Answer:
The answer is the reciprocity norm.
Step-by-step explanation:
This norm states that people feel the need to reciprocate (make up for) other people's actions. It is especially common for positive gestures. For example, if someone gives us a gift, we feel the need to repay it.
This norm seems to be an important factor for a harmonious society, as it reinforces phenomenons such as reciprocal altruism.