When does a behavior usually decrease according to reinforcement principles?

A behavior typically decreases when it is not reinforced because, according to the principles of operant conditioning, the absence of reinforcement leads to a decline in the frequency of that behavior. Reinforcement strengthens behaviors by providing a consequence that increases the likelihood of that behavior being repeated in the future. Without reinforcement, there is no incentive for the individual to continue engaging in that behavior, and as a result, the behavior is likely to diminish over time.

In contrast, behaviors that are consistently rewarded or followed by reinforcement are more likely to increase in frequency. Frequent practice of a behavior can also lead to improvements or maintenance of that behavior, but without reinforcement, the behavior itself may not sustain.