Types of Forgetting & Memory Decay

Instructor: Emily Cummins
Why do we remember some things and forget others? In this lesson, we'll talk about different kinds of forgetting: memory decay, memories fading as time passes; amnesia, the result of an injury; and repression, an effort to forget trauma.

Memories and Forgetting

Do you remember what you wore yesterday? What you had for breakfast last Saturday? Do you remember what outfit you wore on the first day of elementary school? Why do we remember some things and not others? What causes us to forget?

In this lesson, we'll talk about several different types of memory loss, or our inability to recall information. Sometimes, forgetting is simply due to the passage of time. Other times, forgetting might be caused by a brain injury or by a traumatic event. Let's go over some of the major causes of memory loss.

Memory Decay

One theory of forgetting is known as memory decay, which suggests that our memories decay, or weaken, with time. Basically, this theory explains forgetting as part of the passage of time. So that's why you don't remember what you wore on the first day of elementary school or why you probably don't remember your birthday party from that year. It was a long time ago! Decay theory suggests memories are like traces. The memories we make leave traces on our brains, and as time goes on these traces will fade. Decay theory of memory refers specifically to the forgetting that happens in our short-term memory. Unless we're actively trying to remember something, eventually many traces from our short-term memory will fade away.


Sometimes, though, we forget because we experience traumatic injuries to our brains, rather than simply the passage of time. Amnesia is a condition in which we lose memories, facts, dates, or have a difficult time remembering recent or past events. Amnesia can be caused by things such as a stroke, a major blow to the head, or a tumor impacting part of the brain. Excessive use of drugs or alcohol can also cause amnesia. There are a few major kinds of amnesia: retrograde, anterograde, and infantile.

  • Retrograde amnesia is when we can't remember things that happened before the onset of this amnesia. So, while we might be able to form new memories, we are not able to recall things that happened before we experienced amnesia. For example, if you experience retrograde amnesia, you can probably remember a person you just met, but you might not be able to remember a party you attended six months ago.
  • Anterograde amnesia is the opposite. This refers to an inability to make new memories after the onset of amnesia. Sometimes this might also mean we have a hard time recalling the immediate past. In this kind of amnesia our memories from the past are intact, but we might not be able to remember the name of a person we just met. A good trick to keep these straight is to keep in mind that the prefix retro refers to the past and the prefix ante means front, but think of it as forward or future-oriented.
  • Do you have a lot of memories from when you were 2 or 3 years old? Probably not. This is due to infantile amnesia, or our loss of very early memories. Most of us cannot recall the things we experienced when we were very young. Some have suggested that we lose these memories because they might have been traumatic, and so we repress them. But other explanations are slightly different, suggesting that these memories were not formed and therefore we cannot access them, not necessarily that they were traumatic.

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