Questions about self-harm and suicidal thoughts were included in a self-completion postal questionnaire, sent to study participants when they were aged 16 years (see Additional file 2: Appendix B for the full list of self-harm questions asked). Participants were asked “have you ever hurt yourself on purpose in any way (e.g. by taking an overdose of pills or by cutting yourself)?”, wording which was used in the Childhood Interview for DSM-IV Borderline Personality Disorder (CI-BDP) [17], asked during clinic interviews with the ALSPAC sample aged 11. Those who answered yes were asked further closed response questions regarding frequency (once, 2–5 times. 6–10 times, >10 times), what they did the last time they hurt themselves on purpose (4 response categories) and why they did it that time (6 response categories) - response options were a modified version of those in the CASE questionnaire [18]. Participants were also asked whether they sought medical help and how they felt following the most recent time, and whether they had ever seriously wanted to kill themselves when self-harming. The whole sample were then asked whether
