One of several things Psychology has to teach us. In brief, a random reinforcement schedule maintains interest longer than a fixed schedule (for example, a reward exactly once every five responses).

For the details, read "Behavioral Game Design" by John_Hopson?: http://www.gamasutra.com/features/20010427/hopson_01.htm (free registration required)

Something this article misses is that behavioral researchers do most of their work by watching their test subjects and recording behavior. Do not assume you know what your player is going to consider reinforcement and punishment. Watch and learn. In other words, playtest. And not just yourself, but strangers.

Another point is that a random schedule can enhance both positive and negative reinforcement - AvoidRandomAdversity.

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