honor system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An arrangement by which persons are trusted to act honestly and therefore are not directly monitored, even though persons might obtain a benefit from acting dishonestly.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US A system, policy or understanding based on trust in the honesty, honor, or integrity of others.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a system of conduct in which participants are trusted not to take unfair advantage of others