Reciprocity is a specific psychological behaviour whereby someone responds to actions with similar actions (positive or negative). Although many typically think of this behaviour as a peculiarity or a social construct, evolutionary psychologists approach this as a result of a mutually beneficial agreement. That is to say that reciprocity is not purely altruistic, but rather has value in terms of survival. This hypothesis states that people perform kind acts because it increases the likelihood of a kind act being reciprocated to them.
Robert Trivers is a prominent evolutionary psychologist who helped develop and proposed many of the important concepts surrounding reciprocity. The ideas behind reciprocity can be seen through the lens of game theory. An example of which is the classic prisoner dilemma game played repeatedly with no finite end. Whereas normally, the prisoners end up not cooperating, it is possible that the prisoners can mutually agree to cooperate based on the principles of reciprocity. They will expect the other prisoner to cooperate because they have, and as long as they both expect such then they will both benefit in the long run. From this example we can see that they are not doing this because of a social construct, but rather for survival value.