Liking
We say yes far more readily to people we like.
What it means
Liking, as a principle of influence, is the robust tendency to comply with requests from people we like, and to extend trust, attention, and benefit of the doubt to them. Cialdini catalogued the levers that reliably manufacture liking: physical attractiveness (which triggers a halo of assumed good qualities), similarity in background or attitudes, praise and flattery, increased familiarity through repeated contact, cooperation toward shared goals, and positive associations such as good food or attractive settings. The mechanism is partly that liking serves as a fast heuristic — we assume people we like are trustworthy and have our interests at heart — and partly that we are motivated to maintain warm relationships, so refusing a liked requester feels costly. The principle shades into manipulation when these cues are deliberately engineered by strangers whose interests diverge from ours, which is why the recommended defense is to separate one's feeling about the requester from the merits of the request itself. It matters because liking quietly underwrites persuasion across sales, negotiation, fundraising, leadership, and marketing, often more powerfully than the substance of the offer.
Examples
Tupperware-style home parties succeed because the purchase request comes from a friend you like and want to support, not from a faceless salesperson.
A car salesperson who discovers you both sail, then admires your choice of trim, is not making conversation: similarity and praise are the cheapest levers on the forecourt.
Charities ask supporters to approach their own friends, because a request from someone you like is answered far more often than the identical words from a stranger.
First described in Cialdini (1984).