Approximately 30-40% of patients diagnosed with a mental illness that are treated with a current gold standard psychotherapy fail to adequately respond. Moreover, mental health disorders are consistently associated with decreased well-being. In contrast, social interactions and prosocial behaviors (i.e., acting for the benefit of other people) are known to increase well-being, but are infrequently explored in current clinical theories and interventions. In this research proposal I will examine how mental health can be improved, prosocial behaviors increased, and how the two interrelate. The overall objective of the research described in this grant is to test the effectiveness of interventions designed to promote mental health and prosocial behaviors. This will be conducted in three studies: one continuation study from the initial funding period and two new studies. The continuation study is a clinical effectiveness trial called “Choose Change” that tests the long-term effects of Acceptance and Commitment Therapy (ACT) for a transdiagnostic group of treatment non-responders in both inpatients and outpatients. ACT trains people to increase psychological flexibility as a means to increasing well-being. Recruitment of patients into this trial are 91% complete and follow up assessments are ongoing. Preliminary analyses are promising. Expected results include information on the immediate and long-term effectiveness of training psychological flexibility for treatment non-responders, moderating effects of social variables, and information about mechanisms of change via the use of Event Sampling Methodology (ESM) throughout the trial. Advances in digital technology has led to a rapid growth of apps marketed to help people with mental health problems. Unfortunately, few have been tested and extant studies indicate that poor user utilization and adherence limits their effectiveness. Building on findings from the digital ESM data gathered during the first funding period (e.g., using technology to initiate behavioral intentions and monitor completions, social interactions, burden of items, etc.) this feasibility study will develop a “Virtual Coach” (app with integrated ESM) and test whether the combination of content based on psychological flexibility and state-of-the-art design (gamification, signaling, user options) can generate high adherence and treatment utility (i.e., via direct feedback to the patient and therapist). Expected results include information about adherence, utility, and mechanism of action that can be used to help patients increase well-being and deliver information needed to tailor interventions and apps that fit patients’ current context and needs. The second new study uses an experimental design to test the conditions that are helpful in promoting prosocial behaviors in non-clinical small groups. Three groups of dyad strangers will be exposed to varying levels of a micro-intervention that promotes psychological flexibility: no, one, or both members of the dyad. The outcomes will include economic games that measure prosocial behaviors. The games will include phases that promote or discourage prosocial behaviors. Expected results include information about whether prosocial behaviors can be increased, selfish behaviors decreased, and well-being improved using a micro-intervention. By linking methods with the other studies, the breadth of the theories will be tested and information generated to further improve well-being across settings. These studies will inform clinical theory, intervention science, and the prosocial literature. By linking methods, interventions, and theories, the three studies will also cross-pollinate the literatures of clinical psychology, well-being, social psychology, and behavioral economics. Importantly, results will directly and immediately advance clinical care for patients with mental disorders, with implications for the promotion of well-being in the general population.