Table 1.
Demographic characteristics of the samples
| Study | N | % Female | # Of complete twin pairs | Mean age (SD) | Age range | Cognitive test | |
|---|---|---|---|---|---|---|---|
| MZ | DZ | ||||||
| IGEMS | 7052 | 43.7% | 1525 | 2001 | 57.6 (9.7) | 27.0–91.2 | AN, BD, SYD, WL |
| Swedish studies | |||||||
| SATSA | 138 | 55.1% | 25 | 44 | 74.9 (5.9) | 64.3–91.2 | BD, SYD, WL |
| OCTO-Twin | 338 | 63.9% | 82 | 87 | 82.8 (2.5) | 79.4–90.9 | BD, SYD |
| Danish study | |||||||
| MADT | 3760 | 49.2% | 669 | 1211 | 56.4 (6.3) | 45.0–68.0 | AN, SYD, WL |
| US studies | |||||||
| VETSA | 1282 | 0% | 367 | 274 | 55.9 (2.4) | 51.1–60.7 | AN, WL |
| MTSADA | 738 | 63.3% | 215 | 154 | 55.8 (12.6) | 27.0–86.7 | BD, SYD |
| MIDUS | 796 | 60.1% | 167 | 231 | 54.6 (11.4) | 34.0–81.5 | AN, WL |
AN = Animal Naming, BD = Block Design, SYD = Symbol Digit, WL = Wordlist, MZ = Monozygotic, DZ = Dizygotic. Sample size reflects complete pairs with respect to sleep duration.