Motivation researcher Robert Cialdini conducted an experiment to see what kind of message would prompt people to save energy. He and his students placed signs in several homes in certain neighbourhoods, exhorting people to conserve energy for one of four reasons: (1) to save money; (2) to save the environment; (3) to benefit future generations, and (4) because the majority of your neighbours are doing it. Other houses had no signs posted. After one month, Cialdini found that the message about peer energy use provoked people to be the most efficient. Later, he sent letters to homeowners telling them how their energy use compared with their neighbours, which prompted even stronger energy conservation.
The article in the New York Times explains this research in more detail: