Its possible to imagine other complexities to this model. For example, some people may change their behavior without necessarily changing their attitudes beforehand, because of compulsion or pressure of some kind. So the revised model might look more like this...(brown being participants changing their behavior due to compulsion)
PS: For more reading on set relationships and concepts of sufficient and necessary causal conditions, I highly recommend:
Goertz, Gary, and James Mahoney. 2012. A Tale of Two Cultures: Qualitative and Quantitative Research in the Social Sciences. Princeton University Press. http://www.amazon.co.uk/Tale-Two-Cultures-Qualitative-Quantitative/dp/0691149712/ref=sr_1_1?ie=UTF8&qid=1353850106&sr=8-1.