You don’t know most of what you’ve learned.  Why not?  Because human beings forget.  How and why do they forget?  The main story is that people forget what they don’t use.  A bit of googling turned up a nice meta-analysis, “Factors That Influence Skill Decay and Retention” (Arthur et al., Human Performance 1998).  Defining the problem:

Skill decay refers to the loss or decay of trained or acquired skills (or knowledge) after periods of nonuse. Skill decay is particularly salient and problematic in situations where individuals receive initial training on knowledge and skills that they may not be required to use or exercise for extended periods of time.

So what’s known about skill decay?  Common sense checks out.

1. Time matters.  “There is an increase in the amount of skill decay as the length of the nonpractice interval increases.”

2. Physical skills decay slower than cognitive skills.  “[P]hysical tasks display less skill decay than cognitive tasks, and the difference in decay is close to half a standardized unit… across all retention intervals.”

3. Speed tasks decay more slowly than accuracy tasks.  “Across all retention intervals, the amount of skill decay for accuracy tasks was over three times higher than that of speed tasks (i.e., [effect size] = -1.00 and -0.32, respectively).”  Learning to do something rapidly stays with you longer than learning to do something correctly.

4. Ability to transfer knowledge decays faster than mere retention.  “[S]kill decay [is] negatively associated with the level of similarity between the original learning and retention contexts.”

Big takeaway:

[T]he amount of skill loss ranges from a d [effect size] of -0.1 immediately after training (less than one day) to a d of -1.4 after more than 365 days of nonuse. That is, after more than 365 days of nonuse or nonpractice, the average participant was performing at less than 92% of their performance level before the nonpractice interval.


The results of this study suggest that the similarity of the training (acquisition) and work (retention) environments plays a major role in the retention of skills and knowledge over periods of nonuse or nonpractice, providing additional support for a basic tenet in training-program design – that is, to enhance retention, trainers should try to ensure the functional similarity of both the training device (acquisition) and actual job equipment (retention) and the environment in which both are performed.

The authors don’t connect their findings to pedagogical reform, but I’m happy to pick up the slack.  If you really want kids to acquire a cognitive skill, don’t just teach it to them and move on.  You have to maintain the skill not just with practice, but with distributed practice.  The iconoclastic flip side: Cognitive skills that aren’t worth endlessly practicing probably aren’t worth learning in the first place.

P.S. Don’t forget these facts.  They’re important!