The 1970 British cohort study (BCS70) is a cohort study that enrolled 16 567 babies born in England, Scotland, and Wales on 5-11 April 1970. Participants were followed up at 5 (n = 13 135), 10 (n = 14 875), 16 (n = 11 622), and 29-30 (n = 11 261) years of age. As the survey at 16 years coincided with a national teachers' strike and national school leaving examinations, sample sizes for some questions were considerably smaller. Between 10 and 30 years, loss to follow up was highest in those from lower socioeconomic groups and those with a disability (loss of about 5% in both groups).14
CFS/ME was identified by self report questionnaire completed at 30 years. Participants were asked whether they had ever had CFS/ME, whether they currently had CFS/ME, and at which age their condition developed. Measures of socioeconomic status (father's social class and maternal educational achievement) in childhood were obtained from interviews with parents at 10 years. Socioeconomic status at 30 years was derived from participants reported current occupation. Data on ethnicity were recorded at 30 years.
At 5, 10, and 16 years, mothers were asked to complete 19 items from the Rutter parental A scale of behaviour disorder. At 10 years, teachers completed a 53 item social development scale, with items drawn from the Conner teacher rating scale and the Rutter teaching scale. A 14 item subscale for problems/impulsivity/hyperactivity in conduct and an eight item subscale for attention deficit has been previously identified by factor analysis.15
As thresholds indicative of likely disorder were not available for these modified instruments, we defined high scorers for all child behaviour scales as scores greater than 1 SD from the sample mean in the direction of greater problems. At 16 years, teenagers completed the 12 item self report general health questionnaire (GHQ 12) to screen for recent psychological distress. Scores of ≥ 4 indicated psychological disturbance.16
Self esteem at 10 and 16 years was measured in school with the Lawrence self esteem questionnaire (Lawseq) for children, validated for these age groups.17
Scores > 1 SD below the population mean defined low self esteem.
At 5, 10, and 16 years mothers completed the Rutter malaise inventory, a 24 item self completion scale designed to assess psychiatric morbidity. In the standard scoring system used at 5 years, positive or negative responses are obtained for each item and a total score between 0 and 24 is then obtained by summation of all positive responses.18
Alternative scoring systems were used in the 10 and 16 year surveys. At 10 years, mothers were asked to rate themselves on each item on a Likert scale between 0 (“seldom or never”) to 100 (“most of the time”). We derived a total malaise score by summation of all responses divided by 100, providing a score from 0 to 24 somewhat comparable with that produced by the standard scoring system. At 16 years, mothers were asked to rate their response to each question as “most of the time,” “some of the time,” and “rarely or never.” We added the positive (“most of the time”) answers to produce a total score between 0-24. Participants also completed the malaise inventory at the age of 30 years using the standard binary scoring system. A score ≥ 7 on the binary scoring system has been suggested to indicate a high risk of psychiatric disorder, and we used this to define high scorers for maternal malaise scores at 5 and 16 years and for the participant's own score at 30 years.18
As this threshold is not appropriate for the Likert scoring system, we defined a high score for maternal malaise at 10 years as a score > 1 SD above the mean.
At 10 years participants completed two verbal and two non-verbal subscales of the British ability scales for the assessment of cognitive attainment. Age appropriate T scores were calculated with reference norms appropriate for 1980.19
High intellectual ability was defined as a mean T score in the highest 10%, which approximates to a mean T score ≥ 60 (equivalent to IQ ≥ 120).
Also at 10 years parents were interviewed to obtain a history of severe or prolonged illness in either parent in the previous 5 years. Mothers provided information on the child's history of atopy (ever had asthma, eczema, or allergic rhinitis), how often the child played sport in his or her spare time, and the number of days off school that term because of health or emotional problems. Teachers' reports provided data on the usual number of hours of sport played by the child at school each week. The presence of a longstanding condition in the child was recorded from both medical examination and maternal report at 10 years, with maternal report of whether the condition considerably limited the child's daily life. School doctors measured the child's height and weight at 10 years. Obesity was defined as body mass index ≥ 95th centile.20
We explored differences between those with any CFS/ME and the remainder of the cohort with t tests for continuous variables and χ2 test for categorical variables. We then used logistic regression to assess factors associated with the risk of ever having CFS/ME. Factors were initially assessed at univariate level and then examined after adjustment for sex, father's social class in childhood, and mother's educational status. We used a final multivariable model to determine which factors were independently associated with CFS/ME, entering all variables shown to be significantly (P < 0.05) associated simultaneously, including sex, father's social class, and mother's education. Adult psychological morbidity (high scorer on malaise inventory at 30 years) was also included a priori in the multivariable model to control for potential confounding of self report CFS/ME status. We used Stata 8 (StataCorp, College Station, TX) for analyses.