The Psychology of Forgetting and Why Memory Fails

Memory is imperfect, and forgetting is more common than you might think.

Forgetting in daily life
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Forgetting is an all too common part of daily life. Sometimes these memory slips are simple and fairly innocuous, such as forgetting to return a phone call. Other times, forgetting can be much direr and even have serious consequences, such as an eyewitness forgetting important details about a crime.

Why do we forget? From forgetting where you left your keys to forgetting to return a phone call, memory failures are an almost daily occurrence.

Forgetting is so common that you probably rely on numerous methods to help you remember important information such as jotting down notes in a daily planner or scheduling important events on your phone's calendar.

As you are frantically searching for your missing car keys, it may seem that the information about where you left them is permanently gone from your memory. However, forgetting is generally not about actually losing or erasing this information from your long-term memory. Forgetting typically involves a failure in memory retrieval. While the information is somewhere in your long-term memory, you are not able to actually retrieve and remember it.

Why Time Plays Such a Key Role in Forgetting

Psychologist Hermann Ebbinghaus was one of the first to scientifically study forgetting. In experiments where he used himself as the subject, Ebbinghaus tested his memory using three-letter nonsense syllables.

He relied on such nonsense words because using previously known words would have involved drawing on his existing knowledge and associations in his memory.

In order to test for new information, Ebbinghaus tested his memory for periods of time ranging from 20 minutes to 31 days. He then published his findings in 1885 in Memory: A Contribution to Experimental Psychology.

His results, plotted in what is known as the Ebbinghaus forgetting curve, revealed a relationship between forgetting and time. Initially, information is often lost very quickly after it is learned. Factors such as how the information was learned and how frequently it was rehearsed play a role in how quickly these memories are lost.

The forgetting curve also showed that forgetting does not continue to decline until all of the information is lost. At a certain point, the amount of forgetting levels off. What exactly does this mean? It indicates that information stored in long-term memory is surprisingly stable.

How to Measure Forgetting

Sometimes it might seem that information has been forgotten, but even a subtle cue can help trigger the memory. Imagine the last time you took an exam for school. While you might have initially felt forgetful and unprepared, seeing the information presented on the test probably helped cue the retrieval of information you might not have known you even remembered.

So how do we know when something has been forgotten?

There are a few different ways to measure this:

  • Recall: People who have been asked to memorize something, such as a list of terms, might be asked to recall the list from memory. By seeing how many items are remembered, researchers are able to identify how much information has been forgotten. This method might involve the use of free recall (recalling items without hints) or prompted recall (utilizing hints to trigger memories).
  • Recognition: This method involves identifying information that was previously learned. On a test, for example, students might have to recognize which terms they learned about in a chapter of their assigned reading.

So Why Do We Forget? 

Of course, many factors can contribute to forgetting. Sometimes you might be distracted when you learn new information, which might mean that you never truly retain the information long enough to remember it later. Well-known memory researcher Elizabeth Loftus has proposed four key explanations for why forgetting occurs.

The four major reasons for forgetting that she cites are:

  • Retrieval failure
  • Interference
  • Failure to store
  • Motivated forgetting

A few of the major theories of forgetting include:

The Interference Theory 

What did you have for dinner Tuesday night of last week? Is that difficult to recall? If someone had asked you that question Wednesday morning you probably would have had no problem recalling what you had for dinner the night before. But as intervening days pass, the memories of all the other meals you have eaten since then start to interfere with your memory of that one particular meal. This is a good example of what psychologists call the interference theory of forgetting.

According to interference theory, forgetting is the result of different memories interfering with one another. It is difficult to remember what happened on an average school day two months ago because so many other days have occurred since then. The more similar two or more events are to one another, the more likely interference will occur.

Unique and distinctive events, however, are less likely to suffer from interference. Your 12th-grade prom, high school graduation, wedding, and the birth of your first child are much more likely to be recalled because they are singular events - days like no other.

Interference also plays a role in what is known as the serial position effect, or the tendency to recall the first and last items of a list.

For example, imagine that you wrote down a shopping list but forgot to take it with you to the store. In all likelihood, you will probably be able to easily recall the first and last items on your list, but you might forget many of the items that were in the middle. The first thing you wrote down and the last thing you wrote down stand out as being more distinct, while the fourth item and seventh item might seem so similar that they interfere with each other.

