Past research suggests that inequality has been the norm throughout most of human history (Piketty, 2014) until the shocks of the 20th century—especially mass mobilization for the world wars—spurred support for progressive public policies around taxation and social welfare (Scheve & Stasavage, 2010, 2012). The present research investigates why these collective events activated people’s beliefs about fairness. We propose that these collective events imbued people with a powerful sense of interdependence, and that this feeling of interdependence gave motivational force to the belief that group members have a moral responsibility to share and contribute to the welfare of the group. We test this link between interdependence and social justice across five studies (N = 1,646) using diverse methodologies and quantitative techniques. As predicted, our analyses revealed that social justice is of greater importance in more interdependent groups of everyday life (Study 1) and for Americans whom feel a stronger sense of interdependence (Study 2). In fact, manipulating interdependence in well-mixed groups fosters social justice behavior, which in turn nurtures stronger perceptions of interdependence (Studies 3 – 5). The implications of this virtuous cycle between interdependence and social justice for social psychology, public policy, and contemporary events are discussed.