Marketing Social Norms.
A Robert Cialdini study conducted in Phoenix hotels couched the green benefits of recycling one's room-towels in two different ways. One stressed co-operation with the hotel and suggested that if visitors recycled towels then the hotel would donate a portion of the economic savings to environmental charities. The other stressed the actual environmental impacts of not recycling towels.
The co-operation argument achieved much smaller success and Cialidini argued that this is because there is no sense of social obligation to co-operate with someone who offers you something provided you perform that favour first. He went on to prove this by achieving greater customer action in relation to signs that declared that the hotel had already made the donations in anticipation of their visitors joining with them in this effort.
But the greatest success of all was achieved in reaction to signs that asked visitors to join with the majority of their fellow guests who have, in the past, agreed to participate in the recycling programme. If they are like us, then we are much more likely to act as they do.