Christmas time is not just traffic, snow and sweets.. it is also the time for exchanging wishes and gifts. And in doing that, we all show evidence of a behavioral rule that is one of the most widespread across different cultures and latitudes: reciprocation.
The scientific interest on Christmas started in mid seventies with a famous sociometric field experiment: a researcher sent Christmas cards to 600 strangers, and he almost immediately discovered that more than one third reciprocated by sending him back kind and warm greetings (Kunz & Woolcot 1976). Most of respondents, moreover, kept doing that for several years after the experiment. Since then the awareness about the importance of the reciprocating mechanism has increased both in science and society. For instance, in society reciprocation has been "exploited" by charities (if you send a small gift along a request for funds, you raise more money) and it has become a main argument against politicians' excessive fundraising.
But studies on reciprocation have also played a key role in the understanding of society, in particular on the evolution of social norms. Furthermore, studies on reciprocation and altruism have been the backbone of Neuroeconomics development (for instance pioneering works like Fehr et al. 2005), and they have allowed to identify several neural circuits involved in prosocial behavior. In a few words, emotions, strategic behavior, reputation, reciprocation, fairness and altruism are all at play, but the neural evidence provided by most recent works is in favour of a prominent role played by substrates for reward processing (Phan et al. 2010), and thus supporting the theory of social exchange (Krasnow et al. 2012): in our brains giving is rewarding because so is receiving.