honor system
From Longman Dictionary of Contemporary Englishhonor systemˈhonor ˌsystem noun [singular] American English OBEYan agreement between members of a group to obey rules, although no one checks to make sure that they are being followedExamples from the Corpushonor system• In the past, an honor system prevented this contagion of broken trust.• Ticket buying is on the honor system.