The Powerful Role of Forgetting in Learning


Image by Alice Xu


Scientific research on memory and forgetting can be traced back to the 19th century when German psychologist Hermann Ebbinghaus published his pioneering work on the “forgetting curve,” demonstrating how memory retention declines over time. More concretely, Tulving (1974) described forgetting as “the inability to recall something now that could be recalled on an earlier occasion.” Forgetting serves tremendous positive functions in everyday life. For example, it enables positivity and regulates emotions by making negative autobiographical experiences much less memorable than positive ones (Walker et al., 2003). Nevertheless, many still perceive forgetting as frustrating because of its inevitability—no matter how hard we try to remember something, our memory of it fades away with the passage of time. For learners, forgetting seems to reduce the level of learning because the previously learned knowledge becomes less accessible, and thus forgetting is often treated as an “enemy” that one wants to defeat. However, contrary to one’s intuitive sense, forgetting might play quite the opposite role during learning.

Let’s consider a simple question: if you were going to take an exam in a week, how would you spend time preparing for it? If you are on a tight schedule and only have one hour to study, would you distribute learning across time? For example, is it better to study roughly 10 minutes daily, or study 60 minutes in one sitting the night before the exam? These two approaches are called “spaced” practice and “massed” practice, the scholarly term for cramming. In reality, many students limit their studies to just a day or two before the exam, with a deeply ingrained belief that cramming is an effective way to prepare for a test (McIntyre & Munson, 2008). 

The Spacing Effect in Memorization

Despite the fact that most learners adopt massed practice during learning, the robust memory benefits of spaced learning over massed learning have been widely demonstrated in scientific research. The phenomenon that long-term memory and retention benefit more when the study sessions are spaced out rather than massing learning into one single session is called the “spacing effect” (for a meta-analysis, see Cepeda et al., 2006). The earliest study on the spacing effect was conducted by Ebbinghaus, who served as the subject himself in a number of experiments. The task was to recite a single 12-syllable series without error. It took Ebbinghaus 68 consecutive repetitions of the syllable series to complete the task. While if the repetition practices were distributed over a three-day period, 38 repetitions could achieve the same result (Ebbinghaus, 1964). 

The Spacing Effect in Generalization of Conceptual Knowledge

The spacing effect also extends beyond the realm of simple memorization of nonsense syllables. For example, Kornell and Bjork (2008) found that spaced practice also benefited induction, that is, inferring general principles from instances. In this experiment, college students learned the styles of several artists by viewing multiple drawings of each of them. Students either studied drawings of one artist in consecutive order or studied in an interleaved manner. Interleave means when you are learning two or more topics, and in this case, styles of artists, instead of focusing exclusively on one style at a time, you alternate between them. For example, if you have three artists—A, B, and C, and two examples for each, the learning order would be A1-B1-C1-A2-B2-C3. Students who saw the exemplars for each artist interleaved with other artists’ work outperformed those who observed the painting of each artist in consecutive order in different memory tests following learning. Specifically, participants in the spaced approach were more likely to identify the artist’s name when seeing new paintings created by the same artists in the learning phase, compared to participants using the massed learning approach during the study. In addition, when new paintings that were stylistically similar to what was learned were included as distractors, spaced-learning individuals were more likely to make a correct judgment about whether the painting was studied in the learning phase. The aforementioned findings suggested that a spaced learning practice could enhance not only memory but also complex generalization during learning. 

The spacing effect is robust, and spacing practice seems to be an effective learning strategy across development stages. A recent study investigated whether spaced learning could help early elementary school-aged children learn and generalize science concepts about food chains (Vlach & Sandhofer, 2013). In this experiment, children were randomly placed into one of three learning conditions: 1) massed condition, where children received four consecutive lessons without any break in between; 2) spaced condition, where they learned one lesson per day for four days in succession; or 3) clumped condition, where two lessons were offered in immediate succession on the first day, following another two in the same fashion on the second day. In each lesson, children first learned the general concepts of the food chain and then about the food chain under a particular biome (from arctic, desert, grasslands, ocean, or swamp). Children were assessed before the lessons and one week after the lessons both on the simple and complex generalization of concepts in the food chain, using different contexts than the ones used in previous lessons. For example, a simple generalization question tested whether children acquired the knowledge that bigger animals eat smaller animals and applied this rule to a new scenario—whether they could identify what a particular animal ate when they were presented with members from a food chain in different sizes. Complex generalization questions demanded understanding the dynamic relationships within a food chain induced from learned lessons. For instance, given the scenario that when grass was poisoned and caused animals to die when they ate it, what would happen to the number of crickets who ate the grass? The results demonstrated that spaced schedules resulted in better performance in both simple and complex generalizations, suggesting many practical implications of spaced learning in childhood educational practice. 

