The mental health of children and adolescents is crucial for their overall development. Poor mental health can have varying impacts on their daily lives, academic performance, and prospects, depending on its severity [1]. Severe cases may even lead to suicidal behaviors [2]. It is important to note that the effects of mental ill-health during the early years can persist into adulthood and have a lasting influence on individuals' lives [3]. With the significant prevalence and persistence of mental disorders, it is alarming that approximately one-fifth of children and adolescents worldwide have experienced one or more clinical disorders, and more than half have shown sub-clinical symptoms [4].

Given that children and adolescents typically spend a significant amount of their daily time in educational settings, school-based mental health services have emerged as a critical battleground for addressing their mental well-being [5]. However, the current provision of school-based mental health services falls short of meeting the needs of children and adolescents, particularly in low- and middle-income countries (LMICs). Limited access to services and the compatibility of available mental health services with the specific needs of young individuals contribute to this insufficiency and inefficiency [6].

In recent decades, there has been significant progress in developing mental health promotion programs for children and adolescents [7]. These programs primarily focus on school-based universal interventions, demonstrating positive effects on the mental health of young individuals [8]. However, systematic reviews and meta-analyses examining the efficacy of these interventions have shown mixed results [9]. Moreover, prevailing single-diagnosis protocols limit the effectiveness of these programs in addressing co-occurring disorders that commonly exist among individuals [10].

Despite the potential benefits of universal school-based interventions, there is an ongoing debate regarding their effectiveness compared to targeted interventions. Targeted interventions are designed for individuals or groups identified as at-risk for developing mental health issues, whereas universal interventions are applied to all students regardless of their risk level. Literature suggests that targeted interventions may offer more significant benefits for those at high risk, potentially due to their focus on specific needs and challenges [11,12,13]. However, universal interventions play a crucial role in promoting mental health and well-being across the entire student population, aiming to prevent the onset of mental health issues before they arise. This approach is especially relevant in educational settings, where identifying and targeting at-risk individuals can be challenging. Thus, examining the effectiveness of universal interventions remains worthwhile, particularly in their potential to create inclusive, supportive environments that benefit all students.

To overcome these challenges and enhance the efficiency of mental health promotion, a growing emphasis has been placed on adopting a transdiagnostic framework for children and adolescents. Transdiagnostic interventions offer a promising approach that goes beyond targeting specific disorders and focuses on promoting overall mental health and emotional well-being [14]. By addressing a broader range of factors related to mental health, such as mental health literacy, emotions, behaviors, social management techniques, values, and perceptions, transdiagnostic interventions aim to meet the diverse needs of most children and adolescents [15, 16]. Specifically, 'values' here refer to the core beliefs and principles that influence children and adolescents' attitudes and behaviors towards mental health, encompassing what they deem important in their lives and schooling experience. 'Perceptions,' meanwhile, encompass their understanding and attitudes towards mental health issues, including awareness, stigma, and openness to intervention.

While previous reviews have highlighted the positive influence of school-based universal interventions on important determinants of children and adolescents' mental health, adopting transdiagnostic protocols can further enhance the overall effectiveness [8]. Transdiagnostic interventions have the potential to provide significant benefits by targeting a wider range of mental aspects and addressing co-occurring disorders in a more comprehensive manner. Therefore, there is an urgent and compelling need to conduct a systematic review of existing universal school-based transdiagnostic interventions to evaluate their efficacy in promoting the mental health and well-being of children and adolescents.

This article seeks to contribute to the existing body of knowledge by conducting a thorough systematic review of transdiagnostic interventions. By carefully analyzing the available evidence, our aim is to highlight the potential benefits associated with transdiagnostic interventions and shed light on their ability to address the limitations inherent in conventional single-diagnosis protocols. The insights gained from this comprehensive review will provide policymakers, educators, and mental health professionals with valuable information to inform their decision-making processes and facilitate the development of more effective strategies that promote the mental well-being of children and adolescents in a holistic manner.

Definitions of transdiagnostic constructs

Transdiagnostic treatments are a range of therapies that can be applied broadly across different disorders [17]. The foundation of transdiagnostic treatments lies in transdiagnostic constructs, which are the underlying mechanisms that cut across various disorders (Stanton et al. 2020). In this context, there are two types of transdiagnostic constructs: descriptively transdiagnostic processes and mechanistically transdiagnostic processes.

