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How Quickly We Forget: The Transience of Memory


This is the first in a new series on the 7 deadly sins of memory. First up, how the passage of time affects our memories.

My memory continues to surprise me, and not usually in a good way. I recently reread a book which I first read, and greatly enjoyed, about 13 years ago. It is fiction by one of my favourite authors – the writing is vivid, the story exciting and the set-piece action breathtaking.

Despite all this I had almost no memory of reading the book the first time. Almost everything about the book seems to have seeped away in the intervening years. I couldn’t remember the plot, most of the characters or any of the scenes. The only thing I vaguely remembered was the main character’s name, but I couldn’t be sure I hadn’t invented that memory, after all I couldn’t recall anything else about the book.

This is an example of what Harvard psychologist Daniel L. Schacter calls the first deadly sin of memory: transience (Schacter, 1999). Transience can be seen in both short- and long-term memory. Short-term memory, for psychologists, means the things that are in your mind right now, and only those things. On the other hand long-term memory is anything you store to be retrieved at a later time. Studies have shown that both types of memory can be extremely fragile over their respective timescales.

Short-term memory: fast forgetting

A classic experiment on fast forgetting was carried out by Peterson and Peterson (1959). They asked participants to memorise a three-letter sequence, then count backwards in sets of threes. Participants were then asked to try and recall the three-letter sequence after different lengths of time counting backwards.

Participants did surprisingly poorly on this test. After only six seconds of counting backwards in threes, on average half of the original three letters had disappeared from memory. By the time participants had been counting backwards for 12 seconds, less than 15% of the original memory remained. Finally after 18 seconds it was all but gone.

This experiment clearly shows how quickly information leaks out of short-term memory. So perhaps the book I was reading just went straight in and straight out again? No doubt a lot of it did, but surely some of it must have stuck. Otherwise I wouldn’t have been able to follow the story and would have ended up reading the first page again and again.

Long-term memory: slow forgetting

No, some aspects of the book must have become lodged in my long-term memory, so what types of processes affect how much we retain from long-term memory? In fact relatively little is known about how we forget over substantial periods of time. Thirteen years is a long time for an experimenter to wait just to find out if I can remember the details of that book.

Nevertheless, studies do suggest that forgetting probably follows a power function. That means we lose a lot of information soon after it goes in, then, over time, the rate of forgetting slows down.

Storage, retrieval and rehearsal

Of course not all memories are created equal, and so the reasons why we fail to recall information are many and varied. Indeed, some psychologists have argued that we never really forget anything. Perhaps, they say, the memory is still in our minds but we can no longer access it.

Cues are clearly important to retrieving memories. The smell of varnish might remind us of the day we spent canoeing in the rain, lost in solitary thought. Conversely some experiences can hinder the retrieval of certain memories. The memory of a parent’s anger at our childish misdemeanour might completely block out the memory of what we actually did.

Memory is certainly more likely to fade if we don’t use it. The retrieval and rehearsal of memories has been shown to enhance their storage. Interestingly there’s no actual evidence in humans that memories which remain unrehearsed or unretrieved actually do dissipate over time. Perhaps all our memories really are still in there.

Gone, and forgotten

But even if my memory of reading that book the first time is still in there, it’s doing a very good job of hiding. Especially since rereading the book should be a massive cue to its recall. Maybe we do completely forget or maybe I have just forgotten that I didn’t actually read the book in the first place. Either way, perhaps I’ll be able to enjoy the same book all over again in another 13 years!

→ Continue reading: Memory and Recall: 10 Amazing Facts You Should Know

[Image credit: Peter Bowers]


Peterson, L. R., & Peterson, M. J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58, 193-198.

Schacter, D. L. (1999). The seven sins of memory. Insights from psychology and cognitive neuroscience. American Psychologist, 54, 182-203.



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