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An 8-month-long experimental study examined the immediate and longer term effects of regularly practicing two assigned positive activities (expressing optimism and gratitude) on well-being. More important, this intervention allowed us to explore the impact of two metafactors that are likely to influence the success of any positive activity: whether one self-selects into the study knowing that it is about increasing happiness and whether one invests effort into the activity over time. Our results indicate that initial self-selection makes a difference, but only in the two positive activity conditions, not the control, and that continued effort also makes a difference, but, again, only in the treatment conditions. We conclude that happiness interventions are more than just placebos, but that they are most successful when participants know about, endorse, and commit to the intervention.
Lyubomirsky et al. (Fri,) studied this question.