Developmental course of psychopathology in youths with and without intellectual disabilities Karen P. de Ruiter, 1,2 Marielle C. Dekker, 1 Frank C. Verhulst, 1 and Hans M. Koot 2 1 Department of Child and Adolescent Psychiatry, Erasmus MC-Sophia, Rotterdam, The Netherlands; 2 Department of Developmental Psychology, Vrije Universiteit, Amsterdam, The Netherlands Background: We aimed to describe similarities and differences in the developmental course of psy- chopathology between children with and without intellectual disabilities (ID). Methods: Multilevel growth curve analysis was used to analyse the developmental course of psychopathology, using the Child Behavior Checklist (CBCL), in two longitudinal multiple-birth-cohort samples of 6- to 18-year-old children with ID (N ¼ 978) and without ID (N ¼ 2,047) using three repeated measurements across a 6- year period. Results: Children with ID showed a higher level of problem behaviours across all ages compared to children without ID. A significant difference between the samples in the developmental courses was found for Aggressive Behaviour and Attention Problems, where children with ID showed a significantly larger decrease. Gender differences in the development of psychopathology were similar in both samples, except for Social Problems where males with ID showed a larger decrease in problem behaviour across time than females with ID and males and females without ID. Conclusions: Results indicate that children with ID continue to show a greater risk for psychopathology compared to typically developing children, although this higher risk is less pronounced at age 18 than it is at age 6 for Aggressive Behaviour. Contrary to our expectations, the developmental course of psychopathology in children with ID was quite similar from age 6 to 18 compared to children without ID. The normative developmental trajectories of psychopathology in children with ID, presented here, can serve as a yardstick against which development of childhood psychopathology can be detected as deviant. Key- words: Intellectual disability, behaviour problems, development, longitudinal studies. Abbrevia- tions: CBCL: Child Behavior Checklist; GP: general population; ID: intellectual disabilities; SES: socio- economic status. Both pioneering (Corbett, 1985; Rutter, Tizard, & Whitmore, 1970) and recent (Dekker, Koot, van der Ende, & Verhulst, 2002; Dykens, 2000; Emerson, 2003) studies showed a 3–7 times higher prevalence of emotional and behavioural problems in children and adolescents with intellectual disabilities (ID), com- pared with typically developing youths. Whether the developmental course of psychopathology is similar, despite elevated levels in children with ID compared to typically developing youths, remains unanswered. Traditionally, the developmental course of psycho- pathology is reported by stability, persistence and remission (e.g., Richardson, Koller, & Katz, 1985; Tonge & Einfeld, 2003). Few studies have investigated changes in the level of psychopathology in children with ID, showing a decrease of overall level of problem behaviours over time, despite high levels of stability and persistence (McCarthy & Boyd, 2001; Richardson et al., 1985; Wallander, Dekker, & Koot, 2003). In a recent longitudinal study in people with ID, Einfeld, Tonge, and Turner (1999) and Tonge and Einfeld (2000, 2003) found that behavioural problems tend to decline whereas emotional problems tend to increase over time, and that approximately 65% of the children with deviant levels of problem behaviour have persistent deviant problems. Similar findings were reported in studies on children with ID representing different behavioural phenotypes (Cornish et al., 2004). Although most of these studies included a wide age-range, none investigated developmental traject- ories of psychopathology in children with ID. Multi- cohort longitudinal studies are necessary to show whether the same changes with age are observed in different cohorts studied in different time periods. In order to understand more fully the stability, growth and continuity of psychopathology we need to investigate trajectories of behaviour over time. Several studies in children without ID, using accelerated longitudinal designs (Bongers, Koot, van der Ende, & Verhulst, 2003; Kraatz Keiley, Bates, Dodge, & Pettit, 2000; Stanger, Achenbach, & Ver- hulst, 1997) show that developmental trajectories vary by type of psychopathology, viz. internalising behaviours increasing, and externalising behaviours decreasing with age (Bongers et al., 2003; Kraatz Keiley et al., 2000; McConaughy, Stanger, & Achenbach, 1992). The developmental trajectories of psychopathology are also found to vary by gender. The adolescent increase of emotional problems in girls, like depression, is not seen in boys. By con- trast, disruptive behaviours emerge in early and middle childhood and decrease after adolescence Journal of Child Psychology and Psychiatry 48:5 (2007), pp 498–507 doi:10.1111/j.1469-7610.2006.01712.x Ó 2007 The Authors Journal compilation Ó 2007 Association for Child and Adolescent Mental Health. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA