Take the shopper discount cards that you get at your local grocery store or pharmacy. These cards are certainly useful. They save consumers money and sometimes even give them free gifts if they have accumulated enough purchases. But one problem is that the practical value is not very visible. The only information people get about how much they saved is hidden among a half dozen other pieces of information on a lenghty receipt.

But what if stores made the practical value easier to see? They could put up a sign at checkout that shows other people in line how much the person checking out saved. This would make two things happen. First, people would get a better sense of how much they could save by getting the card, encouraging anyone who doesn’t have one yet to get one. Second, it would allow people to see the impressive amounts that some other shoppers were able to save, encouraging them to transmit these remarkable stories of practical value. […], it’s hard to talk about something you can’t see.

-Contagious, p.171