Author contributions: M.B., J. Cahlíková, J. Chytilová, and T.Ž. designed research, performed research, analyzed data, and wrote the paper. Data deposition: The data and statistical code allowing replication of reported figures and tables are available at Harvard Dataverse (

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SignificanceWe provide experimental evidence on peer effects and show that behavior that harms members of a different ethnic group is twice as contagious as behavior that harms coethnics. The findings may help to explain why ethnic hostilities can spread quickly (even in societies with few visible signs of interethnic hatred) and why many countries have adopted hate crime laws, and illustrate the need to study not only the existence of discrimination, but also the stability of attitudes and behaviors toward outgroup members.

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