Efficacy of Positive Psychology Interventions to Increase Well-Being: Examining the Role of Dispositional Mindfulness Kimberley H. Seear • Dianne A. Vella-Brodrick Accepted: 30 October 2012 Ó Springer Science+Business Media Dordrecht 2012 Abstract There is increasing evidence that positive interventions enhance well-being, although benefits for individuals will partly depend on the congruence between their unique characteristics and the requirements of an intervention. In this study, dispositional mindfulness was examined as a potential moderator of the efficacy of two interventions: three good things and best possible selves. These interventions were expected to increase positive affect and mental well-being and decrease negative affect, and it was hypothesised that participants high on mindfulness would benefit most. In an online randomised con- trolled trial, 211 participants (159 women, 52 men, M age = 34.00 years) completed baseline measures of well-being and mindfulness, and were randomly assigned into a 7-day intervention or a no-activity control group. Results from 73 participants who completed Time 2 indicated that best possible selves significantly lowered negative affect, with a medium effect size. Follow-up data from 37 participants 2 weeks later indicated that for participants with lower initial mindfulness, those allocated to the best possible selves group experienced increased positive affect, with a large effect size. Motivation to do the assigned intervention was strongly and positively associated with practice, and both motivation and frequency were strongly related to increased mental well-being. These results provide additional support for the efficacy of the best possible selves intervention and indicate that it can be beneficial for individuals with low levels of dispositional mindfulness. Keywords Best possible selves Á Three good things Á Positive interventions Á Positive psychology Á Motivation Á Well-being Á Dispositional mindfulness K. H. Seear Á D. A. Vella-Brodrick (&) School of Psychology and Psychiatry, Monash University, Wellington Road, Clayton, VIC 3800, Australia e-mail: dianne.vella-brodrick@monash.edu 123 Soc Indic Res DOI 10.1007/s11205-012-0193-7