Abstract
The present study examines the role of beliefs about the malleability of well-being in a randomized, placebo-controlled, online positive psychology intervention targeting 267 German-speaking adults (83% women, mean age = 43.16 years). The participants of the experimental group ("three good things" intervention) and placebo control group ("early memories") reported their levels of happiness before and immediately after the 1-week intervention, as well as 2, 4, and 12 weeks after the intervention. Furthermore, the researchers recorded how participants completed the exercises and to what extent they liked the exercises. This study also assesses the participants' beliefs surrounding two different aspects of malleability (well-being is modifiable in general, and one knows how to change their well-being). Although both aspects of malleability were strongly intercorrelated, beliefs about how to change one's well-being seemed particularly relevant in the context of positive psychology interventions: Those who reported a stronger belief about how to change their well-being liked the intervention better and more often completed the activity as instructed. Further, they reported greater increases in well-being as compared with the control group. We conclude that beliefs about the malleability of well-being might represent an important moderating variable in the effectiveness of positive psychology interventions.