Dissertation of the University of Utrecht Volunteer work, membership of voluntary associations, philanthropy and donation of blood and organs are studied by scholars from a wide range of disciplines. Sociologists and economists assume that good intentions are universal, but that some people have a stock of human and social capital that allows them to fulfill these intentions while others lack the resources to do so. Thus, they have studied the effects of financial capital (income and wealth), human capital (level of education and health), and social capital (community and religious involvement) on giving and volunteering. Personality and social psychologists, on the other hand, argue that across a variety of social conditions, some people are more likely to contribute because they have an ‘altruistic personality’: they are more helpful, empathic, or more able to take the perspective of people in need, while others are more likely to refuse, evade, or forget their duties. However, we know very little about the relative effects of personality characteristics and social conditions on prosocial behavior. Therefore, the first research question of this dissertation is: To what extent can giving and volunteering behavior be explained by prosocial motives and other psychological characteristics of people and the social conditions in which they live? But we know even less about the interactive effects of personality characteristics and social conditions. This is the second research question: In which conditions are prosocial motives and other psychological characteristics more strongly related to giving and volunteering? An economist would say: especially if it does not cost them too much. This idea is called the ‘low cost hypothesis’. Another idea in this regard originated in personality psychology, and predicts that people act upon their individual preferences when the expectations of others about their behavior are rather unclear. This idea is called the ‘weak situation hypothesis’. Using data of the Family Survey of ...