Influence: The Psychology of Persuasion

Cialdini, Robert B. Influence: The Psychology of Persuasion. Revised ed. New York: HarperCollins, 2006. E-book.

Abstract:

Influence, the classic book on persuasion, explains the psychology of why people say yes – and how to apply these understandings. Dr. Robert Cialdini is the seminal expert in the rapidly expanding field of influence and persuasion. His thirty-five years of rigorous, evidence-based research along with a three-year program of study on what moves people to change behavior has resulted in this highly acclaimed book. Youll learn the six universal principles, how to use them to become a skilled persuader – and how to defend yourself against them. Perfect for people in all walks of life, the principles of Influence will move you toward profound personal change and act as a driving force for your success.

Social proof is important because people often look to others to decide how to act, especially when unsure. The more relatable the person, the stronger the influence, though this can vary from person to person. In eco games, showing that others are making sustainable choices can encourage players to do the same.
(Often used in Nudging)