The material for the study is drawn from a Scottish longitudinal community health and lifestyle survey of young people, administered first in-school via questionnaire (ages 11, 13, and 15), with an additional parental questionnaire at age 11, and then in the post-school period by nurse interview at age 19. The focus here is on mental health outcomes at age 15 (1999) and age 19 (2003) within the framework of the 'West of Scotland 11 to 16 Study/16+' . The study received approval from Glasgow University's Ethics Committee, participating Education Authorities and schools, and informed consent was obtained from the parents of all participants via 'opt-out' consent forms at ages 11, 13 and 15, verbal consent from participants at each wave and written consent at age 19.
Due to the school-based nature of the sample the sampling scheme involved several elements to ensure a representative sample at both the primary and secondary school stages . Briefly, the survey used a reverse sampling procedure which randomly selected 43 secondary schools stratified by religious denomination and deprivation, with a separate stratum for independent vs. local authority run schools. These 43 secondary schools were used to select a random sample of 135 primary schools, comprising 'feeder schools', together with those making a high number of placing requests. From these primary schools, classes were randomly selected with all pupils in the classes eligible to participate. Of the 2793 pupils who attended the 43 targeted secondary schools, 2586 (93%) participated in the baseline (age 11) survey. At age 13, the number of participants reduced to 2371 (85%), and by 15 to 2196 (79%). At age 15, 1,860 (67%) of respondents completed a psychiatric interview , which included questions about suicidal thoughts and attempts. As expected losses in the post-school period substantially reducing the sample size at age 19 to 1256 (45%). Full details of the sampling strategy are available elsewhere .
At age 11 the sample was representative (in terms of sex and social class composition) of 11 year olds in the study area . Differential attrition made later waves less representative, with attrition greater among lower social class groups, school truants, pupils of lower ability and with greater emotional and behavioural problems. To compensate for these biases, a weighting scheme was derived . Use of these weights did not substantively alter any of the results presented here. The data used in this paper refer to 2196 pupils in their final year of compulsory education in 43 mainstream secondary schools in the Glasgow area, 1256 of whom provided information when aged 19. Parents provided information on perinatal circumstances such as maternal age and birth weight and on pupils' religious background and family socioeconomic status via a questionnaire in the first wave (age 11) of the study. The final sample included 16 twin births, but excluded adopted children (n = 39), reducing the sample to 2157.
In 1996 (pupils aged 11) parents were asked questions about the child's birth history and perinatal circumstances. This included questions on maternal parity, categorised as first or later born; family size (including parents), categorised as 2 or more family members; subjective recall of the size of the baby (small, average, or large); birth weight (in grams), categorized <2500, 2500-3249, 3250-3749, or 3750g+; number of birth complications (e.g. breech birth, pre-eclampsia, etc), categorized none, one, two or more; birth spacing (both prior to and post index birth), categorised as under 2 years, 2-5 years, or 6+ years between births (or singleton); maternal age, categorised age 15-19, 20-24, 25-29, 30-34, or 35-46. Finally, parents provided information on breastfeeding practices, categorised as none, breastfeeding for less than six months, or six months or longer. Whenever feasible we derived categories compatible with established cut-points or expert recommendations. For example, in relation to the six month cut-point for breastfeeding, the American Academy of Pediatrics recommendation is 'exclusive breastfeeding is sufficient to support optimal growth and development for approximately the first 6 months of life' .
Several background factors relevant to either socio-demographic circumstances or psychiatric outcomes at age 11 and age 15 were recorded. At age 11 an area deprivation score, range 1 (least) to 7 (most deprived), was derived from pupils' postal codes using the 'Carstairs'  index, a standard measure based upon census data. Social class of the head of household was derived from parental questionnaires completed at wave one (age 11), coded using the standard UK classification system  and categorized as non-manual or manual. Religious affiliation was obtained from parents and categorized, Church of Scotland (Protestant), Catholic, Muslim, other (Jewish, Methodist, Baptist, etc) and 'none, atheist/agnostic'. At age 15, pupil's family structure was coded as 2-parent, 1-parent, reconstituted (one 'birth' parent and new partner) or other (relative, foster parent, or other carer). Principal component (varimax) analysis of the (age 15) 8-item Brief Parental Bonding Instrument , produced two scales representing perceived (low) parental care, e.g. 'My parents help me as much as I need' (reversed) and (high) control, e.g. 'My parents treat me like a baby'.
Sexual orientation is an established psychosocial risk factor for suicide and other psychiatric disorders and early gender nonconformity is a strong predictor of future sexual orientation [30, 31]. Gender diagnosticity (GD) is an established measure of gender nonconformity, previously measured in this cohort at age 15 and linked with poor psychological wellbeing . GD score is the probability of belonging to a particular gender, as predicted by the logistic regression of multiple items of gendered behaviours, interests and hobbies. The result is a score (0-1 scale) of how 'typically' male or female an individual is (within a limited domain) compared to their peers. An arbitrary cut-point of 0.1 (10%) has been used to indicate 'gender conformist' (top 10%), 'Gender typical' (middle 80%) and gender nonconformists (bottom 10%) respectively. At age 19, participants were asked 'Have you ever had any kind of sexual experience or sexual contact with someone of the same gender as yourself' and asked to response 'yes' or 'no' using a 'show card procedure' designed to reduced response bias. While not a comprehensive measure of sexual preference, this item is arguably a good indicator of same-sexual orientation and behaviour.
Several dichotomous mental health outcomes were reported at ages 15 and 19. At age 15 participants were asked if they had ever seriously thought about taking their own life and if they had attempted to do so, categorised as suicidal thoughts and attempted suicide respectively, with a positive response to either question classified as 'suicide risk'. At age 19 participants were asked if they had ever 'tried to hurt or harm yourself deliberately' and to indicate which method(s) the had used, such as cutting, scratch or scoring or burning; a positive response was classified as Nonsuicidal Self-Injury NSSI. To assess previous psychiatric problems, parents were asked about the child's use of psychiatric services until age 11 and at age 19 participants were asked about their use of psychiatric services since age 11. Due to both budget and ethical constrains suicidality was only measured at age 15 and 19', self-harm at age 19 and psychiatric contact only measured at age 11 and 19.
The analysis used logistic regression to determine the association between perinatal and psychosocial circumstances and age 15 and age 19 psychiatric outcomes. Analysis was conducted both unadjusted and mutually adjusted for covariates. We constructed weights to compensate for differential attrition (21), but use of these weights did not alter results. The influence of missing data was further explored by comparing results for models using complete data only and multiple imputation methods. Multiple imputation was implemented using the STATA 'ice' procedure and included all variables from the relevant model, with separate imputation runs for every analysis. Birth size was included as a supplementary variable, because it contained information useful in estimating birth weight. Categorical variables were imputed using logit or multiple logit, continuous variables using regression and deprivation using ordinal logit commands. Ten imputed datasets were used to calculate the final combined estimates. Although the results for each method were not substantively different, we report results based on multiple imputation. Results from all other models are available upon request.