An unasked-for favour creates a debt most people feel compelled to repay

In a classic field study, restaurant servers who left a single mint with the bill saw tips rise by about 3%; two mints handed over 'extra, just for you' raised tips by over 20%, far more than the value of the candy. Cialdini's reciprocity principle: receiving something unprompted creates a felt obligation to give something back, often disproportionate to what was received.

— Robert Cialdini, Influence: The Psychology of Persuasion — 1984
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