Regression, in psychology, reverting to behavior that is less mature than is normal for a person's age and personality. Regression is probably caused by frustration, or the thwarting of some desire or mature way of acting. When a new child is born into a family, for example, a five-year-old may revert to using baby talk and in other ways act like a two-year-old. Such behavior is regression, probably caused by the frustration the older child feels when he receives less attention from his parents. Adults also may regress, as when they pout or go into fits of childish anger. Occasional regression, particularly when some immediate cause is apparent, is not unusual for most normal persons. But regression throughout all behavior of a person is a sign of mental illness.

The name regression is also given to loss of memory associated with old age. Such forgetting is called regression because the more recent memories are lost first, often making older memories seem clearer. Under hypnosis some persons may be made to regress in their memories so that they re-member long-forgotten events.