Bullying victimisation was assessed during interviews with mothers when the children were aged 7 and 10 years , and separately in private interviews with the children during home visits when the children were 12 years old . Details of psychometric measurements for the bullying measures reported here have been described previously . 13 14 We explained to the mother or child that : " someone is being bullied when another child says mean and hurtful things , makes fun , or calls a person mean and hurtful names ; completely ignores or excludes someone from their group of friends or leaves them out of things on purpose ; hits , kicks , or shoves a person , or locks them in a room ; tells lies or spreads rumours about them ; or does other hurtful things like these . We call it bullying when these things happen often and it is difficult for the person being bullied to stop it happening . We do not call it bullying when it is done in a friendly or playful way . " When bullying was reported , the interviewer asked the mother or child to describe what happened . An independent rater later checked notes taken by the interviewers to verify that the events described related to instances of bullying , operationally defined as evidence of repeated harmful actions between children where a power differential existed between the bully and the victim . This review was done blind to data on self harm . Mothers ' and children 's narratives of bullying experiences were coded as " never , " " yes , but isolated incidents , " or " frequently . " Children were also asked directly if they had been bullied " a lot . " Of the cohort children , 16.5 % ( 350/2127 ) were reported by their mothers to have been frequently bullied before age 10 , and 11.2 % ( 237/2124 ) of the children reported themselves to have been bullied a lot before age 12 .