Motivation and habit
Variable rewards
Also called Variable ratio reinforcement
Rewards that arrive unpredictably keep people coming back more than rewards that arrive every time.
When people can't predict what they'll get, checking becomes a habit. A feed that might hold something interesting, or a notification that might be important, gets checked far more often than one that's always the same.
Use it gently, for things people want more of: a new tip, a fresh recommendation, a surprising result. Predictable rewards are better when people need to rely on something, like a payment or a delivery.
In practice
A learning app that sometimes unlocks a bonus lesson after a streak, rather than every time, keeps the moment of finishing interesting.
Common mistake
Building a slot machine. The same mechanism drives compulsive checking and gambling, and people who feel hooked against their will tend to leave and resent it.
Where it comes from
From B. F. Skinner's studies of reinforcement schedules, described with Charles Ferster in Schedules of Reinforcement (1957). Nir Eyal made it part of the hook model in Hooked (2014).