Heterotypic trajectories of dimensional psychopathology across the lifespan: the case of youth-onset attention deficit/hyperactivity disorder Arthur Gus Manfro, 1,2 Marcos Santoro, 2,3 Guilherme Vanoni Polanczyk, 2,4 Ary Gadelha, 2,5 Pedro Mario Pan, 2,5 Rodrigo Affonseca Bressan, 2,5 Elisa Brietzke, 2,5 Fernanda Talarico, 2,3 Sintia Belangero, 2,3 Luis Augusto Rohde, 2,4,6 and Giovanni Abrah~ ao Salum 2,6 1 Graduate program in Psychiatry and Behavioral Science, UFRGS, Porto Alegre, Brazil; 2 National Institute of Developmental Psychiatry for Children and Adolescents (INCT-CNPq), S~ ao Paulo, Brazil; 3 LiNC – Interdisciplinary Laboratory of Clinical Neurosciences, Universidade Federal de S~ ao Paulo, S~ ao Paulo, Brazil; 4 Department & Institute of Psychiatry, Universidade de S~ ao Paulo, S~ ao Paulo, Brazil; 5 Department of Psychiatry, Universidade Federal de S~ ao Paulo, S~ ao Paulo, Brazil; 6 Department of Psychiatry, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil Background: Recent studies have demonstrated the existence of a distinct late-onset attention deficit/hyperactivity disorder (ADHD) trajectory. Our objective is to test if there are distinct ADHD trajectories regarding age of onset from childhood to adolescence and to compare clinical manifestations, cognitive functions and genetic risk for ADHD among distinct longitudinal groups. Method: Nine hundred and twenty four children and adolescents from the community participated in the study. We compared clinical, cognitive features and genetic risk among four groups of participants: (a) childhood-limited, (b) youth-onset, (c) childhood-onset with youth persistence, and (d) community comparisons without ADHD. Symptomatic and diagnostic assessments were performed using the Development and Well-Being Behavior Assessment, the Strengths and Difficulties Questionnaire, and the Child Behavior Checklist. Cognitive functions were measured using a battery of standardized tests. Genetic risk for ADHD was calculating using summary statistics from the Psychiatric Genomics Consortium. Results: Half of the adolescents (52%) with ADHD had their symptom onset in adolescence. The impairment level of this group in adolescence is similar to the persistent group. Despite not having ADHD, the youth-onset group already presented in childhood more symptoms from other domains of psychopathology, higher shared variance in psychiatric symptomatology (p-factor), school impairment, and executive dysfunctions than community comparisons. Furthermore, the youth-onset group presented lower levels of genetic risk for ADHD compared to other cases. Conclusions: A significant proportion of adolescents with ADHD were youth-onset cases and presented similar impairment levels as those cases with early- onset ADHD. The presence of cognitive impairments and higher levels of clinical symptoms in the youth-onset group already at childhood speaks in favor of a heterotypic trajectory of psychopathology suggesting that youth-onset ADHD might be an artificial consequence of categorizing dimensional psychopathology into discrete diagnostic groups. Keywords: Youth-onset; executive function; cognition; polygenic risk scores; p-factor. Introduction Attention deficit/hyperactivity disorder (ADHD) is traditionally conceptualized as a neurodevelopmen- tal disorder with onset before the age of 12 that can persist into adolescence and adulthood (American Psychiatric Association, 2013). Therefore, current classifications state that ADHD cases observed in adolescence or adulthood represent the symptomatic persistence from childhood through the individual’s lifespan. In the last years, however, new evidence has emerged questioning this traditional view of the disorder, and the late-onset ADHD started to be a focus of empirical interest. In 2015, Moffit and collaborators published a pioneering study demonstrating that nearly 90% of adult ADHD cases in the Dunedin cohort were de novo cases (Moffitt et al., 2015). Investigators repli- cated the findings on the existence of late-onset ADHD in three other large cohort samples: E-Risk (Agnew-Blais et al., 2016), Pelotas Birth Cohort (Caye et al., 2016) and ALSPAC (Cooper et al., 2018; Riglin et al., 2016). These findings raised a debate on whether adult ADHD could be considered a childhood-onset neurodevelopmental disorder, questioning whether late-onset ADHD might repre- sent a distinct disorder from childhood-onset ADHD (Castellanos, 2015; Moffitt et al., 2015). Other reports, however, are somewhat more conservative, raising concerns about unidentified subthreshold ADHD (Faraone & Biederman, 2016) or better diag- nostic explanation by other psychiatric comorbidi- ties (Sibley et al., 2018). The current literature presents some limitations. First, most community studies did not investigate if the ADHD onset was in adulthood or adolescence. Analysis of the participants of the Dunedin, E-Risk, and Pelotas studies considered a first assessment in childhood and posterior assessments already in adulthood. Thus, the emergence of ADHD symptoms might have occurred any time during adolescence or Conflict of interest statement: See Acknowledgements for full disclosures. © 2018 Association for Child and Adolescent Mental Health. Published by John Wiley & Sons Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main St, Malden, MA 02148, USA Journal of Child Psychology and Psychiatry **:* (2018), pp **–** doi:10.1111/jcpp.12987