Desirable Difficulties: Why Harder Studying Sticks Longer

The study session that feels best is often the one that teaches you least. Rereading a chapter feels smooth. Doing ten problems of the same type feels like progress. Then the exam comes and much of it is gone. The idea of desirable difficulties, introduced by UCLA psychologist Robert Bjork in 1994, explains why: some kinds of struggle during study are exactly what make memories last.

The catch is in the first word. Not every difficulty is desirable. A blurry font, a noisy room or a topic far beyond your level just makes studying harder without making it better. This guide shows how to tell the two apart.

In a chapter written for students, Elizabeth and Robert Bjork separate two things. Performance is what you can observe during study: how fast you answer, how many you get right. Learning is the lasting change in what you know, which you can only see after a delay or in a new setting. Their key point is that conditions that make performance improve quickly often fail to support long-term retention, while conditions that slow apparent progress often produce the most durable learning.

You can see both sides in a single experiment. In the 2006 study by Roediger and Karpicke, students who reread a passage four times beat a recall-practice group on a test five minutes later. One week later the order flipped completely.

Spacing

Reviewing material after a gap means you have to work harder to recall it, and that work builds a sturdier memory. See spaced repetition for how long the gaps should be.

Interleaving

Mixing problem types forces you to pick the right method each time. The Bjorks cite a math study where students who mixed problem types got 63% of new problems right a week later, against 20% for students who practiced in blocks. Details are in interleaved practice.

Testing and generating

Producing an answer yourself, rather than reading it, is one of the strongest difficulties of all. The Bjorks put it bluntly: whenever you look up an answer you could have generated, you rob yourself of a learning opportunity. This is the heart of active recall.

Varying conditions

Practice in slightly different ways: different rooms, different question formats, problems in a different order. The Bjorks describe a classic finding that studying the same material in two different rooms led to better recall than studying it twice in the same room. It does not mean your study space should be chaotic. It means that some variety helps memories work outside the exact place you learned them.

The Bjorks are explicit: a difficulty is only desirable if you can respond to it successfully. Mixing three methods you have never learned is confusion, not interleaving. Spacing reviews so far apart that nothing is left is just forgetting. The struggle has to be about retrieving and connecting the material itself.

Hard-to-read fonts show how easy it is to get this wrong. A 2011 study reported that students remembered more when material was printed in harder-to-read fonts, and the idea spread quickly. Later studies often failed to find the effect, and a 2018 meta-analysis pooled the results and found the effect on recall and transfer was close to zero, a finding later reanalyzed in detail. Squinting at letters is effort, but it is not effort spent on the ideas.

1. Close the book first. Start every session by writing what you remember from last time before you look at any notes. The blurting method is an easy format.
2. Split each topic across days. Three 20-minute sessions on different days beat one 60-minute session.
3. Mix your problem sets. Once you know each method, combine problems from two to four sections and remove the labels.
4. Change the format. Rotate between flashcards, open questions, explaining aloud and past paper questions.
5. Expect the dip. Your in-session scores will drop. Write that on a sticky note if you need to.

Here is what one week can look like for a single course. Monday, learn a new topic and end with a two-minute blurt. Wednesday, start with a closed-book quiz on Monday's topic before learning anything new. Friday, do a mixed problem set drawn from this week and last week. Sunday, explain the hardest idea aloud without notes, then check. None of these sessions is longer than usual. Each one simply swaps an easy move for a slightly harder one that pays off later.

Self-testing is the easiest difficulty to add because it takes no planning beyond having questions. If you do not have time to write them, LearnKit can turn your notes or a chapter into a quiz in a minute, so the effort goes into answering rather than preparing. Answer before you reveal anything, and keep the questions you miss for your next spaced session.