Efficacy of a Positive Psychology Intervention Through a Mobile App

The aim of this study is to analyze the efficacy of several variations of an already validated Positive Psychology Intervention (PPI) called Best Possible Self in not flourishing population. This will be done through a mobile app specially designed for this aim. Concretely, the design will permit to compare the classic intervention (BPS Future), a new variation (BPS Past), and a combination of both components (BPS Past+Future). Participants will be randomly assigned to one of 3 conditions, in which they will practice the intervention for two weeks: * BPS-PAST + BPS FUT condition: Participants will practice BPS Past for one week, and then they will switch to BPS future for another week. * BPS-FUT condition: Participants will practice BPS Future for two weeks. * CONTROL condition: Participants will practice DAILY ACTIVITIES for two weeks.