Exploring the nature, development and effects of complex emotions in Sheffield and Plymouth, UK: a longitudinal mixed methods study protocol

Introduction: Complex emotions include difficulties with emotional regulation, impulsivity, identity, relationships and functioning. These experiences are often construed as borderline personality disorder, a highly stigmatised diagnosis. We will use a transdiagnostic approach to investigate the nature, course and lived experience of complex emotions, with lived experience involvement embedded throughout the programme.

We aim to (1) model symptom and functional trajectories to identify subgroups with distinct needs; (2) elucidate mechanisms underlying outcomes, including interactions with social, cultural and environmental factors; (3) investigate associations between complex emotions and physiological and behavioural measures; and (4) integrate findings to inform interventions and service delivery.

Methods and analysis: We will undertake a mixed methods longitudinal study co-developed with our Lived Experience Advisory Panel (LEAP) and informed by evidence synthesis. We aim to recruit 300 participants who self-identify as experiencing complex emotions through health services, third-sector organisations and probation services, with baseline assessments and 6 monthly follow-up. Two nested substudies will comprise (1) longitudinal biographical interviews and (2) ecological momentary assessment (EMA) of experiential, physiological and behavioural data. Quantitative data will be analysed using multivariate and longitudinal trajectory modelling methods to identify underlying dimensions and phenotypic subgroups. Qualitative data will undergo reflexive thematic and narrative approaches to identify shared experiences, coping strategies and meaning-making. EMA data will be analysed using network and time-series approaches to examine within-person and between-person associations and temporal instability. Findings will be integrated to develop an overall understanding of the nature, trajectories and mechanisms of complex emotions.

Ethics and dissemination: The cohort and qualitative studies were approved by the East Midlands (Derby) National Health Service (NHS) Research Ethics Committee (REC) and the Health Research Authority (HRA) (IRAS 353789, REC 25/EM/0128; 4 July 2025). The EMA substudy was approved by Cambridge East NHS REC and HRA (IRAS 364085, REC 26/EE/0171; 11 August 2026). Dissemination strategies will be co-developed by researchers, LEAP members and lived experience co-investigators. All partners will contribute to peer-reviewed publications, conference presentations and accessible outputs. The anticipated date of study completion is 31 March 2029.