This is a photo I recently took in Paris - a truly unforgetable city! I've traveled to Paris several times and I have fond memories from each trip.
Nearly everything we know today about learning and forgetting was "discovered" by Herman Ebbinghaus in the late 1800's.
Ebbinghaus, a psychologist, had a keen interest in memory and higher cognitive processes. He was the first to describe the "learning curve" and characterize retention. His experiments were quite clever. He was able to systematically measure how long it takes for people to "memorize" new information and how much was retain over time by developing a series of nonsense syllables and words. During his experiments, subjects repeated a series of nonsense syllables and/or words as many times as was necessary to reach an a priori level of accuracy (for example, three perfect reproductions without being prompted verbally or looking at it in writing). What he discover was that the time required to memorize nonsense syllables increased sharply as the number of syllables increased. I think we can all appreciate that it requires much more time and effort to memorize a 13-digit overseas telephone number than it does a 5-digit zip code. Ebbinghaus also discovered that people are able to memorize more in distributed learning sessions rather than by trying to assimilate everything in a single session.
Ebbinghaus then set out to determine the duration and strength of retention. Using a concept he invented, called the "savings method," he determined the number of repetitions required to relearn material (to the same criterion) and compared it to the number of trials initially required to learn the material. The more repititions you required to relearn the material, the more you had forgotten - he surmised.
What he discovered through these experiments are things we inutitively know. First, items that are associated with one another are more easily remembered together. These associations could be due to congruity (e.g. they appeared next to each other on the list) or due to remote association (e.g. the learner made some connections between the two items in their own mind). Second, we remember best what we FIRST and LAST encounter (the so-called primacy and recency effects) and tend to forget middle items. Third, even small amounts of practice, far less than what is required for mastery to the criterion level of performance, lead to "savings" (e.g. improved retention over time). Finally, most humans tend to forget 50% of newly learned knowledge in a matter of days or weeks. But the speed of forgetting is related to a number of factors. Most importantly, our ability to learn nonsense material (e.g. things we don't understand) is quite poor - requiring a great deal of effort - and the forgetting curve is quite steep. On the other hand, meaningful material (e.g. things that make sense because they relate to things we already know) takes only about one tenth the effort to learn and the forgetting is relatively gradual. Not surprisingly, the forgetting curve is nearly flat for vivid or traumatic experiences - perhaps because the learner "reviews" the memory repetitively in his/her mind.
What are the implications of Ebbinghaus' work? Many of our "best practices" in education are based on these findings and most of us probably take them for granted. For example, stating the important learning points at the being and end of a lecture relates to the primacy and lacency effect of memory. Breaking up material into small chucks of information - rather than massive amounts of information at a single sitting - relates to the inherently limited capacity that most of us have to absorb new information. Opportunities for practice during a lecture or workshop - even when its not mastered - improves retention. And encountering the material repetitively over time - rather than concentrating on it intensely for a short period of time - is a more effective learning strategy.
Cognitive learning theory (aka contructivism) postulates that we construct our own learning by making connections between what we sense (see, hear, feel, taste, touch) and what we already know. Ebbinghaus' experiments certainly provides evidence to support that contention - learning new materials is far more efficient when it is "meaningful" to the learner (not the teacher!!). But how can we, as teachers, make the material "meaningful?" .... That's a topic for another time.