In psychology, the Zeigarnik effect states that people remember uncompleted or interrupted tasks better than completed tasks. In Gestalt psychology, the Zeigarnik effect has been used to demonstrate the general presence of Gestalt phenomena: not just appearing as perceptual effects, but also present in cognition.
Psychologist Bluma Zeigarnik first studied the phenomenon after her professor, Gestalt psychologist Kurt Lewin, noticed that a waiter had better recollections of still unpaid orders. However, after the completion of the task - after everyone had paid - he was unable to remember any more details of the orders. Zeigarnik then designed a series of experiments to uncover the dynamic facts underlying this phenomenon. Her research report was published in 1927.
The advantage of remembrance can be explained by looking at Lewin's field theory: a task that has already been started establishes a task-specific tension, which improves cognitive accessibility of the relevant contents. The tension is relieved upon completion of the task. If the task is interrupted, the reduction of tension is impeded. Through continuous tension, the content is made more easily accessible, and can be easily remembered.
The Zeigarnik effect suggests that students who suspend their study, during which they do unrelated activities (such as studying unrelated subjects or playing games), will remember material better than students who complete study sessions without a break (McKinney 1935; Zeigarnik, 1927).
The Zeigarnik effect should not be confused with the Ovsiankina effect. Maria Ovsiankina was a colleague of Bluma Zeigarnik who investigated the effect of task interruption on the tendency to resume the task at the next opportunity.
The reliability of the effect has been a matter of some controversy. Several studies attempting to replicate Zeigarnik's experiment, done later in other countries, failed to find significant differences in recall between finished and unfinished (interrupted) tasks (e.g. Van Bergen, 1968).
[...] there is controversy regarding the reliability of the Zeigarnik effect [...]