Reciprocity refers to a norm in social psychology that a person feels obligated to respond positively to another positive action taken towards them.

When I was working as a door knocker salesman doing fundraising for different charities in Sydney about back in 2020. We used to persuade donors to sign a contract to donate to charities on a monthly basis, but when we tried asking for donations from people who will receive Coles petrol vouchers in return, the sales went skyrocket.

Why is that? The reason is obvious.