Field experiments on helping interactions have sometimes failed to demonstrate discrimination against ethnic/racial minorities. Is this evidence that prejudice has become weaker or rather that social norms against the expression of prejudice have become stronger? We propose that the mixed evidence points to variation in the inhibiting pressure of social norms. Drawing on the low-cost hypothesis in conjunction with studies on ethnic/racial prejudice, we predict discrimination when cost considerations offer a non-racist justification for withholding help from a non-white person. Combining insights from economics, sociology and social psychology, we propose a middle-range theory of normative action that visualizes the actor as possessing the qualitatively different motives of self-interest, the need for (self-) approval, and racial/ethnic prejudice, which have greater or lesser motivational force depending on situational incentives. We performed a large experiment in the streets of Paris (n>4,000) in which twelve testers commonly recognized as arabe, blanc or noir (as validated through a prior online survey) sought help from randomly selected pedestrians in two cost conditions (pre-registration: https://osf.io/74b6m). When the cost of helping the tester is minimal, pedestrians do not exhibit racial/ethnic discrimination. But when the cost is more tangible, we find substantial evidence that pedestrians discriminate against arabe and noir testers.