Is Childhood Bullying a Distinct and Causal Risk Factor for Psychotic Experiences? A Systematic Review of Longitudinal Studies

Childhood bullying is a prevalent adversity consistently linked with psychotic experiences in longitudinal studies, yet its causal status and distinct mechanisms remain to be rigorously investigated. This review synthesises longitudinal evidence to determine if bullying is a distinct and causal risk factor. Analysing prospective cohorts, twin designs, and causal models uncovers a clear hierarchy of evidence. Crucially, discordant monozygotic twin studies corroborate a causal environmental effect, with bullied twins exhibiting significantly elevated rates of paranoid ideation and cognitive disorganisation, independent of genetic confounders. Furthermore, bullying constitutes a distinct risk factor, often outweighing that of other adversities, with cumulative "double dose" exposure (e.g., from both peers and siblings) drastically elevating the odds of PEs. Mechanistically, this pathway is partially mediated (≈approximately one-third) by cognitive-affective factors, specifically, an external locus of control and low self-esteem, while a significant direct effect points to additional biological pathways, including hypothalamic-pituitary-adrenal (HPA) axis dysregulation and dopamine sensitisation. Synthesis of this evidence firmly establishes childhood bullying as a potent causal and environmental risk factor for psychosis, which operates via a complex network of interdependent psychological and biological pathways.