"In this case, the presence of a salad on the menu has a liberating effect on people who value healthy choices," Fitzsimons said. "We find that simply seeing, and perhaps briefly considering, the healthy option fulfills their need to make healthy choices, freeing the person to give in to temptation and make an unhealthy choice. In fact, when this happens people become so detached from their health-related goals, they go to extremes and choose the least healthy item on the menu." VIA Sciguy
Now imagine what chaos would ensue if Diet Coke was thrown into the picture.