Discrimination has persisted in our society despite steady improvements in explicit attitudes toward marginalized social groups. The most common explanation for this apparent paradox is that due to implicit biases, most individuals behave in slightly discriminatory ways outside of their own awareness (the dispersed discrimination account). Another explanation holds that a numerical minority of individuals who are moderately or highly biased are responsible for most observed discriminatory behaviors (the concentrated discrimination account). We tested these 2 accounts against each other in a series of studies at a large, public university (total N = 16,600). In 4 large-scale surveys, students from marginalized groups reported that they generally felt welcome and respected on campus (albeit less so than nonmarginalized students) and that a numerical minority of their peers (around 20%) engage in subtle or explicit forms of discrimination. In 5 field experiments with 8 different samples, we manipulated the social group membership of trained confederates and measured the behaviors of naïve bystanders. The results showed that between 5% and 20% of the participants treated the confederates belonging to marginalized groups more negatively than nonmarginalized confederates. Our findings are inconsistent with the dispersed discrimination account but support the concentrated discrimination account. The Pareto principle states that, for many events, roughly 80% of the effects come from 20% of the causes. Our results suggest that the Pareto principle also applies to discrimination, at least at the large, public university where the studies were conducted. We discuss implications for prodiversity initiatives.