In 2014 Ayelet Gneezy of UC San Diego and Nicholas Epley of Chicago Booth published “Worth Keeping but Not Exceeding.” In one experiment each participant was paired with a volunteer who promised to solve 10 puzzles. Some volunteers secretly solved 5 or 15 instead.
Participants valued a kept promise more highly than a broken one. A promise exceeded made them no happier than a promise kept. Participants did not notice the extra effort.