Norms regulate cooperation in society but are sometimes threatened by norm violators. To sustain cooperation, people need to be willing to enforce norms by negatively sanctioning norm violators. However, people may also act against the norm by assisting the norm violator’s actions. Why people sometimes react to norm violators by enforcing the norm and other times by acting against the norm is an unresolved puzzle. We study whether responses to norm violations depend on the norm violator’s ethnic group membership. In a field experiment, confederates violate the norm of paying for public transport by attempting to free-ride. Confederates approach travelers who are about to go through check-in gates at Dutch train stations and request to follow them without checking in themselves. We observe whether travelers enforce the norm by rejecting this request or assist violating the norm by helping the confederates to free-ride. In total, 801 travelers were individually approached at 3 train stations by 10 different confederates, 5 with a native-majority background and 5 with an ethnic-minority background. We find that confederates with a native-majority background are more likely to receive help with free-riding than confederates with an ethnic-minority background across all locations, travelers of all age groups, and all experimental sessions. Content analysis of the verbal answers travelers gave show that these answers were more disapproving and less helpful towards confederates with an ethnic-minority background. Our findings reveal double standards in real-life behaviors towards norm violators with different ethnic backgrounds. These double standards are puzzling at best and can promote segregation, increase inequality and undermine cooperation in society.