Odigie, Tracy; (2025) Integrating genetic and social science perspectives to dissect the association between bullying victimisation and mental health. Doctoral thesis (Ph.D), UCL (University College London.

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Abstract

BACKGROUND: Bullying victimisation, which involves repeated and unwanted aggression—such as physical, verbal, relational, and cyberbullying—affects school-aged children and adolescents worldwide. Associations between bullying victimisation and mental health outcomes are extensively documented. However, much remains to be done to dissect and understand those associations further. In particular, the nature of those associations, including the role played by genetics, remain unclear. In addition, how the magnitude of those associations can vary according to individual vulnerabilities and across contexts, remains largely unexplored. This thesis proposes three studies aimed at understanding (i) the role of genetics in the associations between bullying victimisation and mental health outcomes, and (ii) testing whether these associations vary according to genetic predispositions to mental health vulnerabilities and biopsychosocial traits, as well as (iii) contextual factors. AIMS & METHODS: In Chapter 2, I use data from the Norwegian Mother, Father and Child cohort study to conduct a genome-wide analysis of bullying victimisation. I use a triangulation of genetically informed methods to elucidate the role of genetics in explaining the associations between bullying victimisation and mental health outcomes, characterise which individual characteristics heightened the impact of bullying victimisation on mental health and infer causality. In Chapter 3, I use gene-environment interactions to test whether the associations between bullying victimisation and depressive symptoms vary according to individual genetic predispositions. In Chapter 4, I conduct a cross-sectional multi-level analysis using data from the 2018 Program for International Student Assessment survey. I investigate whether the associations between bullying victimisation, psychological distress and life satisfaction varied across 63 countries, and the influence of bullying prevalence, income inequality and national wealth in those associations. RESULTS: The key findings from this thesis are: i) Bullying victimisation is genetically correlated and associated with many traits including attention-deficit/hyperactivity disorder, body mass index, and depression. I detect significant causal effects of ADHD, BMI, and depression on bullying victimisation. ii) The association between bullying victimisation and depressive symptoms is stronger in children with a genetic predisposition to depression and posttraumatic stress disorder. iii) Bullying victimisation is associated with higher psychological distress and lower life satisfaction across national contexts. The association between bullying victimisation and poorer mental health is larger in countries with low bullying prevalence and higher income. CONCLUSION: This thesis presents the first genome-wide analysis of bullying victimisation, establishing a methodological framework that supports both my downstream analyses and future research, thereby advancing understanding in the field. This thesis provides the first comprehensive gene-environment interaction study on bullying victimisation, utilising a sufficiently powered sample. Additionally, this thesis demonstrates for the first time that, in 63 countries, bullying prevalence and national wealth moderate the effects of bullying victimisation on psychological distress and life satisfaction. Interventions that aim to reduce bullying prevalence should be complemented by dedicated support to individuals who still experience bullying as they may be at risk of experiencing heightened mental health consequences.

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