Table 1.
Demographic characteristics of the sample
| MZ twins | DZ twins | Sibs of twins | Singletons | Total sample | |
| N (% of total) | 214 (35.7) | 94 (15.7) | 64 (10.7) | 228 (38.0) | 600 |
| Age | |||||
| Mean (SD): | 11.03 (3.16) | 11.20 (3.80) | 11.62 (3.53) | 10.92 (3.48) | 11.08 (3.43) |
| Range: | 5.37–18.72 | 5.55–19.34 | 4.99–19.11 | 5.16–18.88 | 4.99–19.34 |
| Sex (% of subsample) | |||||
| Male | 117 (55) | 53 (56) | 31 (48) | 132 (58) | 332 (55) |
| Female | 97 (45) | 41 (44) | 33 (52) | 96 (42) | 268 (45) |
| Race (% of subsample) | |||||
| Caucasian | 202 (94) | 92 (98) | 63 (98) | 172 (75) | 529 (88) |
| African-American | 6 (3) | 0 (0) | 0 (0) | 36 (16) | 42 (7) |
| Asian-American | 2 (<1) | 0 (0) | 0 (0) | 8 (4) | 10 (2) |
| Hispanic | 4 (2) | 2 (2) | 1 (2) | 11 (5) | 18 (3) |
| Unspecified | 0 (0) | 0 (0) | 0 (0) | 1 (<1) | 1 (<1) |
| Handedness (% of subsample) | |||||
| Right | 189 (88) | 78 (83) | 56 (88) | 199 (87) | 513 (86) |
| Mixed | 19 (9) | 11 (12) | 7 (11) | 21 (9) | 36 (6) |
| Left | 18 (8) | 9 (10) | 10 (16) | 32 (14) | 37 (6) |
| Undetermined | 6 (3) | 2 (2) | 4 (6) | 2 (<1) | 14 (2) |
| SES | |||||
| Mean (SD) | 43.49 (18.40) | 42.92 (13.87) | 40.11 (16.89) | 40.70 (20.57) | 41.99 (18.49) |
| Range | 20–89 | 20–70 | 20–77 | 20–95 | 20–95 |