Generally, economists use models based on the idea that people are motivated only by their material self-interest. In recent years, economists discovered that self-interest is an important human motivation, but it is not the only one. Several results from Experimental Economics have shown that we must consider other kinds of motivations, because the behaviour of people involved in experiments is interpretable only if we take into account, at least, the two paradigms of "Homo altruisticans" and "Homo reciprocans". In this paper I try to contribute at the building of a theory of reciprocity, using one of the most important feature of Experimental Economics: the possibility to compare results from slightly different experiments, in such way to link the specific behaviour registered in each experiment with its particular scheme of incentives. This is a useful exercise to go in deep in the knowledge of different aspects of reciprocity. A particular attention is given to the study of some implications of the reciprocity paradigm on contracts and labour market features.