The file(s) associated with this record are no longer available. [JC][04.02.2013]One of the most interesting aspects of human beings is their ability to choose a course of action and strive to achieve it. When participating in therapeutic interventions involving physical activities, people may allocate different amounts of effort, persistence and commitment to succeed in them. The reason for this difference lies in their motivation. The present thesis focuses on the energising of behaviour, that is, the differential effort and motivation that people put into therapeutic activities. Placebo responses are generally explained by the mechanisms of response expectancy, conditioning and motivational concordance. Findings presented in this thesis partially supported motivational concordance, testing for the first time that therapeutic outcome after engagement in intrinsically motivated tasks requiring physical activity was explained both by response expectancy and motivational concordance . The effects of response expectancy, perceptions of effort and intrinsic motivation on therapeutic benefit and mood change were investigated in both laboratory (Studies 1-4) and real-life therapeutic contexts (Study 5). Study 1 showed that effort mediated the effects of expectancy on perceived benefit, and effort predicted both positive and negative affect following the performance of a breathing exercise. Study 2 showed that differences in outcome between guided imagery and meditation were very small, but that non-specific factors play the major role in outcome. Study 3 showed that perceiving a task as difficult enhances effort perceptions, intrinsic motivation and therapeutic outcome. Motivated behaviour predicted therapeutic outcome but not expectancy. Study 4 showed that the provision of success feedback enhances outcome expectancies, motivated behaviour and mood change. Expectancies, motivation and effort predicted positive affect, whereas only effort predicted negative affect. Finally, results from Study 5 suggest that placebo responses may differ in real-life therapeutic interventions according to the strength motivational factors are elicited within the intervention. Both expectancy and motivated behaviour predicted change in positive affect, whereas motivated behaviour predicted change in negative affect and empowerment. Therapeutic outcome and its underlying mechanisms are likely to reflect a mixture of response expectancies and intervening motivational factors.CONACYT (Mexican council for Science and Technology)