Body dissatisfaction and adolescent mental illness. Understanding causal associations and mechanisms to inform preventative interventions.

Background. Incidence of adolescent depression, anxiety, self-harm, and eating disorders has increased in the past 20-years. During this time, adolescent body dissatisfaction has also soared.

Body dissatisfaction is a key risk factor for eating disorders, but robust longitudinal evidence on whether it is also a risk factor for depression, anxiety, and self-harm, that trans- diagnostic universal preventative interventions could target, is limited.

There is also limited longitudinal research on risk factors - and their underlying mechanisms - for body dissatisfaction in general population adolescent samples. This evidence is needed to adapt existing school-based interventions and increase their effectiveness. Aims.

To: - Investigate the causal role of body dissatisfaction in the aetiology of adolescent mental illness. - Identify novel risk factors for adolescent body dissatisfaction. - Understand aetiological mechanisms underpinning body dissatisfaction.

Methods. - Aim 1: I will triangulate robust methodological approaches for causal inference in large twin cohorts and UK electronic health records. - Aims 2-3: I will conduct a four-year school-based cohort study co-produced with adolescents, teachers, parents.

I will collect observational, experimental, and digital data to gain a novel understanding of aetiological mechanisms unpinning adolescent body dissatisfaction. Implications. My research will inform policy recommendations and lead to the development of new preventative interventions.