Table 1.
Characteristics and summary statistics for the academic performance scores of twins and singletons born in Denmark during 1986–1990. Values are numbers (percentages) unless otherwise stated.
| Characteristics | Opposite-sex females | Same-sex females | Female twins | Singletona females | Opposite-sex males | Same-sex males | Male twins | Singletona males | Totals |
|---|---|---|---|---|---|---|---|---|---|
| Number of live births in Denmark | 968 (9.5) | 2198 (21.6) | 3166 (31.1) | 7010 (68.9) | 966 (9.2) | 2194 (20.8) | 3160 (30.0) | 7391 (70.1) | 20,727 (100.0) |
| Deaths before age 17b | 23 (2.4) | 76 (3.5) | 99 (3.1) | 58 (0.8) | 19 (2.0) | 78 (3.6) | 97 (3.1) | 92 (1.2) | 346 (1.7) |
| 1–28 days after birth | 17 (1.8) | 58 (2.7) | 75 (2.4) | 17 (0.2) | 14 (1.5) | 54 (2.5) | 68 (2.2) | 36 (0.5) | 196 (1.0) |
| 29–365 days after birth | 3 (0.3) | 12 (0.6) | 15 (0.5) | 18 (0.3) | 3 (0.3) | 18 (0.8) | 21 (0.7) | 28 (0.4) | 82 (0.4) |
| Death of co-twin before age 17 | 8 (0.9) | 38 (1.8) | 46 (1.5) | – | 12 (1.3) | 48 (2.3) | 60 (2.0) | – | 106 (1.7) |
| Emigratedb | 30 (3.2) | 50 (2.4) | 80 (2.7) | 181 (2.6) | 30 (3.2) | 48 (2.3) | 77 (2.6) | 170 (2.3) | 508 (2.5) |
| Study base (% of live born)c | 907 (93.7) | 2034 (92.5) | 2941 (92.9) | 6771 (96.6) | 905 (93.7) | 2020 (92.1) | 2926 (92.6) | 7129 (96.5) | 19,766 (95.4) |
| Mean (SD) birth weight | 2562 (521) n = 905 |
2491 (495) n = 2029 |
2513 (504) n = 2934 |
3389 (517) n = 6754 |
2648 (542) n = 904 |
2586 (557) n = 2017 |
2605 (553) n = 2921 |
3514 (558) n = 7104 |
3188 (676) n = 19,713 |
| Mean (SD) maternal age | 29.8 (4.5); n = 908 | 28.9 (4.6); n = 2035 | 29.2 (4.6); n = 2943 | 28.0 (4.8); n = 6769 | 29.8 (4.5); n = 907 | 28.9 (4.5); n = 2023 | 29.2 (4.5); n = 2930 | 28.0 (4.8); n = 7122 | 28.38 (4.8) n = 19,757 |
| Mean (SD) paternal age | 32.7 (5.9); n = 906 | 31.9 (5.7); n = 2025 | 32.1 (5.8); n = 2931 | 31.0 (5.9); n = 6712 | 32.7 (5.8); n = 905 | 31.4 (5.7); n = 2015 | 31.8 (5.8); n = 2920 | 30.9 (5.7); n = 7072 | 31.3 (5.8) n = 19,628 |
| Maternal education | |||||||||
| Basic school | 225 (25.8) | 574 (28.9) | 799 (28.0) | 1823 (27.6) | 224 (25.8) | 558 (28.0) | 782 (27.3) | 1900 (27.3) | 5304 (27.5) |
| Vocational school | 260 (29.9) | 644 (32.4) | 904 (31.6) | 2192 (33.1) | 259 (29.8) | 670 (33.6) | 929 (32.5) | 2394 (34.4) | 6419 (33.3) |
| Secondary education | 69 (7.9) | 152 (7.7) | 221 (7.7) | 603 (9.1) | 69 (7.9) | 182 (9.1) | 251 (8.8) | 606 (8.7) | 1681 (8.7) |
| Short higher education | 37 (4.3) | 100 (5.0) | 137 (4.8) | 269 (4.1) | 37 (4.3) | 80 (4.0) | 117 (4.1) | 304 (4.4) | 827 (4.3) |
| Medium higher education and Bachelor’s degree | 223 (25.6) | 430 (21.6) | 653 (22.8) | 1442 (21.8) | 223 (25.7) | 422 (21.2) | 645 (22.5) | 1466 (21.1) | 4206 (21.8) |
| Higher academic or professional degree | 57 (6.5) | 88 (4.4) | 145 (5.1) | 286 (4.3) | 57 (6.6) | 80 (4.0) | 137 (4.8) | 292 (4.2) | 860 (4.5) |
| Paternal education | |||||||||
| Basic school | 219 (25.3) | 490 (25.4) | 709 (25.4) | 1659 (26.1) | 219 (25.4) | 488 (25.8) | 707 (25.6) | 1818 (27.9) | 4893 (26.2) |
| Vocational school | 341 (39.4) | 818 (42.4) | 1159 (41.5) | 2772 (43.6) | 339 (39.2) | 839 (44.3) | 1178 (42.7) | 2824 (41.9) | 7933 (43.0) |
| Secondary education | 64 (7.4) | 146 (7.6) | 210 (7.5) | 394 (6.2) | 64 (7.4) | 102 (5.4) | 166 (6.0) | 441 (6.6) | 1211 (6.5) |
