Horizontal inequalities are inequalities among groups of people. This is to be contrasted with vertical inequality which is inequality among all the individuals in a society. Relevant group categories include, among others, race, ethnicity, religion, class, gender and age, with the relevance and importance of any category varying across societies. Horizontal inequalities are important above all because of their implications for justice and social stability. Large horizontal inequalities among groups are likely to undermine pluralism in a society, because they generate grievances between groups and disaffection; however, while reducing horizontal inequalities is a necessary condition for flourishing pluralist societies, by itself it is insufficient.