BackgroundUniversal social and emotional learning interventions can produce significant practical improvements in children’s social skills and other outcomes. However, the UK evidence base remains limited.ObjectivesTo investigate the implementation, impact and cost-effectiveness of the Promoting Alternative THinking Strategies (PATHS) curriculum.DesignCluster randomised controlled trial. Primary schools (n = 45) were randomly assigned to implement PATHS or to continue with their usual provision for 2 years.SettingPrimary schools in seven local authorities in Greater Manchester.ParticipantsChildren (n = 5218) in Years 3–5 (aged 7–9 years) attending participating schools.InterventionPATHS aims to promote children’s social skills via a taught curriculum, which is delivered by the class teacher, generalisation activities and techniques, and supplementary materials for parents. Schools in the usual provision group delivered the Social and Emotional Aspects of Learning programme and related interventions.Main outcome measuresChildren’s social skills (primary outcome, assessed by the Social Skills Improvement System); pro-social behaviour and mental health difficulties (Strengths and Difficulties Questionnaire); psychological well-being, perceptions of peer and social support, and school environment (Kidscreen-27); exclusions, attendance and attainment (National Pupil Database records); and quality-adjusted life-years (QALYs) (Child Health Utility 9 Dimensions). A comprehensive implementation and process evaluation was undertaken, involving usual provision surveys, structured observations of PATHS lessons, interviews with school staff and parents, and focus groups with children.ResultsThere was tentative evidence (at ap-value of < 0.10) that PATHS led to very small improvements in children’s social skills, perceptions of peer and social support, and reductions in exclusions immediately following implementation. A very small but statistically significant improvement in children’s psychological well-being [d = 0.12, 95% confidence interval (CI) –0.02 to 0.25;p < 0.05) was also found. No lasting improvements in any outcomes were observed at 12- or 24-month post-intervention follow-up. PATHS was implemented well, but not at the recommended frequency; our qualitative analysis revealed that this was primarily due to competing priorities and pressure to focus on the core academic curriculum. Higher levels of implementation quality and participant responsiveness were associated with significant improvements in psychological well-being. Finally, the mean incremental cost of PATHS compared with usual provision was determined to be £29.93 per child. Mean incremental QALYs were positive and statistically significant (adjusted mean 0.0019, 95% CI 0.0009 to 0.0029;p < 0.05), and the incremental net benefit of introducing PATHS was determined to be £7.64. The probability of cost-effectiveness in our base-case scenario was 88%.LimitationsModerate attrition through the course of the main trial, and significant attrition thereafter (although this was mitigated by the use of multiple imputation of missing data); suboptimal frequency of delivery of PATHS lessons.ConclusionsThe impact of PATHS was modest and limited, although that which was observed may still represent value for money. Future work should examine the possibility of further modifications to the intervention to improve goodness of fit with the English school context without compromising its efficacy, and identify whether or not particular subgroups benefit differentially from PATHS.Trial registrationCurrent Controlled Trials ISRCTN85087674 (the study protocol can be found at:www.journalslibrary.nihr.ac.uk/programmes/phr/10300601/#/).FundingThis project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full inPublic Health Research; Vol 6, No. 10. See the NIHR Journals Library website for further project information.