Reciprocity
Reciprocity is the deeply embedded social norm that compels people to return favours, gifts, concessions, and other positive actions. One of Robert Cialdini's six core principles of influence, reciprocity is the basis of gift-giving rituals, the effectiveness of free samples, the social dynamics of negotiation, and the foundation of cooperative social structures. It operates across cultures, appears early in child development, and functions even when the initial gift was uninvited.