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In response to these trends, on 10 December 2025, the Australian government implemented Social Media Minimum Age legislation prohibiting Australians under the age of 16 years from having social media accounts. The introduction of this legislation allows for a unique quasi-experimental investigation of the impact of social media use on adolescent mental health and the mechanisms through which social media may impact young Australian adolescents\u2019 mental health, well-being and cognition.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods and analysis<\/jats:title>\n                    <jats:p>To assess the relationship between social media use and young adolescents\u2019 mental health, well-being and cognition, a longitudinal mixed-methods approach will be taken. 422 young people (aged 10\u201315 years) and parent\/carer dyads were recruited and invited to complete a baseline assessment before the implementation of the Social Media Minimum Age legislation (October\u2013December 2025). Three follow-up assessments will then be completed after 6 (April\u2013May 2026), 12 (October\u2013November 2026) and 18\u2009months (April\u2013May 2027). Data from parent\/carer and adolescent surveys, ecological momentary assessments (EMAs), cognitive tasks and smartphone-recorded screenshots of social media and messaging app use across each of these timepoints will be analysed. Linear regression analyses will be used to assess correlational relationships between social media experiences and responses and mental health outcomes as well as how individual factors moderate these relationships. To assess these relationships over time and between momentary experiences and responses (EMA data) and more stable trait-based (individual) and environmental (parenting and government restrictions) factors, we will use linear mixed-effects models and mixed-effects segmented regression.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Ethics and dissemination<\/jats:title>\n                    <jats:p>This study has been approved by the UNSW Human Research Ethics Committee (iRECS 9197). Findings from this study will help identify potential opportunities for interventions at individual, micro (parental) and macro (governmental) levels to mitigate any negative impacts of social media for younger adolescents. Results will be disseminated through peer-reviewed publications, conference presentations, public engagement opportunities and researcher-associated websites. After each assessment timepoint, core findings will also be distilled into engaging formats such as infographics and videos, which will be shared with young people and their parent\/carer.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1136\/bmjopen-2026-120216","type":"journal-article","created":{"date-parts":[[2026,8,5]],"date-time":"2026-08-05T11:55:17Z","timestamp":1785930917000},"page":"e120216","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["Tracking mental health and cognitive changes following the Australian social media minimum age legislation: a 4-wave longitudinal study protocol"],"prefix":"10.1136","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0009-0002-7865-287X","authenticated-orcid":false,"given":"Anna","family":"Bilston","sequence":"first","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lyndsay","family":"Brown","sequence":"additional","affiliation":[{"name":"Black Dog Institute, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xu","family":"Qi","sequence":"additional","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0001-6607-2520","authenticated-orcid":false,"given":"Emily","family":"Morgan","sequence":"additional","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yasmin","family":"Hasan","sequence":"additional","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarah","family":"Stevens","sequence":"additional","affiliation":[{"name":"Black Dog Institute, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Weike","family":"Wang","sequence":"additional","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9046-6159","authenticated-orcid":false,"given":"Aliza","family":"Werner-Seidler","sequence":"additional","affiliation":[{"name":"Black Dog Institute, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Susanne","family":"Schweizer","sequence":"additional","affiliation":[{"name":"School of Psychology, University of New South Wales, Sydney, New South Wales, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"239","published-online":{"date-parts":[[2026,8,5]]},"reference":[{"key":"2026080504551203000_16.8.e120216.1","unstructured":"Haidt J . 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