Bjork's deliberate difficulty boosts how students really learn science

The Weight of the Vest

Imagine training for a marathon while wearing a weighted vest. Your times look worse during practice. Your legs burn faster. The effort feels disproportionate to the reward. But when race day arrives and you shed the weight, you discover you are faster and stronger than the runner who trained without resistance.

This is the central metaphor Elizabeth Bjork and her husband Robert Bjork use to explain a counterintuitive finding in cognitive psychology: the struggle during practice is not a sign of failure. It is the mechanism of improvement. Their work, spanning more than three decades from their lab at the University of California, Los Angeles, has quietly become the scientific backbone of modern evidence-based learning strategies.

In the early 1990s, the Bjorks introduced a term that would eventually appear in teacher training curricula, learning science podcasts, and research papers across the English-speaking world: desirable difficulties. The concept captures one of the most counterintuitive findings in their field conditions that slow down learning and increase errors during practice actually produce stronger, more durable, and more transferable knowledge in the long run.

Two Psychologists, One Lab, A Lifetime of Questions

Elizabeth Ligon Bjork and Robert A. Bjork are American cognitive psychologists who have been active in their field from the 1970s through the present day. Robert Bjork was born in 1939; Elizabeth Bjork was born in the 1940s. Together, they run the Bjork Learning and Forgetting Lab at UCLA, where they have produced decades of experimental work on memory, learning, and the conditions that make both stronger.

Their partnership is both intellectual and personal a collaboration that has produced numerous co-authored papers, including a 2020 piece titled "Desirable difficulties in theory and practice" published in the Journal of Applied Research in Memory and Cognition. That paper, along with their earlier work, represents the culmination of a research agenda that began decades before the term "desirable difficulties" entered the educational lexicon.

Robert Bjork first introduced the concept in a seminal 1994 publication, building on experimental psychology's long tradition of studying how memory works and how it fails. The idea grew out of the Bjorks' shared concern about a fundamental misunderstanding they saw in students and educators alike: the belief that memory works like a tape or video recorder, with repeated exposure to the same material enabling it to be written into the brain.

The Fluency Illusion

That belief leads learners to engage in strategies that feel productive but produce fragile results. Re-reading notes, for instance, creates what the Bjorks call perceptual fluency a smoothness of processing that the brain misinterprets as understanding. When material feels familiar, when the words flow easily on the second or third read, learners often conclude that they have mastered it.

"Students can often misjudge their understanding and comprehension due to the subjective impression that can be created when re-reading notes, as it creates a sense of perceptual fluency that can be misinterpreted as understanding," according to Durrington Research School's analysis of the Bjorks' work.

The problem is that this fluency creates what the Bjorks describe as a "nice feeling" and such practices often create observable and measurable performance gains in the short term that lead students to misinterpret their actual level of learning. A student who can read through their notes fluently, who finds practice problems easy, who feels the material is familiar these are the conditions that most learners and teachers assume indicate effective learning.

But the Bjorks' research reveals that this fluency is often an illusion of competence. It confuses recognition with recall and familiarity with understanding. The conditions that make learning feel easy massed practice, re-reading, blocked problem sets produce rapid initial performance gains that fade quickly. The conditions that make learning feel hard produce slower initial progress but dramatically better retention over time.

Storage Strength and Retrieval Strength

To understand why this paradox exists, the Bjorks developed what they call the new theory of disuse. Every memory, they argue, has two independent strengths that follow different rules: storage strength and retrieval strength.

Storage strength refers to how well something is encoded in memory how firmly it is lodged in the brain's architecture. Retrieval strength refers to how easily that information can be accessed at any given moment. A piece of information can have high storage strength but low retrieval strength, which explains why you might know something deeply but struggle to recall it in the moment. Conversely, information with high retrieval strength but low storage strength comes easily to mind but is fragile it fades quickly once the environmental cues that trigger it are removed.

This distinction is the key to understanding why desirable difficulties work. When you encounter material that feels easy when re-reading creates fluency, when blocked practice keeps you in the same context retrieval strength may rise rapidly while storage strength barely changes. The information is accessible, but it is not deeply encoded. Remove the familiar context, wait a few days, and the retrieval strength collapses because the storage was never strong enough to support it.

When you engage with material that feels harder spacing your practice across time, interleaving different topics, forcing yourself to retrieve information more than re-expose yourself to it retrieval strength may rise more slowly. But the effortful processing required builds stronger storage traces. The knowledge becomes embedded in a richer network of associations, making it more durable and more transferable to new contexts.

The Five Desirable Difficulties

The Bjorks identified five primary desirable difficulties that have strong experimental support. These are not merely theoretical constructs; they are practical conditions that educators and learners can deliberately introduce into study and instruction.

The first is spacing, or distributing practice over time. more than massing all study into a single session, spacing breaks practice into multiple sessions separated by intervals. The gaps between sessions feel unproductive because retrieval is harder when time has passed but the effort of reconstructing knowledge after a delay strengthens storage in ways that massed practice cannot match.

The second is interleaving, or mixing different topics or problem types during practice. Blocked practice working through all problems of one type before moving to the next feels more organized and produces faster initial gains. Interleaving feels chaotic and confusing. But research has shown that interleaving produces superior long-term learning, particularly when learners need to discriminate between similar concepts or apply different procedures to different problem types.

The third is testing, or retrieval practice the act of forcing oneself to recall information beyond simply reviewing it. Flashcards, low-stakes quizzes, and practice tests are all forms of retrieval practice. They feel harder than re-reading because they require active reconstruction of knowledge. But that effortful reconstruction is precisely what strengthens memory. As Yiuno's overview of desirable difficulties explains, "flashcards require the student to actively recall the information. This is a desirable difficulty because it requires more effort and forces the student to do more complex processing."

