Last month I finished a book I had been looking forward to for weeks. Two weeks later, a friend asked me what it was about, and I could recall almost nothing. The plot was there, somewhere, but the details felt ephemeral, like a dream that fades a few minutes after you wake up.

This is not a personal failing. In the 1880s, the German psychologist Hermann Ebbinghaus ran a meticulous series of experiments on his own memory. He memorized lists of nonsense syllables, waited, and then measured how much work it took to learn each list again. A day later, relearning a list saved him only about a third of the original work; a month later, about a fifth. Plotted over time, the savings drop steeply at first and then flatten out, a shape now known as the forgetting curve (Ebbinghaus, 1885, ch. 7). In 2015, Jaap Murre and Joeri Dros repeated the experiment and got results very close to his (Murre & Dros, 2015).

“In one ear and out the other,” as the saying goes, is less a character flaw than the default setting of the human brain. Nonsense syllables are an extreme case, and a book with a story is easier to hold on to, but some forgetting is inevitable. What we can change is how fast it happens.

Ebbinghaus also found, under the limited conditions he tested, that spreading repetitions over several days was “decidedly more advantageous” than massing them in a single sitting (Ebbinghaus, 1885, ch. 8). Each time you return to the material, you reinforce it. A 2006 review of 317 experiments confirmed the pattern, and found that the best gap between study sessions grows with how long you need to remember something (Cepeda et al., 2006). Reviewing at growing intervals like this is called spaced repetition, and it is a well-tested way to retain what you learn.

Sleep matters too. During sleep, and especially deep slow-wave sleep, the brain appears to reactivate the day’s new memories and redistribute them from the hippocampus to the neocortex, where they become more stable (Diekelmann & Born, 2010). In other words, sleep helps consolidate what you learned while you were awake.

What about new words in a foreign language? A reader who meets an unfamiliar word usually guesses its meaning from the context first, and then, if the guess feels shaky, checks it in a dictionary or another source. Jan-Arjen Mondria studied these steps and found that each one, guessing, checking, and then a deliberate effort to memorize, added to how well learners remembered a word. Guessing and checking together did not lead to better retention than simply being given the meaning, though, and took considerably longer (Mondria, 2003).

Meeting the same word again matters as well. In a study by Stuart Webb, learners who met unknown words more often in context learned more about them; ten encounters produced sizeable gains, and full knowledge of a word may take more than ten (Webb, 2007). Each encounter can reinforce the last one, and that is how ephemeral knowledge becomes something you retain.

The book I forgot is still on my shelf. I plan to read it again, slowly this time, and to make the second reading a deliberate one. If some forgetting is inevitable, the least I can do is meet it halfway, with a meticulous reader’s habits and enough sleep to consolidate what I read.

References

  • Ebbinghaus, H. (1885). Memory: A Contribution to Experimental Psychology (H. A. Ruger & C. E. Bussenius, Trans., 1913). Full text
  • Murre, J. M. J., & Dros, J. (2015). Replication and analysis of Ebbinghaus’ forgetting curve. PLOS ONE, 10(7), e0120644. doi: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. doi:10.1037/0033-2909.132.3.354
  • Diekelmann, S., & Born, J. (2010). The memory function of sleep. Nature Reviews Neuroscience, 11(2), 114–126. doi:10.1038/nrn2762
  • Mondria, J.-A. (2003). The effects of inferring, verifying, and memorizing on the retention of L2 word meanings. Studies in Second Language Acquisition, 25(4), 473–499. doi:10.1017/S0272263103000202
  • Webb, S. (2007). The effects of repetition on vocabulary knowledge. Applied Linguistics, 28(1), 46–65. doi:10.1093/applin/aml048

I wrote this post partly as a demo text for Catglish, a browser extension I’m building that underlines the words you’ve looked up wherever they come back.