The norm of reciprocity is the expectation that a person's actions will be repaid in kind by other competent social participants. This means that giving something good to another person will eventually yield benefits for the giver. In contrast, it is usually expected that harm or hostility will be met with negative responses. Many people attempt to use the norm of reciprocity to explain why socialized individuals can be tricked by confidence games, as well as how this rule makes individuals more receptive to advertising and sales.
There are many cases in which the norm of reciprocity is demonstrated, but one of the most interesting uses is in confidence games. In this type of con, the con artist typically makes a gesture of goodwill toward the target that the con artist can safely know will never need to be fulfilled. The target, obeying the norm of reciprocity, is then inclined to make a gesture of goodwill toward the con artist, typically involving money. This con is extremely effective on well-socialized individuals who do not suspect a trick, and the desire to demonstrate reciprocity is often highly compelling.
What is possibly most interesting about this case is that the gesture alone is a sufficient demonstration of goodwill to invoke the rule, meaning that tangible gain is not necessarily a component of the norm of reciprocity. In advertising campaigns where a small gift is given in order to make the potential client more likely to listen, it is possible that the actual type of gift given is not nearly as important as the gesture being made. The value or desirability of the gift is not important, and something as small as a button or a pen is enough to demonstrate the positive thoughts necessary to invoke this rule and make a potential customer more receptive.
People who are considered unsocialized participants, such as children or the mentally ill, are typically exempt from these expectations in the short term, although arguably altruism is expected to be returned in the long run. This is the reason that adults interacting with children do not often get angry when this rule is violated by the selfishness of a child, whereas the adult would get very upset with another adult. It is generally expected that through socialization children are taught to interact with others according to this rule. On the other hand, some people argue that the norm of reciprocity has deeper evolutionary roots that explain the compelling revulsion many people feel when this rule is violated.