The Forgetting Curve and the Classroom
A
In the 1880s, the German psychologist Hermann Ebbinghaus spent months memorising lists of nonsense syllables and testing himself at different intervals. His results produced what is now called the forgetting curve: without review, much of what is learned is lost within days, with the steepest losses occurring soon after learning. More than a century later, his basic finding has been repeated many times with more meaningful material, from vocabulary to medical facts, and it remains one of the most reliable results in the study of memory.
B
What Ebbinghaus also noticed was that each review made the curve flatter. Material that was revisited after a gap was forgotten more slowly the next time. Researchers now call this the spacing effect: the same amount of study time produces longer-lasting learning when it is spread across several sessions rather than concentrated in one. A student who studies a list of words for ten minutes on three separate days will usually remember more a month later than one who studies the same list for thirty minutes in a single evening.
C
Why should a gap help? One widely discussed explanation is that a little forgetting is useful. When learners return to material they have partly forgotten, they must work harder to retrieve it, and this effort appears to strengthen the memory. Psychologists describe such conditions as 'desirable difficulties': they make learning feel slower and less fluent in the moment, yet they improve long-term retention. The difficulty must be manageable, however; if too much has been forgotten, the review becomes relearning from the beginning.
D
Closely related is the testing effect, sometimes called retrieval practice. In many studies, students who tried to recall information, for example by answering low-stakes quiz questions, remembered more on a final test than students who simply reread the same material for the same length of time. Rereading tends to create a feeling of familiarity, which learners easily mistake for understanding. Recall, by contrast, shows both the learner and the teacher what has actually been retained.
E
Despite this evidence, spacing is far from universal in schools. Textbooks are usually organised in blocks, so that each unit is taught, tested and then left behind. Students themselves often prefer cramming because it produces quick, visible gains just before an examination. Teachers, meanwhile, face crowded syllabuses and may feel that returning to old topics wastes time that is needed for new content. As a result, the conditions that make learning last are often the ones that are hardest to fit into a school year.
F
Some practical adjustments, however, require little extra time. A teacher might begin each lesson with three or four short questions on material from the previous week and the previous month, or set homework that mixes new items with older ones. Neither approach demands new resources. What they do require is a shift in how progress is judged: a class that struggles slightly with a starter quiz may be learning more durably than a class that answers easily because the content was taught yesterday.