Theories of Forgetting in Psychology

The Psychology of Forgetting and Why Memory Fails

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 summarize the main theories of forgetting developed by psychologists. The first explanation usually applies to forgetting in short-term memory. The second applies 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, retrieval failure 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 a physical or chemical change in the nervous system. That trace can decay.

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 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 trace decay theory, the events between learning and recall have no effect on recall. What matters is the length of time the information is retained.

The longer that time, the more the memory trace decays. As a result, more information is forgotten.

There are a number of methodological problems confronting researchers trying to investigate trace decay theory. A major one 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. Real life is messy. This makes it hard to know whether any forgetting is the result of decay, or simply a consequence of those intervening events.

Support for the idea that forgetting from short-term memory results from decay came from Brown (1958) in the United Kingdom. Peterson and Peterson (1959) in the United States found similar results. Their technique became known as the Brown-Peterson task.

Evaluation

There is very little direct support for decay theory as an explanation for forgetting in short-term and long-term memory. It is also hard to test.

In practice, researchers cannot create a truly blank period between presenting material and testing recall. Once information is presented, participants rehearse it. Blocking that rehearsal with a distractor task introduces interference instead of a clean test of decay.

Decay theory struggles to explain why people can recall events from many years ago with great clarity. Yet they have not thought about them in the meantime.

If memories gradually decayed over time, people should not have clear memories of distant events that have lain dormant for years. Some decay does happen, though: evidence shows that information is lost from sensory memory through 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 offers a simple explanation of forgetting. STM has a limited capacity, suggested by Miller (1956) to be seven items, plus or minus two. It can only hold a small amount of information at once. That’s a tight limit.

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 whichever information had been in the short-term store the longest was the first to be displaced by new information.

Think of a conveyor belt of boxes. As new boxes are placed on one end, the boxes that have been on the belt longest drop off the other 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 uses the following procedure. Participants listen to a list of words read out at a steady rate, usually two seconds per word. They are then asked to recall as many of the words as possible, in any order, which is why it is called “free recall”.

The findings from studies using free recall are fairly reliable, producing similar results each time. That’s useful.

For each item in the list, researchers calculate the probability of participants recalling it, averaged across all participants. Plotting this probability against each item’s position in the list produces the serial position curve (Figure 1).

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

Good recall of items at the beginning of the list is called the primacy effect. Good recall of items at the end of the list is called the recency effect: the last few words in a list tend to be remembered especially well.

Displacement theory explains the recency effect easily. Those final words have not yet been displaced from short-term memory, so they are still 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 get rehearsed more often. When they are presented, they do not yet have to compete with other words for the short-term store’s limited capacity. This means words early in the list are more likely to be transferred into long-term memory.

So the primacy effect reflects items available for recall from long-term memory. Words in the middle tell a different story. They were once in short-term memory, but were pushed out, or displaced, by the words at the end of the list.

Key Study: Waugh and Norman (1965)

Aim: Waugh and Norman wanted to know why we forget from short-term memory. Is it the number of new items that arrive afterward (displacement), or simply the time that passes (decay)?

Method: Participants heard lists of 16 digits. Some lists were read quickly. Others were read slowly. One digit from partway through the list, the “probe,” was repeated back to them. Participants had to recall the digit that had originally followed it.

The probe technique was new.

Results: Recall got worse as more digits came after the target. Fewer digits meant better recall. More digits meant worse recall. The faster rate helped a little too, when less time had passed overall.

Conclusion: Forgetting from short-term memory depends on how many new items enter the store. This favors displacement. Decay was not ruled out, though. The faster rate’s advantage is exactly what a decay account would also predict.

This remains the central evidence for displacement.

Evaluation

Displacement theory gave a good account of how forgetting might occur in Atkinson and Shiffrin’s (1968) model of short-term memory. Later research complicated that picture. The short-term store turned out to be far more complex than their original model proposed, as the working memory model later showed.

The theory needed updating.

Murdock’s (1962) serial position experiment supports the idea that forgetting from short-term memory is due to displacement, although it could equally be due to decay. In practice, it is often very difficult to tell the two apart.

Newer evidence adds more.

Contemporary research continues to test this capacity assumption directly. Adam, Vogel, and Awh (2017) found that visual working memory has a genuine item limit of around three to four items.

This is a real limit, not a resource that simply spreads more thinly as items are added. This supports the kind of fixed-capacity “slots” that displacement theory’s full-store mechanism depends on.

Interference Theory

If you had asked psychologists during the 1930s, 1940s, or 1950s what caused forgetting you would probably have received the answer “ Interference “.