Descriptively transdiagnostic processes are observed across multiple disorders, indicating their presence in a range of conditions [18]. For instance, the study by Mehrdadfar et al. [19] revealed that an online transdiagnostic treatment effectively enhanced social-emotional skills, such as emotional competence, in children with cochlear implants, illustrating emotional competence's broad relevance across various conditions.

On the other hand, mechanistically transdiagnostic processes are the ones that provide causal and functional explanations for the co-occurrence of disorders [20]. An example of a mechanistically transdiagnostic process is the online Brief Emotion Regulation Training (BERT) program for emerging adults developed by Gatto et al. [21]. The program, rooted in cognitive-behavioral practices, aimed to improve emotional competence, demonstrating its transdiagnostic significance by showing that improved emotion regulation can causally influence mental wellness.

To further illustrate the distinction between these concepts, two common transdiagnostic constructs are exemplified: self-esteem and rumination [22]. Self-esteem has been found to be present in various mental disorders such as social anxiety disorder and schizophrenia, indicating its descriptively transdiagnostic nature [23, 24]. However, whether self-esteem is mechanistically transdiagnostic remains uncertain without evidence supporting its causal impact on mental disorders. In contrast, rumination is believed to have a detrimental effect on emotional well-being and can causally influence it, leading to a vicious cycle of worsening behavior [25, 26], namely, normally believed to be mechanistically transdiagnostic.

Transdiagnostic interventions

Based on the identification of transdiagnostic constructs, transdiagnostic interventions have been developed in recent years [22]. These interventions can be broadly categorized into two types based on their developmental basis. The first type involves targeting the identified transdiagnostic constructs to improve treatment efficacy from a psychopathology perspective [27]. The second type involves extending existing single-diagnosis treatments by adapting their protocols to be applicable across disorders, with cognitive behavioral therapy (CBT) being a primary example [28]. While many universal school-based interventions aimed at promoting children and adolescents' mental health by nurturing their social-emotional learning (SEL) abilities fall into the first type [29], they are often not explicitly identified as transdiagnostic in previous review articles [30, 31].

The efficacy of transdiagnostic interventions developed from single-diagnosis treatments is intertwined with their effectiveness in treating psychopathology. Since these interventions build upon the foundation of existing single-diagnosis protocols, psychologists may already possess a significant portion of the necessary knowledge [32]. In contrast, teachers, who are often the conductors of school-based interventions, need training to acquire the relevant intervention knowledge, and it is unclear whether this new acquisition would affect intervention efficiency. Considering the complexity of transdiagnostic theory, it is essential to examine whether teacher-led or psychologist-led interventions are more effective.

However, the binary classification of transdiagnostic interventions mentioned above does not adequately explain the advantages and disadvantages of different interventions in a methodological manner. Therefore, this chapter will adopt the classification proposed by Sauer-Zavala et al. [22] to present the current transdiagnostic approaches and existing debates. Under this framework, transdiagnostic interventions are divided into three types: those with universally applied therapeutic principles, modular treatments, and shared mechanism treatments. Modular treatments, observed at an individual level, will not be discussed in the present study.

Universally applied therapeutic principles

Interventions that utilize universally applied therapeutic principles are grounded in specific theoretical frameworks, such as cognitive-behavioral treatment [33]. These interventions are characterized by their ability to be applied across a variety of psychopathologies using a single protocol, showcasing their universal applicability [34]. While these types of interventions have long been used in the treatment of mental disorders, their classification as transdiagnostic is a more recent development [35]. This shift in understanding has been driven by an increased awareness of their impact on co-occurring disorders. Many widely recognized transdiagnostic treatments fall into this category, as they incorporate existing treatments and minimize the need for the creation of new interventions [10].

Interventions based on universally applied therapeutic principles are structured to broadly address psychological disorders, utilizing standardized protocols that are effective across a diverse range of conditions. This differs from grouping them based on shared mechanisms or psychological processes. This "top-down" approach, referred to as a "one size fits all" strategy by Sauer-Zavala et al. [22], contrasts with the "bottom-up" approach of shared mechanism treatments, which target shared mechanisms across disorders and formulate treatment protocols based on these mechanisms.

A potential limitation of interventions with universally applied therapeutic principles is that their impact on co-occurring disorders is not always intentional. Many of these interventions were originally designed to address specific mental disorders, and their effects on other co-existing disorders may be incidental rather than by design. However, with the rise of transdiagnostic awareness, some of these interventions have been adapted to more effectively address co-occurring disorders, compensating for their initial lack of specificity. Despite this limitation, interventions with universally applied therapeutic principles offer several advantages. Firstly, as they are often based on existing protocols, their development is less time-consuming compared to creating entirely new approaches. Secondly, it is easier for practitioners, such as teachers or psychologists, to acquire the necessary knowledge, which is especially beneficial for school-based interventions. As a result, interventions with universally applied therapeutic principles are of significant interest and are expected to be widely utilized in universal school-based practice.

