Why Do We Forget? Exploring Some Common Theories

FROM THE LECTURE SERIES: Understanding the Mysteries of Human Behavior

By Mark Leary, Ph.D., Duke University

Forgetting is such a common phenomenon in our daily lives. Despite having genuinely amazing memories, we routinely forget things. For years now, cognitive psychologists have been trying to explain why we forget things, using various theories, the most common of them being decay and interference.

Portrait of young student sitting at her desk looking worried and overwhelmed, forgetful.
The trace-decay and interference theories can help us understand why we forget things. (Image: Asier Romero/Shutterstock)

There would be a number of instances where you knew a historical fact, a piece of trivia, or name of your elementary school principal in the past but now are no longer able to recall it. For years, cognitive psychologists have attempted to explain why we forget, using two common phenomenon namely the trace-decay theory and the interference theory. While the trace theory explains that forgetting occurs because memories decay or deteriorate over time, the other explanation is that memories compete and interfere with each other. This article attempts to explain some of the common theories.

Encoding Failures

Sometimes, within a few minutes of meeting someone for the first time, we forget their names. The natural and obvious instinct is to think that we forgot the name of the person. Interestingly, this inability to recall the name doesn’t involve forgetting at all. The name was never stored in the memory to be retrieved later. These instances are called ‘encoding failures’ by cognitive psychologists. In other words, encoding failures occur when information is not stored in memory though it should have been. This makes retrieval of the information difficult.

A research study asked participants to recognize the image of a real penny from 15 drawings. The drawings had Lincoln facing in different directions, the year in different locations, different mottos on the coin’s face, and other changes in the details. The study found that most of the participants were unable to identify the real penny. This is because the information on details of the penny was never registered in the memory. Only sufficient information to distinguish a penny from other coins was encoded in the memory. Thus encoding plays an important role in the retrieval of information from the memory.

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Trace-Decay Theory on Why We Forget

The trace-decay theory states that a memory will fade away or disappear with the passage of time. According to this theory, a memory trace is created every time a new memory is formed. This trace reinforces connections that assist in retaining the new memory in the brain. But if the memory is not recalled regularly, the trace will become weaker and weaker, leading to memory decay and ultimately retrieval failure. For example, regular use of a computer password keeps your memory trace strong. But, occasional use of the same weakens the memory trace, making it difficult to remember the password.

Scientists have identified that changes occur in the individual neurons and brain circuits when we remember something. Neurons communicate with each other at the synapse and memory involves changes to the molecular structure of neurons at the synapse. However, research, so far, is limited to simple neuronal circuits, and the mystery behind neuronal changes involved in complex types of memory such as memories for facts and events has not yet been completely unraveled.

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Problems with the Trace-Decay Theory

Though these theories seem quite plausible, many scientists think that memory decay has little to do with everyday forgetting. For instance, many memories that have not been recalled for a long time remain strong and stable. Further, research has also proved that sometimes people do recollect memories which they were not able to retrieve earlier. This contradicts the idea of slow and natural decay.

Synapse and neuron cells sending electrical chemical signals.
The brain synthesizes information based on electrical and chemical processes and anything that interferes with these processes in the brain has the potential to affect memory. (Image: Andrii Vodolazhskyi/Shutterstock)

It might be difficult to establish if memories weaken due to decay of memory traces or something else. But it is possible to ascertain that memory traces can be disturbed at the time of being encoded in the brain. Our brain needs time to make permanent changes and store the memory properly. This process of stabilizing a memory trace into a long-term memory is called ‘memory consolidation’. The brain synthesizes information based on electrical and chemical processes and anything that interferes with these processes in the brain has the potential to affect memory. Hence, when events such as concussion interfere with the process of memory consolidation, the memory trace cannot be properly encoded in the brain. As a result, the memory trace may either remain weak or may not be stored at all. 

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Interference Theory on Why We Forget

According to the interference theory, memories can compete and interfere with each other and hinder our ability to retrieve information. Interference among memories is common when two or more events are similar to each other. So, when new information is encoded in the brain it interferes with the older information or with things in the future and can get mixed up. For instance, if you use different passwords for different accounts, the memory of one password may interfere with others and affect your ability to recall the passwords.

To demonstrate the effect, suppose you have to remember two sets of a list of four words. If you had to remember just one list of words, you stand a better chance of remembering the first set of words correctly. But when you have a second list of words, it interferes with your memory for the first list.

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 Cue-dependent Retrieval

Researchers suggest sometimes information cannot be retrieved in the absence of a suitable cue to trigger the memory. Memories often cannot be retrieved spontaneously but need elements that were available when the memory was encoded. In a study, two groups of participants were asked to remember a long list of words. While one group was asked to remember the words, the other group was given cues about the categories to be remembered. The outcome suggested that the group that received retrieval cues remembered more words than the one which received none. This study demonstrates that we have much more information in the memory than we think, but need the right cue to trigger the memory. For instance, after seeing your house and street where you lived as a six year old, you are flooded with forgotten memories. Though you hadn’t thought about those for many years, memories were triggered by a cue.

Common Questions about Why We Forget

Q: What is forgetting?

Forgetting is the loss of information from one’s memory. It is a very common phenomenon and we often forget an appointment or someone’s birthday or where we put the car keys.

Q: What are the different ways in which memory consolidation is disrupted?

Memory consolidations are disrupted by concussion, consumption of alcohol and other drugs, infections, lack of sleep, and strong emotions, leading to forgetfulness.

Q: Why do people with concussion report a brief period of forgetfulness?

All the older memories that were already consolidated remain intact but there was not sufficient time to consolidate memories before the physical blow to the head. Hence people with concussion experience a brief period of forgetfulness.

Q: Why can’t the trace-decay theory be tested in controlled experiments?

With time, memory traces decay and new ones form simultaneously. There is no way to measure the decaying memories without the interference from new ones. So, if memories weaken over time, it cannot be determined if it is due to memory trace or something else.

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