Since the publication of the Brundtland Report in 1987, social inequality has been a topic of concern for the international development community. In the last decade, given the rise of global inequality the subject gained even more prominence as several international organizations (UNDP, World Bank, OECD) began emphasizing the negative impact of social inequality on human well- being. The Agenda 2030, the current development strategy adopted by the United Nations in 2015, elevated “reducing inequality” to one of the 17 Sustainable Development Goals (Goal No. 10). This paper connects with this growing concern over the impact of social inequalities on the opportunities for sustainable development. It proposes a research agenda for the social sciences to contribute to the debate by identifying the causal mechanisms that constitute the nexus between social inequalities and sustainable development. The focus on these intermediary steps is important in order to understand in more detail the barriers that social inequalities pose for more sustainable social, economic and ecological arrangements. This is especially necessary when it comes to designing or implementing strategies (political or technological) that aim to promote sustainable development, above all in highly unequal societies