Mindfulness-based interventions

Mindfulness-based interventions aim to cultivate a state of peaceful awareness and encourage individuals to focus on the present moment of internal and external experiences (Greenberg & Harris, 2012). These interventions typically involve body-focused meditation or movement practices where recipients are instructed to focus on their breathing or current movements [36]. Mindfulness is considered a form of emotion regulation and has been widely disseminated in the adult population to promote mental health and emotional well-being, particularly in non-clinical settings [37]. Recent studies have shown that mindfulness-based interventions can be effective as transdiagnostic treatments, addressing a variety of mental health problems by targeting underlying mechanisms such as rumination and worry [38].

Mindfulness-based interventions have advantages in terms of simplicity and group-friendliness. The instructions are relatively straightforward, reducing the requirements for conducting the intervention. Additionally, the instructions can be applied to a broad population, making these interventions suitable for group settings. In the context of school-based interventions, mindfulness-based approaches do not require a lengthy adjustment period, can be easily adopted by individuals, and do not pose risks of worsening mental health. Furthermore, teachers or psychologists can acquire the necessary knowledge to deliver these interventions with relative ease. Therefore, mindfulness-based interventions have a clear advantage in universal school-based practice and are expected to be widely employed.

Acceptance and commitment therapy (ACT)

Acceptance and Commitment Therapy (ACT) is a comprehensive modality that focuses on experiential avoidance as a central difficulty associated with emotional distress, such as anxiety, depression, and stress [39]. ACT aims to enhance individuals' psychological flexibility, which involves their ability to persist in or maintain their current mental states in response to challenging environments [40]. ACT employs various core techniques that collectively work to develop psychological flexibility. Recent research has shown that ACT can be used as a transdiagnostic intervention, addressing a broad range of mental states and disorders by targeting underlying mechanisms such as psychological inflexibility [41].

The core techniques of ACT include:

1.

Encouraging individuals to maintain balance and broaden their scope of reaction to negative thoughts and feelings, thereby protecting them from maladaptive avoidance behaviors.

2.

Using psychoeducation to strengthen individuals' commitment to positive values in their lives, such as family, work, and friendships.

3.

Emphasizing commitment and consistency in actions aligned with personal values.

4.

Promoting willingness to accept adverse events and feelings by utilizing emotional resources developed through previous skills.

5.

Fostering workability through psychological flexibility.

ACT, as a newly developed transdiagnostic intervention, addresses a broader range of mental states than many other transdiagnostic approaches, making it promising for meeting heterogeneous needs across single-diagnosis and general mental health contexts [42]. It has been associated with improvements in mental health and overall well-being [43]. Furthermore, ACT is structured in sessions, making it suitable for integration into school curricula. Therefore, ACT is expected to be frequently encountered in the current systematic review, as it offers a comprehensive approach to address various mental health concerns.

Social emotional learning (SEL) interventions

Social Emotional Learning (SEL) interventions are another type of universally applied therapeutic principles that aim to foster the development of five core competencies: self-awareness, self-management, social awareness, relationship skills, and responsible decision-making [44]. These competencies are believed to be critical for mental health, well-being, and success in life [29].

SEL interventions are designed to be universally applicable and can be implemented in various settings, including schools, to promote the emotional and social competence of children and adolescents [29]. They are based on the understanding that learning is a social and emotional process, and that these skills can be taught in a coordinated manner [45].

A study by Mehrdadfar et al. [19] demonstrated the transdiagnostic nature of SEL interventions. The study found that a unified protocol for online transdiagnostic treatment improved social-emotional skills, including emotional competence, in children with cochlear implants. This suggests that SEL interventions can be effective across a range of conditions and disorders, highlighting their descriptively transdiagnostic nature.

Furthermore, Gatto et al. [21] developed an online Brief Emotion Regulation Training (BERT) program for emerging adults, rooted in cognitive-behavioral practices, which aimed to improve emotional competence. The program demonstrated its transdiagnostic significance by showing that improved emotion regulation can causally influence mental wellness, indicating the mechanistically transdiagnostic nature of SEL interventions.

