Robert Bjork has been studying learning in different settings throughout his career. His lab has a resourceful website (Applying Cognitive Psychology to Enhance Educational Practice) with videos explaining the different aspects of his research and major findings. Most of his findings, advice, and explanations are counterintuitive to common knowledge of what learning is. One of these findings is how items in long-term memory become inaccessible due to their disuse over time.
Sometimes people cannot access information that was well learned earlier (e.g., the address of the house where they grew up). And students find that although they can recall information over and over again the day before a test, they cannot always recall it at the time of examination. Finally, sometimes people cannot recall information at one point in time, but can recall it later. In looking at these situations, it seems that our memories work in strange and unpredictable ways. The function of our memories, however, may be predictable. The New Theory of Disuse (R. A. Bjork & E. L. Bjork, 1992) posits that there are two indices of memory strength: storage strength (SS) and retrieval strength (RS). Storage strength is how well learned something is; retrieval strength is how accessible (or retrievable) something is. To illustrate, imagine four possible situations. If something is well learned (e.g., the address where you have lived for several years), it has both high SS and high RS: You know it well and can retrieve it readily. The address of a friend that you visited for the first time this afternoon, however, may only have high RS (and low SS) because the address, although practiced recently, was not well learned. Thus, although you know the address now, you will be unlikely to be able to recall it in a few days because RS will decrease over time, especially for information with low SS. Sometimes information has high SS (due to it having been well learned), but cannot be retrieved (e.g., the address where you lived as a child). If you were provided with this address again, however, you would have the feeling that that information was somewhere in the recesses of your memory, and in fact, you would be likely to relearn it very quickly. Finally, information can have both low RS and low SS. This information would include things that you heard in class earlier today, but did not learn well and cannot recall now.
In the book Development of Professional Expertise, edited by Anders Ericsson, Bjork explains what role forgetting plays in memory and learning, he states:
The prevalence of forgetting and its role in relearning
Finally, before turning to comments on the individual chapters in this section, I cannot resist chiming in on the discussion (Chatham, Chapter 2; Hunt, Chapter 5) on the importance of understanding the nature of forgetting and the interaction of forgetting and learning. Chatham rightly emphasizes the importance of acknowledging that skills and knowledge, once apparently acquired, will often not remain accessible over time -that is, they will be forgotten -and often at a rapid rate. It is, in fact, a fundamental -and even adaptive -property of human memory that skills and knowledge, with disuse, become inaccessible.
Forgetting, of course, is often frustrating to us, but it is arguably as important to our day-to-day functioning as is remembering. To be efficient requires that we continually update procedures, facts, and episodes in our memories: We need to remember, for example, how the current version of a given software program works, or how it works on a new computer, not how the old program worked on our old computer; we need to remember current names and numbers, not the names and numbers they replaced; we need -as military personnel -to remember the details of a current military engagement or how a new piece of military hardware works, not the details of an earlier and similar engagement, or how the old hardware worked; we need to remember where we parked the car today, not yesterday or a week ago; and on and on. Any efficient information-processing system, whether living or non-living, needs some means to set aside or erase information that is no longer relevant and a possible source of confusion. In human memory, forgetting plays that role.
Research on forgetting, tracing back more than 120 years, has demonstrated that forgetting, rather than being a process of decay, analogous to footprints fading in the sand, is a consequence of interference and competition among memories (for a brief history of research on forgetting, see Bjork, 2003). Learning new information and using that information renders inaccessible information that is no longer being used. The acceleration of forgetting that accompanies disuse is an especially pertinent consideration in military contexts. Much of military training focuses on procedures, skills, and knowledge that -under typical conditions -are accessed only rarely and intermittently, such as in actual combat situations. Without systematic relearning and rehearsal procedures, much of what has been “learned” during military training will be inaccessible when needed.
There's a good reason why irrelevant information becomes inaccessible and it might have to do with Orgel's second rule:
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Evolution is cleverer than you are.