The onset of mental disorders mostly occurs in adolescence and young adulthood. An estimated 62.5% of disorders begin before the age of 25, with a peak age at 14.5 years [1]. These disorders are associated with negative outcomes on educational, occupational, and social domains [2,3,4,5,6,7,8]. Compared to those without psychiatric problems, individuals with a psychiatric disorder in their youth are nine times more likely to face negative outcomes on these domains in the transition to adulthood. For youth with subthreshold problems this is five times [9]. Early onset of psychiatric problems is associated with high persistence and negative prognosis [8, 10,11,12].

The etiology of psychiatric disorders is complex. Studies have shown that psychiatric disorders have a multifactorial etiology and that risk factors are pleiotropic [13,14,15]. Particularly in adolescence, the symptoms in the early stages of disorders tend to be non-specific [16, 17]. Subsequently, there are high comorbidity rates between psychiatric disorders as well as heterotypic continuity over time [11, 18,19,20]. This underlines the importance of a transdiagnostic approach in research [21].

To advance a more preventive and transdiagnostic approach in psychiatry, more epidemiological knowledge, especially on individual and environmental exposures, is necessary [7]. This knowledge could be used in individual prediction models for targeted prevention strategies [22, 23]. Considerable effort has been made to study the etiology of psychopathology, but this has been complicated by selective drop-out bias in general population studies, referral bias in patient-based samples, and a focus on a specific diagnosis or inheritance pattern in familial loading studies [24]. More accurate, large-sample deep-phenotype data in a high-risk population is likely to overcome these difficulties [16, 21].

The design of the iBerry (Investigating Behavioral and Emotional Risk in Rotterdam Youth) Study follows a cross-diagnostic approach that cuts across traditional diagnostic boundaries to examine the etiology and course of psychopathology instead of maintaining nosological boundaries with a focus on a single diagnostic category. The main aim of the iBerry Study is to examine the developmental course of psychiatric disorders and associated risk factors to contribute to the development of preventive interventions. The current paper discusses the design and protocol of the first follow-up measurement and gives a cohort profile update, details on the (non) response, and the prevalence of adolescent and parental psychopathology. Furthermore, the long-term effectiveness of using a screening questionnaire to select a cohort oversampled on their self-reported emotional and behavioral problems is discussed.

The iBerry Study is a cohort study of adolescents from the general population who were oversampled on their self-reported emotional and behavioral problems. The study is conducted in the greater Rotterdam area in the Netherlands, this region contains a combination of the highly urbanized city of Rotterdam, the surrounding suburban cities, and more rural villages [25]. The current study discusses the details from the first follow-up measurement (T1), the screening procedure at age 13 and baseline measurement at age 15 were described in detail elsewhere [24]. A concise graphical overview of the iBerry Study is presented in Fig. 1.

Overview of the design and summary of the different phases of the iBerry Study

As described previously by Grootendorst-van Mil and Bouter et al. [24], adolescents were selected for participation in the iBerry Study based on a questionnaire administered in the first year of secondary school as part of standard preventive healthcare performed by community Child and Family Centers in the Netherlands. All adolescents (mean age 13.1 years) filled out the Strengths and Difficulties Questionnaire–Youth (SDQ-Y), to assess their emotional and behavioral problems [26]. Unless the adolescent or their parent(s)/guardian(s) objected, all questionnaires from the school years 2014–2015 and 2015–2016 were screened. From these 16,736 screened questionnaires, adolescents with the highest 15% problem scores were selected, together with a random selection of adolescents with the lower 85% problem scores, resulting in the inclusion of 1,022 adolescents at baseline (September 2015–September 2019, response rate at enrollment 54%).

Participants from the baseline measurement were contacted for the first follow-up measurement and 807 adolescents (79.0%) participated at T1. Data were collected between March 2019 and June 2022. Because the COVID-19 pandemic occurred during this measurement, we added two additional online measurements to collect data on emotional and behavioral problems during the lockdowns [27]. The median interval between the SDQ-Y screening and T1 was 4.7 years (IQR 4.5–5.4). The time between baseline and T1 had a median interval of 3.1 years (IQR 3.0–3.5).

215 adolescents (21.0%) included at baseline did not participate at T1. A small number of participants objected to being contacted for follow-up measurements (n = 13, 1.3%). 71 adolescents (6.9%) declined participation in the first follow-up. The most common reasons for declining were a lack of interest (n = 48, 4.7%) or time (n = 10, 1.0%). The remaining 131 adolescents (12.8%) could not be reached.

Response rates were comparable for the high-risk adolescents (78.3%) and low-risk adolescents (80.6%), and the distributions of high-risk and low-risk adolescents were approximately equal in the responders and the non-responders (χ2 = 0.676, p = 0.411). A detailed overview of the baseline characteristics of responders and non-responders is provided in Supplementary table S1. Non-responders more often were male, had a higher age at baseline, had a non-Dutch ethnic background, had a lower educational level, and belonged to a lower income household. Non-responders were not more likely to score above the borderline cut-off for emotional and behavioral problems (measured with the Youth Self-Report) at baseline. Significant differences showed small effect sizes. Importantly, those non-responders at T1 were more frequently associated with incomplete baseline measurements, indicating that obtaining comprehensive data from this group was already challenging during the cohort's initial assessment.

The iBerry Study aims to investigate the transition of subclinical symptoms to full-blown psychiatric disorders in adolescents who enter young adulthood. We use a cross-diagnostic approach to study all psychiatric disorders. The aim is to identify risk and protective factors and examine the mechanisms underlying the development of psychopathology.