It was assumed that memory can be disrupted by what we have previously learned, or by what we will learn in the future. Under this idea, information in long-term memory can become confused or combined with other information during encoding. This distorts or disrupts the original memory.

Interference theory states that forgetting occurs because memories interfere with, and disrupt, one another (Baddeley, 1999). There are two ways this can happen:

Proactive and retroactive interference are both more likely when the memories involved are similar, such as confusing old and new phone numbers. Chandler (1989) found that students who study similar subjects at the same time often experience this kind of interference. Similarity is the key.

Previous learning can sometimes interfere with new learning, such as struggling with foreign currency when traveling abroad. New learning can also cause confusion with previous learning: starting to learn French, for example, may disrupt memory for previously learned Spanish vocabulary.

In short-term memory, interference can take the form of distraction. A loud drill outside the classroom door, for example, can stop us processing information properly in the first place.

Key study: Underwood and Postman (1960)

Aim: To investigate how retroactive interference affects learning.  In other words, to investigate whether information you have recently received interferes with the ability to recall something you learned earlier.

Method: In a lab experiment, participants were split into two groups. Both learned a list of paired words (e.g., cat–tree, jelly–moss).

The experimental group also learned a second list with new second words (e.g., cat–glass, jelly–time). The control group did not. All participants then tried to recall the first list.

Results: The recall of the control group was more accurate than that of the experimental group.

Conclusion: This suggests that learning items in the second list interfered with participants’ ability to recall the list.  This is an example of retroactive interference.

Evaluation

Although proactive and retroactive interference are reliable and robust effects, there are a number of problems with interference theory as an explanation of forgetting.

Interference theory has two main weaknesses. First, it tells us little about the cognitive processes involved in forgetting.

Second, most research on interference has used word lists in a laboratory, a situation rarely found in everyday life, meaning the findings may not generalize well.

Baddeley (1990) notes that in these lab studies, the two learning tasks are given too close together, whereas in real life such events are more spread out. That is a fair critique. More recent research addressing this by studying real-life events has still found support for interference theory.

However, there is no doubt that interference plays a role in forgetting, but how much forgetting can be attributed to interference remains unclear (Anderson, 2000).

Lack of Consolidation

The previous accounts of forgetting have focused primarily on psychological evidence, but memory also relies on biological processes.  For example, we can define a memory trace as:

Some permanent alteration of the brain substrate in order to represent some aspect of a past experience”.

When we take in new information, changes to the nervous system are needed to record it properly. This is called the consolidation process. It takes time. During this period, information moves from short-term memory into more permanent long-term memory.

The brain consists of a vast number of cells called neurons, connected to each other by synapses. Synapses enable chemicals to be passed from one neuron to another. Chemicals cross the gap. These chemicals, called neurotransmitters, can either inhibit or stimulate the performance of neurons.

Picture a network of neurons connected via synapses, forming a pattern of stimulation and inhibition. This pattern can act as a basis for storing information. Modifying neurons in this way to form new permanent memories is called consolidation (Parkin, 1993).

There is evidence that the consolidation process is impaired if there is damage to the hippocampus (a region of the brain). This was shown by a famous case. In 1953, HM had brain surgery to treat his epilepsy, which had become extremely severe.

The surgery removed parts of his brain and destroyed the hippocampus, and although it relieved his epilepsy, it left him with a range of memory problems.  Although his STM functioned well, he was unable to process information into LTM.

HM’s main problem was his inability to remember and learn new things. This is called anterograde amnesia. More revealing for consolidation research, though, is what HM could still remember: his memory for events before the surgery.

In general, his memory for events before the surgery remained intact. That part survived. He did, however, have some memory loss for events in the two years leading up to surgery.

Pinel (1993) argues that this challenges Hebb’s (1949) idea that consolidation takes only about 30 minutes. HM’s memory was disrupted for the two years before his surgery. That gap suggests consolidation actually continues for years, not minutes.

Finally, aging can also impair our ability to consolidate information.

Evaluation

Research into consolidation reminds us that memory relies on biological processes. Exactly how neurons are altered during the formation of new memories, though, has not yet been fully explained.

However, there is no doubt that investigating the role of neurons and neurotransmitters will provide new and important insights into memory and forgetting.

Retrieval Failure Theory

Retrieval failure is where information is in long-term memory but cannot be accessed. Such information is available (it is still stored) but not accessible (it cannot be retrieved), because the right retrieval cues are missing.