There are two basic types of interference that can occur:

  • Retroactive interference happens when newly acquired information interferes with old memories. For example, a teacher learning the names of her new class of students at the start of a school year might find it more difficult to recall the names of the students in her class last year. The new information interferes with the old information.
  • Proactive interference occurs when previously learned information makes it more difficult to form new memories. Learning a new phone number or locker combination might be more difficult, for example, because your memories of your old phone number and combination interfere with the new information.

Eliminating interference altogether is impossible, but there are a few things you can do to minimize its effects. One of the best things you can do is rehearse new information in order to better commit it to memory. In fact, many experts recommend overlearning important information, which involves rehearsing the material over and over again until it can be reproduced perfectly with no errors.

Another tactic to fight interference is to switch up your routine and avoid studying similar material back to back. For example, don't try to study vocabulary terms for your Spanish language class right after studying terms for your German class. Break up the material and switch to a completely different subject each study session.

Sleep also plays an essential role in memory formation. Researchers suggest that sleeping after you learn something new is one of the best ways to turn new memories into lasting ones.

The Decay Theory of Forgetting

According to the trace theory of memory, the formation of new memories results in physical and chemical changes in the brain that results in a memory 'trace.' Information in short-term memory lasts approximately 15 to 30 seconds and if it is not rehearsed, the neurochemical memory trace quickly fades.

According to the trace decay theory of forgetting, the events that happen between the formation of a memory and the recall of the memory have no impact on recall. Instead, trace theory proposes that is the length of time between the memory and recalling that information determines whether the information will be retained or forgotten. If the time interval is short, more information will be recalled. If a longer period of time passes, more information will be forgotten and memory will be poorer.

The idea that memories fade over time is hardly new. The Greek philosopher Plato suggested such a thing more than 2,500 years ago. Later, experimental research by psychologists such as Ebbinghaus bolstered this theory.

One of the problems with this theory is that it is difficult to demonstrate that time alone is responsible for declines in the recall. In real-world situations, many things are bound to happen between the formation of a memory and the recall of that information.

A student who learns something in class, for example, might have hundreds of unique and individual experiences between learning that information and having to recall it on an exam.

Was forgetting the date that the American Revolutionary War began due to the length of time between learning the date in your American History class and being tested on it, or did the multitude of information acquired during that interval of time play a role? Testing this can be exceedingly difficult since it is nearly impossible to eliminate all the information that might have an influence on the creation of the memory and the recall of the memory.

Another problem with decay theory is it does not account for why some memories fade so quickly while others linger. Novelty is one factor that plays a role in why some things are remembered while others are forgotten.

For example, you are more likely to remember your very first day of college than all of the intervening days between it and graduation. That first day was new and exciting, but all the following days probably seem quite similar to each other.

The Retrieval Failure Theory

Sometimes the memories are there, we just can't seem to access them. Two of the basic reasons for this failure in memory retrieval are related to encoding failures and lack of retrieval cues. A common reason why we don't remember information is because it never made it into long-term memory in the first place. Try this well-known demonstration first used by researchers Nickerson and Adams. From memory, try to draw the back side of a penny. Once you are done, compare your drawing to an actual penny.

Are you surprised by how poorly you recalled what the back of a penny looks like? While you probably had a good idea about the overall shape and color, the actual details were probably pretty fuzzy. Why? Since you don't actually need to know what the back of a penny looks like to differentiate it from other coins, you only really focus on the information you do need - the overall size, shape, and color of the coin. You aren't able to recall what the back of a penny really looks like because that information was never really encoded into memory in the first place.

The Cue-Dependent Theory of Forgetting

Other researchers have suggested that sometimes information is actually present in memory, but that it cannot be recalled unless retrieval cues are present. These cues are elements that were present at the time that the actual memory was encoded. For example, remembering the details of your first date with your spouse might be easier if you smell the same scent that your partner was wearing on that first date. The retrieval cue (the perfume) was present when that memory was created, so smelling it again can trigger the retrieval of those memories.

Final Thoughts

Numerous theories exist to explain how and why we forget. In many situations, several of these explanations might account for why we cannot remember. The passage of time can make memories more difficult to access (decay theory), while the abundance of information vying for our attention can create competition between old and new memories (interference theory).

While forgetting is simply a part of life, there are a number of things that we can do to improve our memories and become better at recalling information. Next, take a closer look at some of the different things you can do now to improve your memory.


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Tulving, E. Cue-dependent forgetting. American Scientist. 1974;62: 74-82.

Willingham, D. T. Cognition: The thinking animal (3rd ed.). Upper Saddle River, NJ: Pearson/Prentice Hall; 2007.

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