Q.A student learns Hindi vocabulary in the first hour of an evening and French vocabulary in the second hour. The next morning, she finds it very hard to recall the Hindi words she had learnt so carefully. Identify the type of interference at work here and explain, step by step, why it produces this effect.

Forgetting is a routine, universal experience, and psychologists have put forward several theories to explain why it happens — why information we appear to have committed to memory sometimes becomes unavailable when we need it. Three theories in particular have received serious research attention: trace decay, interference, and retrieval failure.

The earliest theory, trace decay (or disuse theory), assumes that learning produces physical changes in the nervous system called memory traces, and that these traces simply fade away when they are not used for a long time. This theory turned out to be inadequate: if forgetting really happened because unused traces decay with time, people who sleep after learning something should forget more than people who stay awake, since sleep offers no chance to 'use' the traces — but the opposite is actually found, with the waking condition producing more forgetting than the sleeping condition. Because of this and other problems, trace decay was largely replaced by interference theory.

Interference theory holds that forgetting happens because different sets of associations stored in memory compete with one another at the moment of retrieval, not because any association decays on its own. Interference can be proactive — earlier learning interfering with the recall of material learned later — or retroactive — later learning interfering with the recall of material learned earlier. This theory has been demonstrated experimentally using designs that compare an experimental group (which learns two lists) against a control group (which learns only one, resting instead of learning the second), and it remains one of the most influential explanations of everyday forgetting.

A third cause is retrieval failure, proposed by Tulving and his associates. This view holds that information stored in long-term memory is, in a strong sense, never actually lost — instead, forgetting reflects an absence or mismatch of retrieval cues at the moment a person tries to recall the information. Cues such as category names, or even the physical setting in which something was originally learned, can dramatically improve access to information that seemed completely unavailable a moment before, which is strong evidence that the information itself was never really 'gone'.

Beyond these three cognitive-level explanations, forgetting can also arise from more extreme causes: Freud proposed that traumatic, painful, or threatening experiences may be actively repressed into the unconscious, making them unavailable for retrieval, and severe emotional crisis can, in rare cases, produce conditions such as the fugue state. More mildly, everyday high anxiety and stress — such as sudden nervousness on receiving an exam paper — can also cause temporary forgetting of well-prepared material.