A social norm of responding to a positive action with another positive action, even if the action received has been unwanted. The interesting thing is that the positive action returned is oftentimes bigger than the initial action received.
Primary origin: Robert Cialdini
One of the persuasion principles from Cialdini’s book Influence.
People feel obliged to give back if they receive something first.
It’s heavily used in sales and marketing.
People are more likely to buy if a brand gives free samples first. Waitresses who bring candy with the bill receive more tips.