Forgetting in Long & Short-Term Memory

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Forgetting is an annoying reality in our lives, but why does it happen? In this lesson, we'll look at forgetting as it applies to both long and short-term memories and see how they differ.


Famed philosopher Friedrich Nietzsche once said that ''The advantage of a bad memory is that one enjoys several times the same good thing for the first time.'' Forgetting can, as it turns out, be useful. Most of the time, however, it's just annoying. The study of the memory is a complex field, which is unsurprising since the mind is such a complex instrument. Psychologists spend lots of time studying how we remember, but they also examine how and why we forget. Where does that information go, and why does a walk down memory lane sometimes leave you with a blank map?

Types of Memory

Before we can start forgetting, we need to understand the ways that the mind stores memories. The first way is in short-term memory (STM). When new information is encountered, it is stored immediately in the STM, just in case your mind needs to recall it right away. For example, your STM lets you remember what the beginning of a sentence was by the time you reach the end of it. However, it's usually only there for about 15-30 seconds (a minute at most) unless conscious effort is put into remembering it.

Once memories are transferred out of short-term memory, they are sent to long-term memory (LTM). Long-term memory is theoretically limitless, holding all the data your mind encounters so that you can recall it later. Think of memories like money. If you get paid, you may keep that money in your wallet temporarily, in case you need it right away. However, if you're not going to use it then it's safer to deposit in the bank. You can access it later, but it takes more effort. So, your short-term memory is your memory wallet, and your long-term memory is your memory bank.

STM is like a wallet for memories

Short-Term Memory and Forgetting

So, what happens when funds are lost? Forgetting is defined as the complete loss of information from the mind. It's a tricky thing to study, but psychologists have determined that this process occurs differently between short and long-term memory. Let's start by looking at short-term memory.

Trace Decay

There are two main theories to explain forgetting in STM. The first is the trace decay theory. This theory relies on the assumption that short-term memories are inherently prone to being forgotten. That's how they work; they stick in your mind for a minute then are either forgotten or transferred to long-term memory. So, short-term memories begin to fade from the moment they are formed, until all traces of that memory have completely disappeared or decayed. In this theory, memory is not impacted by the events that occur between learning information and forgetting it; it's simply a matter of time. However, since this theory is nearly impossible to test it remains controversial.

Trace decay focuses on short-term memory decay over time

Displacement Theory

The other theory psychologists use to explain STM forgetting is the displacement theory. Think back to that analogy of your STM as a wallet. There is limited space, so something has to be removed to make room for new material. That's the basic idea in the displacement theory. The STM is naturally limited in its capacity. Therefore, old information is displaced or pushed out to make room for new short-term memories.

Forgetting in Long-Term Memory

Forgetting is a natural part of short-term memory, but is this also true of long-term memory? After all, this is supposed to be the memory bank, the secure place to deposit information. Long-term memory is actually not limited in its capacity the way short-term memory is, so why do we forget things we remembered for years? There may be a few ways to explain this.

Interference Theory

While the STM is prone to forgetting, the LTM is not, which leads many to believe that forgetting in the LTM isn't true forgetting at all. Rather, it's an inability to correctly access memories. One way to explain this is the interference theory. The interference theory claims that long-term memories can become jumbled, mixed, and distorted due to time or the events in which those memories were formed. Basically, what was once a set of individual and complete memories becomes entangled as a ball of memory that the mind cannot unravel. Old memories are thought to interfere particularly badly with new memories if the information is similar, like when you keep forgetting your new address and confusing it with part of your old address.

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