Well-functioning groups enforce social norms that restrain opportunism, but the social structure of a society may encourage or inhibit norm enforcement. Here we study how the exogenous assignment to different positions in an extreme social hierarchy – the caste system – affects individuals' willingness to punish violations of a cooperation norm. Although we control for individual wealth, education, and political participation, low caste individuals exhibit a much lower willingness to punish norm violations that hurt members of their own caste, suggesting a cultural difference across caste status in the concern for members of one's own community. The lower willingness to punish may inhibit the low caste's ability to sustain collective action and so may contribute to its economic vulnerability.