Suppose you are at a bar, enjoying a conversation with some friends. With one brand you get a calorie-free beer, and with another you get a three-calorie beer. Which brand will make you feel that you are drinking a really light beer? Even though the difference between the two beers is negligible, the zero-calorie beer will increase the feeling that you’re doing the right thing, healthwise. You might even feel so good that you go ahead and order a plate of fries.

Dan Ariely, Predictably irrational: The hidden forces that shape our decisions, New York, 2008, p. 63