Influence, New and Expanded: The Psychology of Persuasion review

Influence, New and Expanded is the 2021 revision of Robert Cialdini's foundational 1984 work on the psychology of persuasion. The original book identified six principles — reciprocity, commitment and consistency, social proof, authority, liking, and scarcity — that reliably move people to say yes. The new edition adds a seventh: unity, the sense of shared identity between the influencer and the influenced. Cialdini spent three years undercover as a sales trainee, phone solicitor, and fundraiser to identify these principles, then spent decades documenting their psychological mechanisms.

Each principle exploits a different feature of human decision-making. Reciprocity works because we are wired to repay what we receive and feel uncomfortable with unresolved debts. Commitment and consistency work because we want to behave in line with what we've already said and done — a small yes creates a path toward a larger yes. Social proof works because in uncertain situations we look to what others are doing as a signal of correct behavior. Authority signals expertise and we are inclined to defer to experts. Liking makes us more likely to say yes to people we know, like, and who seem similar to us. Scarcity creates perceived value through restriction. Unity argues that shared group membership — family, ethnicity, political tribe, team — creates a bond that makes compliance almost automatic within the group.