Motivation, treatment engagement and psychosocial outcomes in outpatients with severe mental illness: a test of Self-Determination Theory.

Currently, it is unclear whether Self-Determination Theory (SDT) applies to the mental health care of patients with severe mental illness (SMI). Therefore, the current study tested the process model of SDT in a sample of outpatients with SMI. Participants were 294 adult outpatients with a primary diagnosis of a psychotic disorder or a personality disorder and their clinicians (n = 57). Structural equation modelling was used to test the hypothesized relationships between autonomy support, perceived competence, types of motivation, treatment engagement, psychosocial functioning and quality of life at two time points and across the two diagnostic groups. The expected relations among the SDT variables were found, but additional direct paths between perceived competence and clinical outcomes were needed to obtain good model fit. The obtained process model was found to be stable across time and different diagnostic patient groups, and was able to explain 18% to 36% of variance in treatment engagement, psychosocial functioning and quality of life. It is concluded that SDT can be a useful basis for interventions in the mental health care for outpatients with SMI. Additional experimental research is needed to confirm the causality of the relations between the SDT constructs and their ability to influence treatment outcomes.