| Short higher education | 44 (5.1) | 84 (4.4) | 128 (4.6) | 307 (4.8) | 44 (5.1) | 83 (4.4) | 127 (4.6) | 325 (4.8) | 887 (4.8) |
| Medium higher education and Bachelor’s degree | 108 (12.5) | 228 (11.8) | 336 (12.0) | 730 (11.5) | 108 (12.5) | 214 (11.3) | 322 (11.7) | 767 (11.4) | 2155 (11.6) |
| Higher academic or professional degree | 90 (10.4) | 164 (8.5) | 254 (9.1) | 501 (7.9) | 90 (10.4) | 168 (8.9) | 258 (9.4) | 558 (8.3) | 1571 (8.4) |
| Mathematics — oral and written averaged,e | |||||||||
| Test score available | 787 (86.8) | 1749 (86.0) | 2536 (86.2) | 5907 (87.2) | 744 (82.2) | 1655 (81.9) | 2399 (82.0) | 5819 (81.6) | 16,661 (84.3) |
| Mean (SD) test score | 7.84 (1.4) | 7.98 (1.4) | 7.94 (1.4) | 7.96 (1.4) | 8.14 (1.4) | 8.13 (1.4) | 8.13 (1.4) | 8.15 (1.5) | 8.05 (1.4) |
| Teacher rating available | 794 (87.5) | 1771 (87.1) | 2565 (87.2) | 6046 (89.3) | 761 (84.1) | 1688 (83.5) | 2449 (83.7) | 5953 (83.5) | 17,013 (86.1) |
| Mean (SD) teacher rating | 7.97 (1.4) | 8.08 (1.4) | 8.05 (1.4) | 8.07 (1.4) | 8.08 (1.4) | 8.06 (1.4) | 8.07 (1.4) | 8.03 (1.4) | 8.05 (1.4) |
| Physics/chemistry — oral | |||||||||
| Test score available | 613 (67.6) | 1424 (70.0) | 2037 (69.3) | 4701 (69.4) | 582 (64.3) | 1360 (67.3) | 1942 (66.4) | 4700 (65.9) | 13,380 (67.7) |
| Mean (SD) test score | 7.90 (1.6) | 8.01 (1.6) | 7.98 (1.6) | 7.98 (1.6) | 8.09 (1.6) | 8.05 (1.7) | 8.06 (1.7) | 7.94 (1.7) | 7.98 (1.7) |
| Teacher rating available | 771 (85.0) | 1715 (84.3) | 2486 (84.5) | 5786 (85.5) | 715 (79.0) | 1632 (80.8) | 2347 (80.2) | 5648 (79.2) | 16,267 (82.3) |
| Mean (SD) teacher rating | 7.94 (1.4) | 8.03 (1.3) | 8.00 (1.4) | 7.94 (1.3) | 8.05 (1.4) | 7.99 (1.4) | 8.01 (1.4) | 7.91 (1.4) | 7.95 (1.4) |
| Danish — oral, written and spelling averagef | |||||||||
| Test score available | 797 (87.9) | 1774 (87.2) | 2571 (87.4) | 5981 (88.3) | 747 (82.5) | 1655 (81.9) | 2402 (82.1) | 5819 (81.6) | 16,773 (84.9) |
| Mean (SD) test score | 8.38 (1.2) | 8.36 (1.2) | 8.37 (1.2) | 8.41 (1.2) | 7.88 (1.3) | 7.83 (1.3) | 7.85 (1.3) | 7.84 (1.3) | 8.13 (1.3) |
| Teacher rating available | 798 (88.0) | 1779 (87.5) | 2577 (87.6) | 6055 (89.4) | 754 (83.3) | 1677 (83.0) | 2431 (83.1) | 5936 (83.3) | 16,999 (86.0) |
| Mean (SD) teacher rating | 8.47 (1.2) | 8.47 (1.2) | 8.47 (1.2) | 8.49 (1.1) | 7.90 (1.3) | 7.84 (1.2) | 7.86 (1.25) | 7.81 (1.2) | 8.16 (1.2) |
| English — oral | |||||||||
| Test score available | 765 (84.3) | 1720 (84.6) | 2485 (84.5) | 5809 (85.8) | 721 (79.7) | 1607 (79.5) | 2328 79.6 | 5664 (79.5) | 16,286 (82.4) |
| Mean (SD) test score | 8.54 (1.8) | 8.48 (1.8) | 8.50 (1.8) | 8.52 (1.8) | 8.18 (1.7) | 8.10 (1.8) | 8.13 (1.8) | 8.22 (1.8) | 8.36 (1.8) |
| Teacher rating available | 792 (87.3) | 1758 (86.4) | 2550 (86.7) | 6007 (88.7) | 744 (82.2) | 1665 (82.4) | 2409 (82.3) | 5885 (82.6) | 16,851 (85.3) |
| Mean (SD) teacher rating | 8.27 (1.4) | 8.23 (1.4) | 8.24 (1.4) | 8.21 (1.4) | 7.96 (1.4) | 7.87 (1.4) | 7.90 (1.4) | 7.85 (1.5) | 8.05 (1.5) |
a
5% random sample of Danish birth cohorts 1986–1990.
b
Before January 1, 2003.
c
Rest of table gives percentage of study base.
d
Average performance was rated as 8.
e
Average scores in oral and written mathematics for individuals with both scores available.
f
Average scores in oral, written and spelling Danish for individuals with all three scores available.