I read an article on Gamasutra a long time ago about this kind of experiment with mice (sadly, my google-fu was weak.) The researchers compared how much effort

I read an article on Gamasutra a long time ago about this kind of experiment with mice (sadly, my google-fu was weak.) The researchers compared how much effort mice would put into pushing a lever N times to get food vs. pushing a lever that had a 1/N chance of giving food. What they saw was that, if they had a fixed goal, their "motivation" would follow a seesaw pattern: right after they got their prize, they would ignore the lever for a while, but when they were a few pushs from getting food, they would go nuts on it. The random lever got constant attention at a higher average. The author translated this to Diablo: the player has leveling up, which is a fixed, visible goal that the player can strive for, and random equipment drops, that keep him hooked even when other incentives are one or two hours away. Another game to consider: Civilization. I know you all experienced the just-one-more-turn, why-are-birds-chirping-outside effect. It does it by interleaving minor and major goals in different areas, so you don't have motivation valleys, only peak after peak.