| N (%) | IQ score (mean (SD)) | |
| Sex | ||
| Male | 3948 (48.3) | 100.8 (15.2) |
| Female | 4223 (51.7) | 99.3 (14.3)† |
| Parental social class‡ | ||
| Professional/managerial | 2245 (27.5) | 106.6 (13.9 |
| Skilled non-manual | 752 (9.2) | 102.9 (14.1) |
| Skilled manual | 3083 (37.7) | 97.6 (14.5) |
| Semi/unskilled | 1204 (14.7) | 94.6 (14.4) |
| Unknown | 887 (10.9) | 97.8 (15.7)‡ |
| Current social class | ||
| Professional/managerial | 2969 (36.3) | 106.7 (13.9) |
| Skilled non-manual | 2103 (25.7) | 99.1 (13.4) |
| Skilled manual | 1649 (20.2) | 95.8 (13.6) |
| Semi/unskilled | 1274 (15.6) | 92.5 (14.4) |
| Unknown | 176 (2.2) | 91.2 (18.7)‡ |
| Highest academic/vocational qualification | ||
| Higher degree/NVQ 5 | 306 (3.7) | 114.6 (12.3) |
| Degree/diploma/equivalent NVQ 4 | 2495 (30.5) | 106.6 (13.8) |
| A level/equivalent NVQ 3 | 1707 (20.9) | 100.2 (13.5) |
| O level/equivalent NVQ 2 | 2321 (28.4) | 96.9 (13.1) |
| CSE/equivalent NVQ 1 | 647 (7.9) | 91.4 (13.4) |
| No qualifications | 695 (8.5) | 87.7 (14.0)‡ |
| Annual gross earnings (£ pa)‡ | ||
| >23 000 | 1486 (24.8) | 107.5 (13.8) |
| −23 000 | 1518 (25.3) | 102.9 (14.3) |
| −16 600 | 1497 (25.0) | 98.8 (13.9) |
| ⩾11 440 | 1495 (24.9) | 95.4 (14.3)‡ |
CSE, certificate of secondary education; NVQ, national vocational qualification.
*Characteristics according to earnings are based on a subset of 5996 participants with these data.
†p Value for difference <0.001.
‡p Value for linear trend <0.001.