Norm of reciprocity

Norm of reciprocity means if someone does something nice for you, you should do something nice for them too. Think of it like a game of give and take - when someone is nice to you and does something for you, you should be nice back and do something for them. For example, when you go to a birthday party, you usually bring a gift to the person who is having the party. That is an example of the norm of reciprocity - the person threw a party for you, now you have to give something back in return.