Ebbinghaus’s Forgetting Curve, and Why One Mention Isn’t Enough
By Noam Shemla · Data & Method
In the early 1880s, Hermann Ebbinghaus ran one of the most patient experiments in the history of psychology, with himself as the only participant. He memorised lists of meaningless syllables, like "DAX" and "BUP," until he could recite them perfectly. Then he waited a set time and measured how much he had kept.1 He repeated this over and over, across years. The pattern he found is now called the forgetting curve, and it is one of the oldest quantified findings in memory research.
How he measured forgetting
Ebbinghaus did not simply ask himself what he remembered. He used a cleverer measure called savings: how much less time it took to relearn a list compared with learning it the first time. If a list that took ten minutes to learn took only six to relearn, four minutes of the original effort had survived.
By that measure, the losses came fast. About twenty minutes after learning, he had saved roughly 58% of the original effort. After an hour, about 44%. After a day, about a third. After six days, about a quarter. After a month, about a fifth.
Fast loss, then slow
The curve is not a straight line down. Most of the loss happens near the beginning, and then the decline flattens out. What survives the first day tends to stay for much longer.
That shape has held up. In 2015, two researchers in Amsterdam replicated the experiment as closely as they could, with one participant spending around seventy hours learning and relearning nonsense syllables.2 The curve came out very similar to Ebbinghaus's, more than a century later. It also showed a small rise at the 24-hour mark, which the authors thought might reflect the role of sleep in holding on to what was learned.
Spacing is the other half of the finding
The forgetting curve tells you how quickly material fades. A second finding tells you what slows it down: spreading repetition out over time.
The spacing effect is one of the most reliable results in memory research. A 2006 review by Nicholas Cepeda and colleagues pulled together hundreds of experiments and found that spaced practice beat the same amount of practice crammed into one session, consistently.3 A follow-up study with more than a thousand participants found that the best gap between sessions depends on how long you need to remember something: the further away the test, the longer the ideal gap.4
Recall beats rereading
A third finding sharpens the picture. In 2006, Henry Roediger and Jeffrey Karpicke had students read short prose passages, then either study them again or take a test asking them to recall what they had read.5 Five minutes later, rereading looked better. Two days and a week later, the students who had practised recalling remembered more.
Pulling information out of memory strengthens it more than putting it back in. For a speaker, that is a direct instruction: getting your audience to retrieve your point does more than simply repeating it at them.
What this means for a talk's key message
A single well-delivered mention of your core point faces the same steep early drop Ebbinghaus measured. That is the research-based case for repeating your key message more than once within a talk. It is not padding. The steepest loss happens in the window right after something is first heard.
It is also the case for follow-up after the talk ends: a recap message the next day, a slide sent to attendees, a theme that returns across a series of sessions. One mention, however good, is a bet against the curve.
How to repeat without sounding repetitive
Say it at the start, return to it at the end. Opening and closing are the positions listeners remember best anyway, so placing your main point there uses the primacy and recency effects as well as repetition.
Change the form, not the message. State it as a claim, then show it in an example, then return to it as a question. The listener meets the same idea three times without hearing the same sentence three times.
Make the audience retrieve it. Before your conclusion, ask the room what the three takeaways were, or what they would tell a colleague about the talk. Retrieval practice works in a meeting room as well as a classroom.
Space the follow-up. A note the next day and a reminder a week later will do more than three messages on the same afternoon.
Syllables versus real messages
Ebbinghaus measured meaningless syllables, chosen precisely because they had no connection to anything he already knew. Real messages, with meaning and structure, generally fade more slowly because they attach to existing knowledge. A memorable story about a customer will outlast a list of figures.
So the exact numbers from 1885 do not apply to your talk. The shape does: fast early loss, slower decline later, and a clear benefit from repetition that is spaced out and makes the listener do some of the remembering.
References
- Ebbinghaus, H. (1885). Über das Gedächtnis (translated as Memory: A Contribution to Experimental Psychology, 1913). https://openlibrary.org/works/OL28477195W
- Murre, J. M. J., & Dros, J. (2015). Replication and Analysis of Ebbinghaus’ Forgetting Curve. PLOS ONE, 10(7), e0120644. https://doi.org/10.1371/journal.pone.0120644
- 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. https://doi.org/10.1037/0033-2909.132.3.354
- Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: a temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. https://doi.org/10.1111/j.1467-9280.2008.02209.x
- Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning: taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255. https://doi.org/10.1111/j.1467-9280.2006.01693.x