Forgetting Is a Feature, Not a Bug
Imagine you’ve got a test in six weeks. You have, let’s say, six hours of revision in you. How should you spend them?
Most people’s instinct is to do them in a big block near the test — maybe a marathon Sunday the day before. Some sensible souls might spread it over a few days the week of. Almost nobody does the thing the evidence overwhelmingly recommends: start now, and revisit the material in small doses over the whole six weeks.
This is the spacing effect, and it’s one of the most reliable findings in the entire science of learning. A 2006 review by Cepeda and colleagues pulled together over 800 studies. The pattern was clear and consistent: information studied across multiple sessions, with gaps in between, is remembered far better than the same total time crammed into one go.
So why does cramming feel like it works?
Because it does — briefly. If you cram on Sunday night, you’ll do alright on Monday morning. By Friday, most of it will be gone. By the time your end-of-year exam comes round, almost all of it will have evaporated.
This is the heart of one of the most important ideas in learning, from the researchers Robert and Elizabeth Bjork: the difference between performance and learning.
Performance is how well you can do something right now. Learning is a durable change — being able to do it next week, next month, next year. These are not the same thing.
These two things are not the same. In fact, they can pull in opposite directions. The conditions that boost short-term performance (cramming, rereading, doing twenty similar problems in a row) often reduce long-term learning. And the conditions that boost long-term learning (spacing, interleaving, struggling to recall) often lower short-term performance. Your brain, watching this, concludes the easy methods are working better. Your brain is wrong. The Bjorks call these “desirable difficulties” — conditions that slow you down in the moment but produce more durable learning.
Forgetting is part of the trick
Here’s the bit that sounds backwards. The spacing effect works because you forget a bit between sessions. When you come back to material after a gap, you have to work to retrieve it. That effortful retrieval is what strengthens the memory. If you study again immediately, there’s nothing to retrieve — it’s all still sitting on the surface of your mind — so no strengthening happens.
In other words: forgetting isn’t a bug in your brain. It’s what makes the next round of learning stick.
The Education Endowment Foundation, summarising the research for UK schools, puts it bluntly: the kind of “challenge” that produces long-term learning often feels unproductive in the moment. That mismatch between feeling and reality is what trips most of us up.
A six-week spaced revision plan
Say you’ve got a topic to revise — the causes of the First World War, the water cycle, French verb endings, whatever. Six weeks out, here’s what a spaced plan looks like:
| Week | What to do | Roughly how long |
|---|---|---|
| 6 weeks out | Learn the material properly for the first time | 60 min |
| 5 weeks out | Brain-dump everything you remember, fill gaps | 30 min |
| 3 weeks out | Practice questions, mixed with another topic | 45 min |
| 1 week out | Brain-dump again, focus on what you forgot | 30 min |
| 2 days out | Quick review, test yourself on weak spots | 20 min |
Total: under three and a half hours. Spread across six weeks. It will outperform six hours crammed into the night before, and it isn’t close.
Two practical rules
- Start earlier than feels necessary. If you only start when you feel the pressure, you’ve already lost the spacing benefit.
- Trust the discomfort. When you come back to something after a week and can’t remember it, that’s not a sign you’ve failed. That’s the spacing effect working. Retrieve what you can, look up what you can’t, and the next gap will be easier.
The strangest, most useful thing about learning is that it doesn’t happen while you’re studying. It happens in the gaps in between.
References
- 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. Abstract · PDF.
- Bjork, E. L. & Bjork, R. A. (2011). “Making things hard on yourself, but in a good way.” Open PDF on the Bjork Lab site.
- The Bjork Learning and Forgetting Lab at UCLA — research page covering disuse theory, spacing, interleaving and desirable difficulties.
Further reading
- The Learning Scientists’ practical guide to spaced practice — free, classroom-tested resource with downloadable posters for students.
- Education Endowment Foundation, Cognitive Science Approaches in the Classroom — a sober UK review of how strongly spacing holds up in real schools.
- Brown, P. C., Roediger, H. L., & McDaniel, M. A. (2014). Make It Stick: The Science of Successful Learning, Harvard University Press — chapter 4 is on spacing and interleaving.
- Benedict Carey (2014). How We Learn, Macmillan — a journalist’s friendly tour of the same evidence, including a good chapter on why forgetting helps.