What explains large and persistent differences in reciprocity across social groups? This paper exploits variation in historical experience of democracy over space and time in Switzerland to highlight its strong positive association with reciprocity today. Individuals from regions that experienced democracy since the Middle Ages display stronger reciprocity than individuals from regions that acquired democracy only after the invasion by Napoleon. Because historical democracy was widespread in Swiss German but limited in Swiss French-speaking regions, individuals from these groups differ widely in their reciprocity. The difference, however, disappears when we compare Swiss Germans and Swiss French from regions without historical democracy. These results are not capturing current institutions, beliefs, migration, historical dynasties, language and other group-specific characteristics. Further results suggest that the emergence of historical democracy was due to idiosyncratic events and that its effect on reciprocity persists due to intergenerational transmission.