Title page 1

Contents 3

Abstract 6

Acknowledgements 7

Executive summary 8

Policy context 8

Key conclusions 8

Main findings 8

Related and future Joint Research Centre work 9

Quick guide 9

1. Introduction 10

1.1. Why social media and youth mental health is a policy priority 10

1.2. What we know from existing literature 11

2. Methods and scope 14

2.1. Overview 14

2.2. Eligibility criteria 14

2.3. Information sources and search strategy 16

2.4. Study selection and record management 16

2.5. Inclusion criteria and appraisal approach 17

2.6. Quality assessment 17

2.7. Data extraction 18

2.8. Synthesis approach 18

3. Results 20

3.1. Evidence base and operationalisation of core constructs 20

3.1.1. Definitions 20

3.1.2. Included studies 20

3.1.3. Exposure operationalisation 21

3.1.4. Outcome operationalisation 22

3.2. Experimental and intervention evidence 22

3.2.1. Overview of intervention designs and exposure verification 22

3.2.2. Effects on internalising symptoms and global well-being 23

3.2.3. Effects on self-evaluation and appearance-related outcomes 24

3.2.4. Findings relevant for implementation 25

3.3. Longitudinal and intensive longitudinal evidence 25

3.3.1. Overview of designs and analytic strategies 25

3.3.2. Time/intensity measures and later well-being/mental health 25

3.3.3. Problematic use and negative consequences as predictors 27

3.3.4. Directionality and reciprocal pathways 29

3.3.5. Wider outcome domains: loneliness, sleep, and self-harm/suicidality 29

3.4. Mechanisms and sources of heterogeneity across the evidence base 30

3.4.1. Social comparison, FoMO, and self-evaluative pathways 30

3.4.2. Displacement, substitution and time use trade-offs 30

3.4.3. Moderators and context 31

3.5. Consistency across designs and measures 32

3.6. Synthesis by outcome domain 32

3.6.1. Depression, anxiety, and broader internalising symptoms 32

3.6.2. Global well-being and life satisfaction 33

3.6.3. Self-evaluation, body image, FoMO and social comparison 34

3.6.4. Loneliness and social connectedness 34

3.6.5. Sleep 34

3.6.6. Self-harm, suicidality, and eating-disorder symptoms 35

4. Discussion 36

4.1. Integrating the evidence 36

4.2. Implications for conceptualising "exposure" in social media and well-being research 37

4.3. Mechanisms: interpreting the most coherent pathways 38

4.4. Heterogeneity and vulnerable groups 38

4.5. Methodological considerations and limitations of the evidence base 38

4.6. Implications for policy and practice 39

4.7. Directions for future research 40

5. Conclusion and key messages 41

5.1. Conclusion 41

5.2. Key messages for policy and practice 41

5.3. Key messages for research and monitoring 42

References 43

List of abbreviations and definitions 49

Annex 1. Database search strategies 53

Annex 2. Table of evidence 56

Annex 3. Quality appraisal summary 80

Table 1. Eligibility criteria. 16

Table 2. Composition of the evidence by design, outcome, geography and publication 21

Table 3. Evidence table 56

Table 4. Quality assessment 80

Figure 1. PRISMA flow diagram 15