Mothers were asked whether each twin had ever deliberately harmed him/herself or attempted suicide in the previous six months, as part of a face to face interview when the children were aged 12. Mothers who responded positively to this question were asked to provide a description of what took place. An independent rater blind to other data later checked the notes taken during the interview to verify that the description provided was clearly an act of self harm. We asked only mothers, and not children, to report the child’s self harm because of ethical considerations. Examples of self harming behaviours included cutting and biting arms, pulling out clumps of hair, banging head against walls, and attempted suicides by strangulation. Of the cohort, 2.9% (62/2141) had self harmed; of these children, 52% (n=32) were girls.
