This thesis investigates experimentally how social identity affects the economic behavior of individuals in collective decision situations. For a long time, social psychology has used the concept of social identity to explain the behavioral patterns of groups. Recently, the study of social identity also began to emerge as an important research field in economics. My PhD thesis consists of three separate papers. The introductory paper contains a comprehensive survey of the literature on social identity. While I also briefly discuss the most essential social psychological works on group identity, priority is given to the quickly emerging literature from behavioral and experimental research in economics. Based on this, the second paper investigates experimentally the effects of social identity on voting for socially beneficial reforms. The different voting situations make it possible to analyze the effects of social identity on subjects voting behavior and therefore also on subjects social preferences. In addition, the experimental design allows to gain some insight into the relationship between relative in-group size and in-group status. Finally, the third paper studies experimentally the effects of social identity on the decision to vote on the enlargement of franchise with subsequent redistribution among the electorate. Here, “enlargement of franchise” means an extension of voting rights towards a further group of people. The experiment is designed in such a way that it is possible to investigate whether social identification affects subjects fairness preferences and whether subjects social identification depends on in-group status and relative in-group size.