According to Robert Cialdini
People want to be with the crowd. It tells them something not only about what’s appropriate, but what’s possible for them.
If we send people in San Diego a message saying the majority of your neighbors are conserving energy on a daily basis, that has more effect than telling them to do it for the environment or to be socially responsible citizens or to save money. If your neighbors are doing it, it means it’s feasible. It’s practicable. You can do it—people like you.
It was very important that we say “people in your neighborhood.” If we said “the majority of Americans,” that wasn’t effective. If we said “the majority of Californians,” that was more effective. If we said “the majority of San Diegans,” that was more effective. But the most effective was “the majority of your neighbors.” That’s how you decide what’s possible for you: what people in your circumstance are able to do.
According to John Farrell in “Solar Power is Contagious, Study Finds”
The study notes that for every 1 percent increase in the number of installations in a single ZIP code, there’s a commensurate 1 percent decrease in the amount of time until the next solar installation.