Where would you like to be in one year? Motivating judicial supervised substance abusers to change

The main aim of this thesis was to compare the effectiveness of probation supervision plus a motivation enhancing intervention (MEI) with that of supervision as usual (SAU). Moreover, we aimed to explore the predictive role of behavioural measures of impulse control in substance use and criminal behaviour, and of motivation in treatment entry. It is discussed that the lack of difference between the SAU plus MEI and SAU primarily raises three questions: (1) what are the working mechanisms of motivational interviewing (MI)? (2) what is needed for a successful implementation of MI? (3) what offender and context factors are predictive of MI outcome? It is proposed that more research is needed not just on if interventions work, but also on how specific mechanisms and what MI aspects affect behaviour change and for whom and when MI is effective. In addition, it is argued that future research on implementation strategies for MI training in the criminal justice setting, such as probation, is needed and will improve the quality of MI training implementation in these contexts. Based on our finding that motivation plays an important role in predicting treatment initiation, it is argued that the use of MI is advised in offenders with substance use disorders. It is further suggested that the finding that sensitivity to punishment and reward is related to cannabis and alcohol use in offenders, respectively, can be a starting point for further research within supervision into which positive or negative reinforcements can be used to encourage offenders to change their alcohol or cannabis use.