Partial reinforcement is the delivery of reinforcement after only some occurrences of a response. The behavior is strengthened, yet not every instance is followed by a reward.

Schedules may be based on the number of responses (ratio) or on the passage of time (interval), and they may be fixed or variable. Variable-ratio schedules, such as those found in gambling, produce high rates of responding that are highly resistant to extinction. Fixed-interval schedules produce the characteristic scalloped pattern of accelerating response as the end of the interval approaches. Partial reinforcement is more common in everyday life than continuous reinforcement and helps explain why certain behaviors persist long after rewards become infrequent.

The partial-reinforcement extinction effect is well established: behavior acquired under partial reinforcement extinguishes more slowly than behavior acquired under continuous reinforcement. The organism has learned that rewards are intermittent, so the absence of reward is less informative.

When reinforcement arrives only some of the time, the behavior often becomes especially persistent.