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Why do we forget?  There are two simple answers to this question. 

First, the memory has disappeared - it is no longer available.  Second, the memory is still stored in the memory system but, for some reason, it cannot be retrieved.

These two answers summaries the main theories of forgetting developed by psychologists.  The first answer is more likely to be applied to forgetting in short term memory, the second to forgetting in long term memory.

    Forgetting information from short term memory (STM) can be explained using the theories of trace decay and displacement.

    Forgetting from long term memory (LTM) can be explained using the theories of interference and lack of consolidation.

Trace Decay Theory of Forgetting

This explanation of forgetting in short term memory assumes that memories leave a trace in the brain. A trace is some form of physical and/or chemical change in the nervous system.  Trace decay theory states that forgetting occurs as a result of the automatic decay or fading of the memory trace. Trace decay theory focuses on time and the limited duration of short term memory.

This theory suggests short term memory can only hold information for between 15 and 30 seconds unless it is rehearsed.  After this time the information / trace decays and fades away.

No one disputes the fact that memory tends to get worse the longer the delay between learning and recall, but there is disagreement about the explanation for this effect. 

According to the trace decay theory of forgetting, the events between learning and recall have no affect whatsoever on recall.  It is the length of time the information has to be retained that is important. The longer the time, the more the memory trace decays and as a consequence more information is forgotten.

There are a number of methodological problems confronting researchers trying to investigate the trace decay theory.  One of the major problems is controlling for the events that occur between learning and recall.  Clearly, in any real-life situation, the time between learning something and recalling it will be filled with all kinds of different events.  This makes it very difficult to be sure that any forgetting which takes place is the result of decay rather than a consequence of the intervening events.

Support for the idea that forgetting from short-term memory might be the result of decay over time came from research carried out by Brown (1958) in the United Kingdom, and Peterson and Peterson (1959) in the United States. The technique they developed has become known as the Brown-Peterson task.


There is very little direct support for decay theory as an explanation for the loss of information from short-term and long-term memory.  One of the problems with decay theory is that it is more or less impossible to test it.  In practice, it is not possible to create a situation in which there is a blank period of time between presentation of material and recall.  Having presented information participants will rehearse it.  If you prevent rehearsal by introducing a distracter task, it results in interference.

Decay theory has difficulty explaining the observation that many people can remember events that happened several years previously with great clarity, even though they haven't thought about them during the intervening period.  If our memories gradually decayed over time, then people should not have clear memories of distant events which have lain dormant for several years.  However, there is evidence to suggest that information is lost from sensory memory through the process of decay (Sperling, 1960).

Displacement from STM

Displacement seeks to explain forgetting in short term memory, and suggests it’s due to a lack of availability.

Displacement theory provides a very simple explanation of forgetting.  Because of its limited capacity, suggested by Miller to be 7+/- 2 items, STM can only hold small amounts of information.

When STM is 'full', new information displaces or 'pushes out’ old information and takes its place.  The old information which is displaced is forgotten in STM.

It was also assumed that the information that had been in the short-term store for the longest was the first to be displaced by new information, similar to the way in which boxes might fail off the end of a conveyor belt - as new boxes are put on one end, the boxes which have been on the conveyor belt the longest drop off the end.

Support for the view that displacement was responsible for the loss of information from short-term memory came from studies using the 'free-recall' method. 

A typical study would use the following procedure: participants listen to a list of words read out a steady rate, usually two seconds per word; they are then asked to recall as many of words as possible. They are free to recall the words in any order, hence the term 'free recall'.

The findings from studies using free recall are fairly reliable and they produce similar results on each occasion.  If you take each item in the list and calculate the probability of participants recalling it (by averaging recall of the word over all participants) and plot this against the item's position in the list, it results in the serial position curve (Figure 1).

serial position effect

Fig 1. Simplified representation of the serial position curve for immediate recall

Good recall of items at the beginning of the list is referred to as the primacy effect and good recall if items at the end of the list are referred to as the recency effect. The displacement theory of forgetting from short-term memory can explain the recency effect quite easily. The last few words that were presented in the list have not yet been displaced from short-term memory and so are available for recall.

The primacy effect can be explained using Atkinson & Shiffrin's (1968) multi-store model which proposes that information is transferred into long-term memory by means of rehearsal.

The first words in the list are rehearsed more frequently because at the time they are presented they do not have to compete with other words for the limited capacity of the short-term store.  This means that words early in the list are more likely to be transferred to long-term memory.

So the primacy effect reflects items that are available for recall from long-term memory. However, words in the middle of the list used to be in short term memory until they were pushed out - or displaced by the words at the end of the list.


Displacement theory provided a good account of how forgetting might take place in Atkinson & Shiffrin's (1968) model of short-term memory.  However, it became clear that the short-term memory store is much more complex than proposed in Atkinson and Shiffrin's model (re: working memory). 

Murdock’s (1962) serial position experiment supports the idea of forgetting due to displacement from short term memory