Williams et al. (2006) conducted a clinical trial to test this
SDT-based therapeutic treatment model for smoking cessation.
Patients’ autonomous motivation for treatment and perceived competence
for change were assessed before treatment and six months
later, and their perceptions of the therapist’s autonomy-support
were assessed one month into treatment. The primary outcome was
smoking status at six months, assessed via a biochemically validated
index. Results showed that the SDT intervention was experienced
as more autonomy-supportive than the community-care
alternative, and it led to significantly greater cessation at six
months. Support was also found across conditions for a process
model of change in which perceived autonomy-support led to
increases in both autonomous motivation and perceived competence,
and these motivation variables led to greater cessation.
Follow-ups at 18 and 32 months showed that the enhanced improvement
in the SDT-treatment group was maintained at each
time point (Williams et al., 2006).