Desirable Difficulties: Why Harder Studying Works Better

Desirable difficulties are study conditions that make learning feel slower and harder in the moment but make it last longer. UCLA psychologist Robert Bjork coined the term in 1994. The classic examples are spacing your study over days, interleaving different topics, testing yourself instead of rereading, and varying how and where you practice.

The idea explains a frustrating fact about studying: the methods that feel most productive are often the ones that work worst on exam day. Rereading and doing twenty identical problems in a row feel smooth. Quizzing yourself on last week's material feels clumsy. The clumsy session usually wins.

The word "desirable" matters, though. Some difficulties just get in the way. This guide covers why the right kind of struggle helps, the main examples with the research behind each, how to tell a desirable difficulty from an undesirable one, and how to build them into your week.

What are desirable difficulties?

They are the gap between performance and learning. Performance is how well you do right now, during practice. Learning is the lasting change that shows up days or weeks later, in a new setting.

In a 2015 review, Nicholas Soderstrom and Robert Bjork showed that the two can move in opposite directions. Some conditions boost performance during practice while producing little lasting learning. Others depress practice performance while producing more learning. And people tend to read their practice performance as a measure of their learning, so they pick the wrong methods.

One study shows the pattern clearly. College students learned to find the volume of four unusual geometric solids. Half practiced the problems in blocks (all of one shape, then the next); half practiced the same problems mixed together.

During practice, the blocked group looked far better. A week later, the mixed group scored three times higher. The study was small (18 students), but large classroom trials have since found the same direction of effect, as covered in our interleaving guide.

Why does harder studying work?

Robert and Elizabeth Bjork explain it with two kinds of memory strength:

• Retrieval strength: how easily you can bring something to mind right now. It's high right after you study and drops quickly.
• Storage strength: how deeply the memory is woven into what you already know. It builds up slowly and lasts.

The conditions that raise retrieval strength fastest are not the ones that build storage strength best. Rereading a definition you read two minutes ago is easy because retrieval strength is already high, and it adds little. Pulling the same definition out of memory three days later, when it has started to fade, is harder and builds much more.

The Bjorks' point is that forgetting creates the opportunity for learning. When something is perfectly fresh in your mind, studying it again adds little or nothing.

That's also why easy study is misleading. A smooth session raises retrieval strength, you mistake that for storage strength, and you stop studying too soon.

Desirable difficulties examples

Bjork's original list had four: varying conditions of practice, spacing, interleaving, and testing. Generation, producing an answer instead of reading it, is often added as a fifth.

Spacing

Study the same material across several days instead of in one block. A 2008 study of more than 1,350 people found that the best gap between sessions grew with the time until the test: roughly 20–40% of a one-week delay, and 5–10% of a one-year delay. Too short a gap helped less; too long a gap also cost recall. The spacing effect guide shows how to match the gaps to your exam date.

Interleaving

Mix different kinds of problems or examples instead of grouping them. Beyond math, Nate Kornell and Robert Bjork had people learn the styles of 12 painters. When each artist's paintings were mixed in with other artists', people identified the artist of new paintings 61% of the time, against 35% when each artist's paintings were shown together.

Retrieval practice (testing)

Answering from memory beats rereading, even though rereading wins on a test a few minutes later. This is the strongest single example and has its own guide: what is active recall. Even failed attempts help if you see the answer afterward, which is the idea behind pre-testing.

Generation

Producing an answer, example, or solution yourself instead of reading it. As the Bjorks put it, whenever you look up something you could have generated, you lose a learning opportunity. See the generation effect for how to use it.

Varied practice

Practice under changing conditions. In a classic 1978 study by Kerr and Booth, 8- and 12-year-olds practiced throwing beanbags at a target. One group always threw at the distance they'd be tested on; the other group practiced only at closer and farther targets. On the test distance, the varied group did better, despite never having practiced it. Even studying the same material in two different rooms has improved later recall compared with studying twice in one room.

For students, variation means practicing the same idea in different forms: a physics principle as a word problem, a graph, and a calculation; a vocabulary word in three different sentences.

When does a difficulty become undesirable?

The Bjorks are explicit: a difficulty is desirable only if you have the background knowledge or skills to respond to it successfully. Otherwise it is just an obstacle.

Signs a difficulty has crossed the line:

• You fail almost every attempt, even after checking answers. Go back to worked examples or a first read, then return to testing.
• You're mixing topics you haven't learned yet. Learn each problem type once before you interleave it.
• The gap was too long. If a review feels like starting over, shorten the next interval.
• You're overloaded. Long, dense material with no structure can swamp working memory. Break it into smaller chunks; cognitive load theory explains why.
• The struggle is superficial. Noise, constant notifications, and confusing explanations make studying harder without making you retrieve, choose, or connect anything.

Hard-to-read fonts are a useful warning. A 2011 study reported better memory for material printed in harder-to-read fonts, and the idea spread quickly. But when the most famous related finding, that a hard-to-read font helped people solve tricky math problems, was pooled with 16 replication attempts, the benefit disappeared. Making the page harder to read is not the same as making your brain do useful work.

A practical rule: aim for effortful success. If you answer everything instantly, increase the gap or mix in harder material. If you fail almost everything, make it easier until you get a fair share right.

Why desirable difficulties feel like they aren't working

Because your feeling of learning tracks performance, not learning.

In the painting study, 78% of participants did better on the mixed paintings, yet 78% said blocking had been as good as or better than mixing. A 2022 study asked 368 undergraduates to design or choose practice schedules for a math class; most rated schedules with little spacing and interleaving as the most effective, and rated the spaced, interleaved ones as harder and less enjoyable.

So expect desirable difficulties to feel worse. Judge them by a delayed test instead. Quiz yourself on Monday's material on Friday, without looking first. That score is your real measure. The habit of checking your own learning this way is the core of metacognition.

How to add desirable difficulties to your study week

Make these swaps one at a time. Each keeps your study time the same and changes only how you use it.

Start each session with five minutes of recall on something from earlier in the week, before any new material. That one habit adds spacing, retrieval, and a bit of interleaving at once. For how these techniques rank overall, see the best study techniques.