Although correlates of mental wellbeing have been extensively studied, relatively little is known about how to effectively raise mental wellbeing in local communities by means of intervention. We conduct a randomised controlled trial of the "Exploring What Matters" course, a scalable social-psychological intervention aimed at raising general adult population mental wellbeing and pro-sociality. The manualised course is run by non-expert volunteers in their local communities and to date has been conducted in more than 26 countries around the world. We find that it has strong, positive causal effects on participants' self-reported subjective wellbeing (life satisfaction increases by about 63% of a standard deviation) and pro-sociality (social trust increases by about 53% of a standard deviation) while reducing measures of mental ill health (PHQ-9 and GAD-7 decrease by about 50% and 42% of a standard deviation, respectively). Impacts seem to be sustained two months post-treatment. We complement self-reported outcomes with biomarkers collected through saliva samples, including cortisol and a range of cytokines involved in inflammatory response. These move consistently into the hypothesised direction but are noisy and do not reach statistical significance at conventional levels.
Note that it's not peer-reviewed, but I didn't see any red flags when looking through the paper. If the estimates in the paper turn out to be reliable, it has external validity, persistence, etc., this seems like a surprisingly impactful and scalable intervention.
TL;DR: Big, if true.
Course material can be found here.