Some of you will be familiar with research that shows that emotional feelings can be triggered by the body, rather than the other way round. We are happy because we smile, rather than smiling because we are happy. For example, if people put a pencil between their teeth and are asked to rate a cartoon or video, they find them funnier than people who place a pencil between their lips (try it and see which is more like a smile). See here for descriptions of such studies.
An American dermatological surgeon, Eric Finzi, has applied this to the treatment of depression. He injects botox into the ‘frowning’ muscle of depressed patients (the corrugator muscle) to prevent it contracting. He suggests that people can cure themselves by just looking happy and has success stories to support this, documented in his book The Face of Emotion: How botox affects our mood and relationships.