I recently read about a study (sorry, I don't have the exact citation or the exact numbers involved), but here was the general details of the study...
The researchers split the group into 2 large groups of people. The first group they gave one of these typical "energy drinks" that you see everywhere.
Now here's the important part... they specifically told this first group of people that this energy drink they were receiving would increase their strength and endurance levels and increase their energy output.