Background and rationale {6a}
Adolescence and young adulthood bear a heightened risk for the development of mental health disorders. For example, the first onset of mood and anxiety disorders often lies in the teenage years or early twenties [
1‐
3]. In addition to reducing the quality of life, mental health disorders can have other severe consequences, such as an increased risk for suicide attempts [
4,
5] and economic costs for society [
6,
7], e.g., more health care utilization, special educational needs, and a high need for family care. Thus, there is an urgent need for effective prevention and treatment of mental health problems in young people.
While traditional (face-to-face) psychological interventions have been shown to be effective in the prevention of mental health disorders, it is usually not feasible to deliver these interventions on a large scale. It has therefore been argued that Internet- and mobile-based interventions (IMIs) are a promising avenue to increase access to prevention programs [
8]. In their review, Ebert et al. [
8] report findings from several randomized controlled trials (RCTs) showing that IMIs decrease the risk for mental health disorders such as depression compared to waiting list control conditions; however, the current evidence is limited by a small number of existing RCTs, and studies testing IMIs in younger age groups (i.e., adolescents) are still scarce. As discussed in a recent meta-review on mobile interventions ranging from commercial mediation apps to mobile interventions based on cognitive behavioral therapy [
9], the content of the interventions might crucially contribute to whether or not they have an effect on mental health outcomes. A relatively novel approach to the prevention of mental health disorders is IMIs focusing on the reduction of repetitive negative thinking (RNT, i.e., rumination and worry) [
10,
11].
Prevention focusing on RNT has been highlighted as a promising strategy [
12] for several reasons. First, substantial evidence suggests that RNT in the form of rumination and/or worry is an important transdiagnostic risk factor for psychopathology [
13‐
16]. The overarching construct of RNT can be defined as repetitive thinking about one or more negative topic(s) that is experienced as difficult to control and unproductive [
13,
17‐
20]. RNT can for example occur in the form of rumination about one’s own sad mood [
21] or worry about potential future problems [
22]. In longitudinal studies, RNT in the form of rumination and/or worry was found to increase the risk for a broad range of future mental health disorders including depression, anxiety disorders, and posttraumatic stress disorder [
13,
15,
16,
23,
24]. Moreover, cross-sectional findings demonstrate that RNT is related to existing mental health disorders [
20,
25] and indicate that RNT is involved in the maintenance of psychopathology. Importantly, experimental studies furthermore support the notion that RNT is not only an epiphenomenon or early sign of psychopathology, but causally involved in development and maintenance of mental health problems [
26‐
28]. Of specific relevance to this trial, RNT was found to play a central role in adolescents’ mental health (for a review, see [
29]). For instance, adolescent RNT was shown to prospectively predict the onset of major depression [
30], explain current depressive and social anxiety symptoms [
31], and mediate the relationship between infant temperament and adolescent depressive symptoms [
32].
Importantly, results from a growing number of clinical trials suggest that RNT is modifiable and support the usefulness of focusing on RNT in the prevention and treatment of mental health disorders. For example, rumination-focused cognitive-behavioral therapy (RFCBT) [
16,
33] is a variant of CBT that specifically targets rumination. RFCBT was originally developed as a treatment for depression but has recently been extended to treat and prevent a broader range of mental disorders by addressing transdiagnostic RNT. In RFCBT and related RNT-focused approaches, different modules are typically combined to reduce RNT as effectively as possible. These modules include identifying warning signs for RNT as well the repeated practice of different helpful habits and alternative strategies that are incompatible with RNT such as being more specific and concrete, relaxation, problem-solving, and self-compassion [
33].
A series of randomized controlled trials have provided evidence for the effectiveness of RFCBT in reducing RNT and depressive symptoms as well as preventing relapse in adults and adolescents with a history of depression [
34‐
36]. Two trials tested RFCBT as a preventive intervention for adolescents [
10,
11]. In one of these trials [
10], group and guided Internet RFCBT were compared to a waiting list control condition in adolescents with high levels of rumination and/or worry but no current depressive or anxiety disorder. Both RFCBT interventions significantly decreased RNT as well as sub-threshold depressive and anxiety symptoms post-intervention and over a 1-year follow-up period. Additionally, both interventions significantly reduced the 1-year incidence of major depression and generalized anxiety disorder. Cook et al. [
11] replicated these findings in a sample of undergraduate university students with elevated RNT. Additionally, this trial included an arm in which participants received an unguided version of Internet RFCBT. Interestingly, unguided and guided RFCBT showed similar effects, which indicates that RFCBT has potential as a scalable online intervention.
The current trial is based on these prior studies [
10,
11] and aims to extend their findings by addressing the following limitations. Firstly, prior findings suggest that RNT-focused interventions are effective in reducing sub-threshold depressive and anxiety symptoms in populations at high risk for psychopathology; however, the evidence is limited by a small number of studies. Hence, further large-scale trials are needed to test the robustness of these promising but preliminary findings. This includes testing whether the results replicate when delivering the interventions via IMIs such as scalable and easy-to-access mobile phone apps. We aim to contribute more decisive evidence based on a well-powered trial. Secondly, the efficacy of the single elements of the intervention has not yet been established. Evidence from experimental psychopathology studies [
37‐
40] and preliminary clinical findings [
41,
42] suggest that training concrete thinking could be an active ingredient of the treatment and might have potential as a stand-alone intervention. Focusing mainly on concreteness training might be particularly efficient in preventing at-risk individuals from developing more severe mental health problems; however, this has not yet been tested empirically. We aim to extend prior findings by exploring concreteness training as an alternative to a more extensive RNT-focused intervention. As both concreteness training and the full intervention have potential benefits, there is no clear rationale for the superiority of one of the two interventions. While the full intervention provides a greater variety of RNT-reducing strategies and therefore potentially more flexibility, the concreteness training intervention offers a more focused training of one strategy and hence potentially leads to better mastery of this strategy. The results from our exploratory analyses of potential differences between the two interventions could therefore yield valuable information for future studies. Finally, prior trials mostly tested the effects of RFCBT and related interventions on depressive symptoms and generalized anxiety symptoms. However, social anxiety symptoms were also found to be related to RNT [
43‐
45] and thus might decrease after RNT-focused interventions. This is especially relevant for trials in younger age groups as social anxiety is prevalent in adolescents [
46]. Thus, we will include social anxiety as an additional outcome measure.