Forgetting is a common experience, and the first systematic attempt to understand it was made by Hermann Ebbinghaus, who plotted the course of forgetting using nonsense syllables. Several theories have been proposed to explain why it takes place.

The earliest theory is Trace Decay, also known as the disuse theory. It suggests that memory leads to physical changes in the brain called memory traces, which fade away if not used for a long time. However, this theory is considered inadequate because people who sleep after memorising actually forget less than those who remain awake, contradicting the idea that disuse causes decay.

A more influential explanation is the Interference Theory. This theory suggests that forgetting occurs because various sets of associations stored in memory compete with each other at the time of retrieval. While these associations rest independently, interference arises when new information disrupts the recall of earlier memories during retrieval.

Finally, forgetting can also result from Retrieval Failure. Proposed by Tulving, this view states that memory contents may become inaccessible not because they are lost, but because the retrieval cues are either absent or inappropriate at the time of recall.