We offered coke vs. Diet Coke and sprite vs. diet sprite, and asked people to "guess" which was which. The percentage of individuals who guessed correctly was only 60%, and considering that even a pure guess would get 50% right and people were "on the look out," that is a surprisingly low number.

The point is, I bet if someone switched your soda without telling you you may not even know the difference. Think about it, how many times has a server brought you a drink at a restaurant in an unmarked cup? What are the chances that the server gets it right every time, they are both the same color. Yet, how often have you said, "Excuse me, I believe you brought me Diet Coke instead of regular?" Most people would just drink it and not even think about it.

I suspect that a lot of the taste difference between diet and regular is just mental.