People work harder as they get closer to a goal. This is the goal-gradient effect, first documented by behaviourist Clark Hull in 1932 through experiments with rats in mazes, animals ran faster as they approached the food reward, not at a constant pace throughout. The principle extends cleanly to human motivation. Kivetz, Urminsky, and Zheng's 2006 research on loyalty programs confirmed it: customers who felt closer to a reward accelerated their purchase frequency as they approached it. The practical implication for designers is significant, proximity to completion is itself motivating.