Reinforcement is any event that follows a behavior and increases the future probability of that behavior. When a rat presses a lever and receives food, lever-pressing becomes more frequent. When a student receives praise for completing homework, homework completion tends to rise. The defining criterion is the strengthening effect, not the subjective pleasantness of the event.

Positive reinforcement adds a desirable stimulus. Negative reinforcement removes an aversive one—taking aspirin removes a headache and thereby reinforces aspirin-taking. Both forms increase behavior. Schedules of reinforcement further shape the pattern: continuous reinforcement produces rapid acquisition; intermittent schedules produce greater resistance to extinction.

The concept is central to operant conditioning. It differs from classical conditioning, which pairs stimuli rather than reinforcing responses. Applied behavior analysis relies heavily on careful delivery of reinforcement to build adaptive skills.

• Increases the likelihood of a preceding behavior
• Positive form adds a stimulus
• Negative form removes an aversive stimulus
• Effectiveness depends on immediacy and schedule

Misapplication—reinforcing the wrong behavior or delivering reinforcement inconsistently—can produce unwanted results.