Why do we forget?

I realised earlier today that whilst I’ve written several posts about memory, for example this one, about the different types of memory, the link between smell and memory, whether our memory is trustworthy, and about those with perfect memory syndrome, I’ve never actually written a post about the opposite – forgetting. Why is it that we often can’t remember something so simple as what we had to eat yesterday, or a piece of information we need to know for an exam? Read on to find out more..


One theory is the Trace Decay Theory of forgetting. This assumes that memories leave a trace in the brain, and if we don’t activate this trace (by thinking about the memory) then it fades, or decays. This theory involves our short term memory, which has a limited duration and can only hold onto information for around 30 seconds. However, it is actually pretty hard to test, meaning there isn’t much evidence to support it. It also doesn’t explain why people can remember things even though they haven’t thought about them for years, which is at odds with trace decay theory.

An alternative theory involving the short term memory is Displacement Theory. This theory is based on evidence which has shown the capacity of the short term memory to be between 5 and 9 items (Miller, 1956). Once new information enters our short term memory, other items in there are displaced. This has been illustrated by asking participants to remember a list of words. Results of experiments using this method have found that people are more likely to remember the words at the beginning and at the end, the ones in the middle have been ‘displaced’.


Interference Theory explains forgetting in terms of our long term memory. Have you ever typed in your old password and wondered why it wasn’t working? That’s an example of proactive interference – old knowledge interfering with what we know now. Or how about if you’ve broken your new phone and have to go back to using your old one, but keep pressing the wrong buttons? That’s retroactive interference – new knowledge interfering with what you used to know. Anderson (2003) explains interference as a failure of inhibition in the brain, whilst it might be useful to forget some things over time (e.g. what you had for dinner 3 weeks ago), there are other things which we need to remember, despite new learning. A single retrieval cue (such as sitting at your computer) can link to more than one memory (your old and new password), meaning the correct memory needs to be selected. However a problem with this mechanism means that as well as forgetting potentially distracting memories, problems with inhibiting other memories triggered by the same cue means that useful things are forgotten too.

The above theories assume that the memory has been forgotten because it no longer exists. But what if the problem isn’t with the memory itself, but the process of remembering known as retrieval? Retrieval failure happens when the memory is still contained in our long term memory, but we are unable to access it because certain cues are not there. These cues can be anything such as context about where you were when you learnt the information (external), or how you were feeling (internal). Goddon & Baddeley (1975) asked a group of divers to take part in a memory experiment. Half learnt a word list on land, and half underwater. Half of the group who learnt the list on land then had to recall the list on land, whilst the other half had to do this task underwater. The same happened to the participants in the underwater learning group. They found that participants who had to recall the words in the same setting as they learnt them in performed significantly better than those whose context had changed.

What about when forgetting is more serious? Amnesia is more severe than the types of forgetting we experience in day to day life, as it can involve forgetting large proportions of previous life events or information and is often caused by trauma to the brain. Perhaps the most famous case of amnesia was in Patient H.M., who had most of his hippocampus (structure in the centre of the brain which is thought to be responsible for long term memory) removed to cure his severe epilepsy. Whilst successful in reducing his seizures, he was left unable to retain any new information for more than a few minutes. If you’d like to read more about what H.M.’s case taught us about human memory, I’ve also written a post about that here.



Anderson, M.C., 2003. Rethinking interference theory: Executive control and the mechanisms of forgetting. Journal of memory and language49(4), pp.415-445.

Godden, D.R. and Baddeley, A.D., 1975. Context‐dependent memory in two natural environments: On land and underwater. British Journal of psychology66(3), pp.325-331.

Miller, G. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. The psychological review, 63, 81-97.

Scoville WB, Milner B. J. 1957. Neurol. Neurosurg. Psychiatry. 20:11–21



One thought on “Why do we forget?

  1. Structural plasticity is required to consolidate long term memory. Chemical change in brain mechanism may cause memory loss. Your article is interesting to know about memory processing.

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