Reciprocity bias describes the impulse to reciprocate actions others have done towards us. The desire to return favours, pay back debts and treat others well could have been a decisive evolutionary advantage for humans as it engenders cooperation. However, reciprocity bias can also work in reverse — in response to unfriendly actions, people will deliver back other unfriendly actions!
Robert Cialdini extensively researched reciprocity bias and considers it powerful since it allows one person to choose the nature of the indebting first favour and steer the nature of the debt-cancelling return favour. Because of that, people tend to avoid asking for a (needed) favour if they are not in a position to repay it. The psychological cost may simply outweigh the material loss.
Marketers and salespeople often make use of positive reciprocity, for example, offering a free sweet at the end of a meal can inspire a higher tip as a result of the customer feeling obligated to reciprocate. In his book Influence, Cialdini describes the story of an Indian supermarket operator who sold one thousand pounds of cheese in a few hours simply by inviting customers to cut off slices for themselves as free samples.