Forgetting: The Underlying Mechanism of The Spacing Effect in Concept Learning

Many psychologists are intrigued to know the underlying mechanism of the spacing effect, and some of them suggest that it is forgetting that enables people to retain more information in memory in the long term.

According to the study-phase retrieval theory, the harder the successful retrieval, the more the original memory trace could be strengthened (Hintzman & Block, 1973). Extended from this prevailing theory, scientists proposed the forgetting-as-abstraction account, which suggests that abstraction and generalization are enhanced in spaced learning because the spaced schedule enables forgetting between learning events (for a review, see Vlach 2014). When learning is distributed across time, the relevant features are reactivated in memory but not the irrelevant features. Gradually, the relevant features become more accessible in memory than irrelevant ones, so learners are more likely to abstract the regularities and rules from the relevant features and apply the rules when they need to categorize new items in the future. In other words, when forgetting occurs, information processing becomes more efficient because it frees up more cognitive resources (Gaissmaier et al., 2018). 

Implications in Education Practice

The ultimate question students may ask is how do I incorporate spaced practice into my study? Here is a tip: make a practical, spaced study and review plan, and stick with it. Instead of cramming, you can preview the content before lectures and review it a while after. During exam preparation, space out and interleave the to-be-reviewed topics. You may feel it becomes harder to recall the information compared to studying them in a massed way; however, “easy” doesn’t mean you remembered them better. The sense of familiarity that occurred during mass practice could usually be attributed to factors independent of how well information is learned and remembered (Bjork et al., 2013). In the era of online learning, many instructors make their lecture recordings available to students, making cramming even more appealing. Some students choose to watch lectures for the entire class during the final week. However, at the same time, it also allows students to space out their restudy sessions, which can lead to better learning. Although cramming for exams may bring a decent grade for one single class and seems to be an “efficient” way of studying, we have to compare the long-term costs and benefits associated with cramming and spacing. Using spaced practice can make us feel difficult in the first place; however, it has long-lasting beneficial effects on memory retention and learning. 


Bjork, R. A., Dunlosky, J., & Kornell, N. (2013). Self-regulated learning: Beliefs, techniques, and illusions. Annual Review of Psychology, 64, 417–444. 

Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. 

Ebbinghaus, H. (1964). Memory: A contribution to experimental psychology. Dover.

Gaissmaier, W., Schooler, L. J., & Mata, R. (2008). An ecological perspective to cognitive limits: Modeling environment-mind interactions with ACT-R. Judgment and Decision Making, 3(3), 278-291.

Hintzman, D. L., & Block, R. A. (1973). Memory for the spacing of repetitions. Journal of Experimental Psychology, 99(1), 70–74. 

Kornell, N., & Bjork, R. A. (2008). Learning Concepts and Categories: Is Spacing the “Enemy of Induction”? Psychological Science, 19(6), 585–592. 

McIntyre, S. H., & Munson, J. M. (2008). Exploring cramming: Student behaviors, beliefs, and learning retention in the Principles of Marketing course. Journal of Marketing Education, 30(3), 226–243. 

Nørby, S. (2015). Why forget? On the adaptive value of memory loss. Perspectives on Psychological Science, 10(5), 551-578. 

Tulving, E. (1974). Cue-dependent forgetting: When we forget something we once knew, it does not necessarily mean that the memory trace has been lost; it may only be inaccessible. American scientist, 62(1), 74-82.

Vlach, H. A. (2014). The spacing effect in children’s generalization of knowledge: Allowing children time to forget promotes their ability to learn. Child Development Perspectives, 8(3), 163-168. 

Vlach, H. A., & Sandhofer, C. M. (2012). Distributing learning over time: the spacing effect in children’s acquisition and generalization of science concepts. Child development, 83(4), 1137–1144. 

Walker, W. R., Skowronski, J. J., & Thompson, C. P. (2003). Life is Pleasant—and Memory Helps to Keep it that Way! Review of General Psychology, 7(2), 203–210.