Reciprocity is here considered as an internalized social norm, and a questionnaire to measure individual differences in the internalized norm of reciprocity is presented. The questionnaire, Personal Norm of Reciprocity (PNR), measures three aspects of reciprocity: positive reciprocity, negative reciprocity, and beliefs in reciprocity. The PNR has been developed and tested in two cultures, British and Italian, for a total of 951 participants. A cross-cultural study provides evidence of good psychometric properties and generalizability of the PNR. Data provide evidence for criterion validity and show that positive and negative reciprocators behave in different ways as a function of the valence (positive or negative) of the other's past behaviour, the type of feasible reaction (reward versus punishment), and the fairness of their reaction. Copyright ©2002 John Wiley & Sons, Ltd.