The Slot Machine

One of Skinner's most important discoveries is that behavior reinforced intermittently (as opposed to consistently) is the most difficult to extinguish. In other words, intermittent rewards beat predictable rewards. It's the basis of most animal training, but applies to humans as well... which is why slot machines are so appealing, and one needn't be addicted to feel it.

From a Time magazine feature story on multitasking:
Patricia Wallace, a techno-psychologist,...believes part of the allure of e-mail--for adults as well as teens--is similar to that of a slot machine.
"You have intermittent, variable reinforcement," she explains. "You are not sure you are going to get a reward every time or how often you will, so you keep pulling that handle."

From Kathy Sierra's Is Twitter TOO good? post. It's really a shame Kathy stopped blogging on Creating Passionate Users, she did a lot of great posts.