At age 18, participants were interviewed face-to-face about exposure to a range of adverse experiences between 12–18 years using the Juvenile Victimization Questionnaire (JVQ) (Finkelhor et al., 2011; Hamby et al., 2004 ), adapted as a clinical interview. The JVQ has good psychometric properties (Finkelhor et al., 2005) and was used in the U.K. National Society for the Prevention of Cruelty to Children (NSPCC) national survey (Radford et al., 2011 ; Radford et al., 2013 (link)), thereby providing important benchmark values for comparisons with our cohort. Our adapted JVQ comprised 45 questions covering different forms of victimization grouped into 7 categories: Crime Victimization, Peer/Sibling Victimization, Internet/Mobile Phone Victimization, Sexual Victimization, Family Violence, Maltreatment, and Neglect. The interview schedule used in this study is provided in Supplementary Materials I.
Within each pair of twins in our cohort, co-twins were interviewed separately by a different research worker and were assured of the confidentiality of their responses. The participants were advised that confidentiality would only be broken if they told the research worker that they were in immediate danger of being hurt, and in such situations the project leader would be informed and would contact the participant to discuss a plan for safety.
Each JVQ question was asked for the period ‘since you were 12’ and participants were given the option to say ‘yes’ or ‘no’ as to whether each type of victimization had occurred in the reporting period. Consistent with the JVQ manual (Finkelhor et al., 2011; Hamby et al., 2004 ), participants were coded as 1 if they reported any experience within each type of victimization category or 0 if none of the experiences within the category were endorsed. If an experience was endorsed within a victimization category, follow-up questions were asked concerning how old the participant was when it (first) happened, whether the participant was physically injured in the event, whether the participant was upset or distressed by the event; and how long it went on for (by marking the number of years on a Life History Calendar; Caspi et al., 1996 ). In addition, the interviewer wrote detailed notes based on the participant’s description of the worst event. If multiple experiences were endorsed within a victimization category, the participant was asked to identify and report about their worst experience.