZ boys | 50 | 78 | 30 |
| DZ girls | 29 | 100 | 32 |
| DZ boys | 33 | 120 | 34 |
Note. Scandinavia = Norway and Sweden; MZ = monozygotic; DZ = Dizygotic