Perspective taking, the ability to take another person's perspective, is instrumental in building successful and harmonious partnerships, from romantic relationships to international cooperation. Yet, failure to achieve perspective taking, or egocentrism, is increasingly observed in clinical and healthy populations. Fortunately, perspective taking is a skill that can be acquired through training, but, to date, existing training has only yielded limited results. This project aims to conduct the first epidemiological study of egocentrism (i.e. to assess its prevalence in Europe, its severity, its psycho-sociological determinants, and its consequences on mental health and well-being) and the first large-scale perspective-taking training intervention for both healthy and clinical populations. By distinguishing the profiles of egocentrism and identifying their key determinants, Work Package (WP) 1 will assess perspective-taking performance (and associated psycho-sociological factors) in Alzheimer, addictive disorder, anorexic, and forensic patients and in matched healthy control participantss. Building on the Supervisor’s team expertise in devising and conducting training interventions, WP2 will devise and conduct intervention programs tailored to the distinct egocentrism profiles in the same populations as WP1. WP3 will export the WP1-WP2 methodology into a free web-based assessment and training tool to conduct the epidemiological study and the large-scale training intervention in the general population, from adolescents to seniors. The assessment and training tool will be made available to all clinicians, researchers and all other actors to foster further uses such as for youth education programs, support programs for caregivers, or mental health promotion programs targeting vulnerable populations.