An idea that sounds backwards
Most of us assume that good teaching makes learning easy. The idea of desirable difficulties suggests that this is only partly true. The term was introduced by the cognitive psychologist Robert Bjork, and it describes study conditions that make practice feel slower or more effortful while leading to better retention and transfer later on.
The key distinction is between performance and learning. Performance is how well you do right now, during practice. Learning is what stays with you afterwards and what you can use in a new setting. Conditions that boost current performance, such as repeating the same task many times in a row, can leave little lasting trace. Conditions that lower current performance, such as leaving longer gaps between attempts, can sometimes produce stronger memory.
Familiar examples
The best-known desirable difficulties are techniques covered elsewhere in this cluster.
- Spacing. Reviewing material over several sessions feels less fluent than massing it into one. You forget part of it between sessions and have to work to reconstruct it. This is the logic behind spaced repetition.
- Testing. Answering questions from memory is harder than rereading. That struggle to retrieve is what strengthens the memory, which is the heart of retrieval practice.
- Mixing. Switching between kinds of problem, as in interleaved practice, makes each problem slightly harder because you must first decide which method applies.
- Generating before being told. Making a reasonable attempt at a question before seeing the answer, even if the attempt is wrong, often helps the later explanation stick. Evidence here is more mixed than for spacing and testing, and it depends on the learner having at least some background.
Why effort can help
No one fully agrees on a single explanation, and several ideas probably contribute. One common account is that retrieving a memory, rather than just recognizing it, strengthens the routes that lead to it. Another is that the longer gap before a review means the memory has weakened slightly, so bringing it back leaves a stronger trace than if you had never let it fade. A third is that mixing and varied practice make you pay attention to what distinguishes one situation from another, rather than just repeating a routine.
The result is often a trade-off that surprises learners. In some experiments, people who practiced under harder conditions did worse during practice but better on a later test. Because the easier group looked better during the session, both students and teachers can come away believing the easier method was superior.
The illusion of fluency
One reason this matters is that our sense of how well we know something is unreliable. When material feels smooth, such as a page you have read three times, it is natural to conclude that you have learned it. Psychologists describe this as an illusion of competence or fluency. Rereading boosts familiarity much more than it boosts the ability to produce the answer without prompts.
Noticing this is a metacognitive skill. A quick test is to close the material and try to explain it. If you cannot, the smooth feeling was misleading. More on this appears in the article on metacognition skills.
Not every difficulty is desirable
It would be a mistake to read this as “make everything harder”. The word desirable is doing real work. A difficulty is helpful when it makes you do thinking that serves the goal and when you have enough knowledge to succeed with effort. It is unhelpful when it is simply an obstacle.
Some examples of difficulty that usually does not help:
- A badly worded question or confusing instructions.
- Material far beyond your current knowledge, where you cannot make a meaningful attempt.
- A cluttered page or distracting setting that adds to cognitive load without teaching anything.
- Stress or time pressure so intense that it prevents thinking.
Complete beginners are the clearest case. When someone has no framework yet, struggling alone with unaided problems tends to overwhelm working memory. For them, worked examples that are gradually faded are often a better first step, and difficulty can be added as skill grows. This is one reason many researchers describe desirable difficulties as depending on the learner’s level, not as a fixed rule.
How to apply it sensibly
A few low-risk adjustments follow from the research, though the effects vary with subject and person.
- Test before you reread. After a study session, try writing down what you remember, then check. Use the gaps to guide the next session.
- Leave gaps. Return to material after a day or several days rather than immediately. A weekly revision plan can build this in.
- Mix problem types. Once you can handle each type separately, practice them in a shuffled order.
- Check the success rate. If you are getting almost everything wrong, the task is too hard and you need more support. If you are getting everything right with no effort, it is probably too easy. Somewhere in between, where you succeed with effort and can check your answers, is a useful target.
- Expect it to feel slower. Judge results over days or weeks, not minutes.
What the evidence can and cannot tell us
The research behind spacing and testing is fairly consistent across many experiments, including in classrooms, but the results are not guaranteed for every learner or topic. Studies often measure memory for facts over short to moderate periods, and it is less clear how well the findings extend to complex skills or to years later. Motivation also matters: a technique that is slightly more effective on paper may not help if it makes you give up studying.
So treat desirable difficulties as a guiding principle rather than a rule. When a method feels uncomfortably effortful but you can still succeed, and you can check your results, it is often worth keeping. When it feels hopeless, reduce the difficulty until the effort is productive again.