ORIGINAL ARTICLE Severe conduct problems in adolescence and risk of schizophrenia in early adulthood Marko Manninen 1 | Reetta Latvala 2 | Minna Torniainen-Holm 1 | Jaana Suvisaari 1 | Maija Lindgren 1 1 National Institute for Health and Welfare, Mental Health Unit, Helsinki, Finland 2 Department of Psychology, University of Helsinki, Helsinki, Finland Correspondence Marko Manninen, National Institute for Health and Welfare, Mannerheimintie 166, 00300 Helsinki, Finland. Email: marko.manninen@thl.fi Funding information Päivikki ja Sakari Sohlbergin Säätiö, Grant/ Award Number: N/A; Yrjö Jahnssonin Säätiö, Grant/Award Number: 6783 Aim: Reform school (RS) is a foster care institution for adolescents with severe conduct prob- lems. Both instability of early rearing environment and severe conduct problems in adolescence may associate with later psychotic disorders. We studied whether the risk of schizophrenia in adulthood is elevated in RS adolescents, and whether it is related to the age at first foster care placement or placement instability. Methods: Adult age schizophrenia spectrum disorder data from RS subjects (N = 1099) were compared to a comparison group matched by age, sex, and place of birth (N = 5437) in a register based follow-up study, with up to 23 years follow-up time. Schizophrenia was also pre- dicted with chosen placement factors. Cox proportional regression model was used in the analysis. Results: RS subjects had an 8-fold (HR = 7.82, 95% CI 5.63-10.87) risk of schizophrenia com- pared to the comparison group. RS subjects also had an earlier age of schizophrenia onset. RS cohort, gender, placement instability, or age during the first out-of-home placement did not pre- dict later schizophrenia. Conclusions: Adolescents with severe conduct problems are a specific high-risk group for later schizophrenia. The risk manifests early, which compromises the pathway to the adult well-being. Specialized screening procedures for psychosis risk should be implemented in the standard clini- cal procedures when working with adolescents with severe behaviour problems, and early inter- vention programs should be available. KEYWORDS adolescents, conduct problems, delinquency, foster care, schizophrenia 1 | INTRODUCTION Out-of-home placement in childhood is associated with an elevated risk for numerous adult psychiatric problems including psychosis (Egelund & Lausten, 2009; Viner & Taylor, 2005; Vinnerljung & Hjern, 2014). Among foster care population, the length of placement corre- lates with an elevated risk for psychosis (Vinnerljung, Hjern, & Lind- blad, 2006), and placement instability associates with mental health problems (Fawley-King & Snowden, 2012), substance use (Stott, 2012), and several other adverse long-term outcomes (Vinnerljung & Sallnäs, 2008). Foster care children placed due to behavioural prob- lems appear to have especially high risk for psychiatric problems (Vinnerljung & Sallnäs, 2008). Numerous studies have confirmed a link between delinquency and psychotic symptoms. Childhood oppositional defiant disorder and conduct disorder (CD) diagnoses associate with over 4-fold risk of schizophrenia in the adulthood (Maibing et al., 2015). The risk for psy- chosis has been shown to be 10-fold for adolescents in detention and correctional facilities compared to the general population (Fazel, Doll, & Långström, 2008). A recent Finnish register-based study showed that 12.8% of 15-to-19-year-old delinquents, who have gone through forensic psychiatric examination, are diagnosed with schizo- phrenia later in life (Lindberg, Miettunen, Heiskala, & Kaltiala-Heino, 2017). In a follow-up study by Gosden, Kramp, Gabrielsen, Andersen, and Sestoft (2005) conviction of violence in late adolescence associ- ated with later schizophrenia. Among adults with schizophrenia, prior Received: 3 January 2018 Revised: 12 September 2018 Accepted: 4 November 2018 DOI: 10.1111/eip.12767 Early Intervention in Psychiatry. 2018;1–7. wileyonlinelibrary.com/journal/eip © 2018 John Wiley & Sons Australia, Ltd 1