When we store a new memory, we also store information about the situation. These are known as retrieval cues. Meeting the same situation again can trigger the memory through these cues. Retrieval cues can be:

• External / Context – in the environment, e.g. smell, place etc.
• Internal / State – inside of us, e.g. physical, emotional, mood, drunk etc

There is considerable evidence that information is more likely to be retrieved from long-term memory when appropriate retrieval cues are present. This comes from both laboratory experiments and everyday experience. A retrieval cue is simply a hint that helps retrieval.

Tulving (1974) argued that information is more readily retrieved when the cues present at encoding are also present at retrieval.

Suppose you proposed to your partner while a certain song was playing on the radio. The song stuck with you. You will be more likely to remember the proposal when you hear that song again. It is a retrieval cue, present both at encoding and at retrieval.

Tulving suggested that two kinds of information are stored alongside a memory as it is learned. Both matter. One is the physical surroundings, or external context; the other is the learner’s physical or psychological state, or internal context.

Reinstating the state or context makes recall easier by providing relevant information. Retrieval failure occurs when appropriate cues are not present, for example when we are in a different context or state from when we learned the information.

Context (external) Cues

Retrieval cues may be based on context-the setting or situation in which information is encoded and retrieved. Examples include a particular room, driving along a motorway, a certain group of people, or a rainy day.

Context also refers to how information is presented: printed, spoken, or sung; grouped by category, or presented as an unrelated jumble. Matching context helps. Retrieval is more likely when the context at encoding matches the context at retrieval.

Context shapes memory in small ways too.

You may have felt this effect if you have ever revisited a place you once lived, or an old school. Such visits often trigger memories you did not realize you still had.

Tulving and Pearlstone (1966): Category Cues

Tulving and Pearlstone (1966) asked participants to learn lists of words belonging to different categories, such as animals, clothing, and sports. Participants then tried to recall the words. Those given the category names recalled substantially more words than those who were not.

The category names were the cue.

The categories provided a context, and naming them provided retrieval cues. Tulving and Pearlstone argued that this cue-dependent forgetting explains the gap between the two groups: those who recalled fewer words simply lacked the right retrieval cues.

Baddeley (1975): The Diving Study

Baddeley (1975) asked deep-sea divers to memorize a list of words, either on the beach or underwater. At recall, half of each group stayed in the same environment and half switched to the other.

The environment itself was the cue.

Divers who recalled in the same environment where they had learned the words did much better. They recalled 40% more words than those who switched environments. Matching context helps retrieval.

State (internal) Dependent Cues

The basic idea behind state-dependent retrieval is simple. Memory works best when a person’s physical or psychological state is similar at encoding and at retrieval.

For example, imagine someone tells you a joke on Saturday night after a few drinks. Monday morning, sober, you have forgotten it completely. But have a few drinks again on a later night, and the joke comes back to you. State-dependent retrieval works like that.

State retrieval cues are based on state. This is the physical or psychological state of the person when information is encoded and retrieved.

A person may be alert, tired, happy, sad, drunk, or sober when information is encoded. They are more likely to retrieve it later if they are in that same state again.

Take Tulving and Pearlstone (1966) again. Their study involved external cues, such as category names. Internal cues work too. Cue-dependent forgetting has also been shown with mood state.

Information about current mood is often stored in the memory trace. Mood matters, in other words. There is more forgetting when the mood state at retrieval differs from the mood state at learning. Less forgetting when the two match is known as mood-state-dependent memory.

A study by Goodwin et al. (1969) investigated the effect of alcohol on state-dependent retrieval. The design was simple. People who encoded information while drunk recalled it better in that same state.

For example, participants who hid money and alcohol while drunk often could not find them again while sober, but could when drunk once more.

When they were drunk again, though, the hiding place came back to them. Memory followed the state. Other studies have found similar effects with other drugs, including marijuana.

Matching moods helps memory. People tend to remember material better when their mood at learning matches their mood at retrieval. The effect is stronger for positive moods than negative ones, and stronger still for personally relevant memories.

Evaluation

According to retrieval-failure theory, forgetting occurs when information is available in LTM but is not accessible.  Accessibility depends in large part on retrieval cues.  Forgetting is greatest when context and state are very different at encoding and retrieval.  In this situation, retrieval cues are absent and the likely result is cue-dependent forgetting.

There is considerable evidence to support this theory of forgetting from laboratory experiments. The ecological validity of these experiments can be questioned, but their findings are supported by evidence from outside the laboratory.

For example, many people say they can”t remember much about their childhood or their school days.  But returning to the house in which they spent their childhood or attending a school reunion often provides retrieval cues which trigger a flood of memories.

References

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