At some point or another, everyone has felt the obligation to return a favour, even if you don’t really want to. Or, maybe you have done someone a favour, keeping in mind that they will have your back in the future. It happens all the time, whether it may be lending a friend money when they forget their wallet, knowing they will return the favour, or asking someone what mark they received on a test; you only ask if you’re willing to share your grade. Cialdidi suggests that this sort of reciprocity determines many of our daily interactions (1997). Essentially, we help those who help us, contributing to the maintenance of balanced relationships. Regan, conducted an experiment demonstrating this reciprocity obligation (1971). In the study, confederates give an acquaintance a free drink, stating that they asked the experimenter if they could get a drink, and when the experimenter said yes, the confederate decided to bring them a drink as well. Later, the confederate asks the person they gave the drink to to the buy raffle tickets, and those who were given a drink were twice as likely to buy tickets (Regan, 1971). Even in a lab setting, a simple and casual demonstration of altruism doubled the chances of someone then later doing them a favour, therefore altruism is reciprocal.
Cialdini, Robert B. (1997) The Psychology of Persuasion.
Regan, Dennis T. (1971). Effects of a Favor and Liking on Compliance. Journal of Experimental Social Psychology 7, 627 - 639.