SEL interventions, like Mindfulness-based interventions and Acceptance and Commitment Therapy (ACT), have the potential to address a broad range of mental states and meet heterogeneous needs across single-diagnosis and general mental health contexts. Therefore, they are expected to be frequently encountered in the current systematic review, as they offer a comprehensive approach to address various mental health concerns.

Shared mechanisms treatment

Shared mechanisms treatment represents a transformative shift in the field of transdiagnostic interventions, aiming to identify and target shared constructs and processes that underlie multiple disorders [22]. It explicitly focuses on identifying and influencing the common mechanisms that cut across different disorders, distinguishing itself from other types of interventions. This approach builds upon the conceptualization of mechanistically transdiagnostic constructs and informs treatment protocols by identifying core processes [22].

Three methods typically represent existing programs of shared mechanisms treatment:

1.

Exposure practice: This approach primarily targets anxiety disorders and involves gradually or intensely exposing individuals to feared stimuli [46]. By addressing the overestimation of potential risks and dangers associated with these stimuli, exposure practice aims to facilitate fear habituation, disconfirmation of fear beliefs, and promote inhibitory learning [32]. While exposure practice may not be suitable for school-based settings due to its environmental requirements and time-consuming nature, it remains a valuable intervention.

2.

Cognitive-Behavioral Therapy-Enhanced (CBT-E): Developed as a shared-mechanism treatment for eating disorders, CBT-E focuses on changing individuals' awareness regarding the overvaluation of weight and shape [47]. This approach involves reducing weight checking frequency and improving diet plans. However, CBT-E requires multifaceted assistance from various sources, making it less applicable in pure school-based delivery modes.

3.

Unified Protocol (UP) for Transdiagnostic Treatment of Emotional Disorders: UP targets shared mechanisms across emotional disorders and explicitly aims to address their co-occurrence. For example, UP focuses on reducing reactive distress triggered by strong negative emotions and preventing reliance on avoidant coping strategies. It incorporates elements of mindfulness-based interventions, psychological flexibility, and raising relevant awareness [48]. While UP is comprehensive, its implementation requires access to relevant training and resources for both conductors and recipients, making it less feasible for widespread school-based use.

Considering the accessibility, training requirements, and available resources, shared mechanisms treatments have found a wide range of applications. However, their use in school-based delivery modes may be limited due to the specific context and resources available [49].

Cognitive behavioral therapy (CBT)

Cognitive Behavioral Therapy (CBT) is a well-established and widely used transdiagnostic treatment approach, which targets shared mechanisms across various mental disorders [28]. CBT is based on the cognitive model, which posits that maladaptive thought patterns contribute to the development and maintenance of psychological disorders. By identifying and challenging these thought patterns, CBT aims to alleviate symptoms and improve functioning across a range of disorders [50].

CBT has been adapted for use in various formats, including individual, group, and internet-delivered therapy. Internet-delivered CBT (i-CBT) has emerged as a promising approach to increase the accessibility of CBT, particularly for individuals who may face barriers to in-person treatment. For instance, a recent study found that guided i-CBT was effective in treating anxiety and depression among university students, demonstrating the potential of i-CBT as a scalable and cost-effective treatment option [51].

Despite its broad applicability, CBT is not a "one size fits all" approach. The effectiveness of CBT can vary depending on the specific disorder, the individual's characteristics, and the context in which it is delivered. Therefore, ongoing research is needed to optimize the delivery of CBT and maximize its benefits across diverse populations and settings.

Potential benefits of transdiagnostic approach

Transdiagnostic approaches, though not universally defined, were conceived as a response to the constraints of traditional diagnostic systems such as the DSM and ICD. These approaches aim to treat mental disorders from a broader perspective, considering higher dimensions and addressing multiple co-occurring conditions [52]. Transdiagnostic approaches have shown promise in treating mental disorders holistically [53]. While they may not produce exceptionally prominent effects on specific disorders or symptoms in individuals with co-occurring conditions, they tend to be more effective than single-diagnosis interventions in reducing a broader range of mental issues, without focusing solely on clinical diagnoses or symptoms [54].

Transdiagnostic approaches are also in line with the current trend of expanding the scope of mental health. It is increasingly recognized that good mental health and mental disorders are not simply two ends of a continuum among children and adolescents [55]. The conventional perspective, which equates the absence of mental disorders with good mental health, stems from the dominance of medical, disease-centered approaches in psychiatric research over the past decades [56]. However, researchers now acknow