The fourth is varying the conditions of practice. Learning under predictable conditions can lead to knowledge becoming context-dependent easily retrieved in the original learning environment but not elsewhere. Research has shown that even teaching the same material in different rooms, compared to the same room, can have measurable positive impacts on later recall. Varying conditions forces the brain to build multiple retrieval routes to the same knowledge, making it more accessible in novel situations.

The fifth is reducing the frequency of feedback. Immediate feedback feels helpful it confirms correct answers and corrects errors quickly. But research suggests that delaying feedback, or reducing how often it is provided, can strengthen learning. When learners must tolerate uncertainty and errors before receiving guidance, they engage in deeper processing that leads to better retention.

Learning alongside Performance

A crucial distinction the Bjorks emphasize is the difference between learning and performance. Performance is what we observe during practice; learning is the progress observed later on. These two measures often tell different stories.

During a study session using desirable difficulties, performance will often look worse than it would under easier conditions. Learners make more errors. Progress feels slower. This creates a metacognitive problem: learners interpret their struggling performance as evidence that they are not learning. They abandon the effective strategy in favor of something that feels better in the moment.

"Students often resist, as these methods feel harder," according to School Success's overview of the Bjorks' work. "Effects can vary depending on subject and context." This resistance is not irrational it is a natural response to the fluency illusion. When re-reading feels productive and interleaving feels confusing, most people will choose the path that feels smoother.

The Bjorks' research suggests that auto-pilot is the enemy of learning. When the brain slips into automaticity when practice becomes routine and effortless less processing occurs. Desirable difficulties keep the brain engaged by introducing variation, forcing retrieval, and preventing the efficiency that comes from too much predictability. Pre-tests and studying to teach others can also take learners out of auto-pilot, making them more active consumers of material.

From Lab to Classroom

The practical applications of desirable difficulties have spread far beyond the walls of the UCLA lab. The Bjorks' work underpins much of what the learning science community calls evidence-based learning strategies. Organizations like The Learning Scientists a collective that translates cognitive research for educators and students have built their popular resources around the framework of spacing, interleaving, retrieval practice, and the other desirable difficulties.

In the United Kingdom, the Bjorks' work has become central to the researchED movement, which promotes evidence-based practice in education. Their concepts appear in teacher training programs and have been directly applied in maths and science revision strategies, including low-stakes quizzes and retrieval grids used in Key Stage 3 and 4 classrooms.

The Bjorks themselves have contributed to public understanding of their work through various channels. They appeared on The Learner Lab podcast, where they discussed how incorporating desirable difficulties into practice can make it substantially better. Their 2011 paper, "Making Things Hard on Yourself, But in a Good Way," remains a touchstone for educators seeking to understand the theoretical rationale behind effortful learning strategies.

In 2019, the Bjorks published "Forgetting as a friend of learning: Implications for teaching and self-regulated learning" in Advances in Physiology Education, arguing that forgetting more than being a failure of memory is actually a mechanism that strengthens subsequent learning. When you forget something and then retrieve it, the act of retrieval strengthens the memory more than simply reviewing it would have.

What This Means for EducationGuide Readers

If you are researching learning strategies, study techniques, or evidence-based educational frameworks, the Bjorks' work offers something valuable: a theoretical foundation for understanding why some study methods feel good but produce fragile results, while others feel frustrating but build durable knowledge.

The practical implication is straightforward. When you design a study session, ask not "does this feel productive?" but "will this produce strong retention over time?" The answer often requires deliberately introducing friction spacing your practice, interleaving topics, testing yourself more than re-reading, varying your study environment, and tolerating delayed feedback. These strategies will make your practice sessions feel harder and your performance during those sessions will look worse. That is not a sign that the strategies are failing. It is evidence that they are working.

For educators, the Bjorks' framework offers a language for explaining to students why struggle is not a problem to be eliminated but a condition to be embraced. It also provides a rationale for designing instruction that includes difficulty not arbitrary difficulty, but the specific kinds of effortful processing that research has shown to enhance long-term learning.

The Myth of Blocking

One of the Bjorks' notable contributions to educational discourse is their direct challenge to a widely held belief about how study and practice should be organized. In a chapter titled "The myth that blocking one's study or practice by topic or skill enhances learning," published in 2019 in Education Myths edited by C. Barton, the Bjorks argue that the common practice of mastering one topic completely before moving to the next is less effective than interleaving topics, even though it feels more logical and produces faster initial progress.

This is characteristic of their approach: identifying intuitions about learning that are widespread, testing them experimentally, and then reporting what the evidence actually shows even when the findings contradict common sense. The resistance to their work that sometimes emerges in educational circles is, in a sense, a predictable response to having one's intuitions challenged by data.

A Legacy Measured in Decades

What does a 30-year research agenda look like when it succeeds? In the Bjorks' case, it looks like a framework that has been absorbed into the vocabulary of learning science, translated into practical tools by organizations around the world, and embedded in teacher training programs across multiple countries. It looks like a 2020 paper on desirable difficulties in theory and practice that draws on decades of experimental work to synthesize what researchers have learned. It looks like a podcast conversation where two psychologists in their eighties explain to a general audience why the struggle they are experiencing during study is actually good news.

The Bjorks continue to publish. Their comprehensive publications list includes work from 2021 on how learners with different working memory capacities benefit from interleaving, on the learning benefits of confidence-weighted testing, and on how using the internet in certain ways can enhance more than impair retention. The research agenda that began in the 1970s shows no signs of slowing.

For learners and educators who encounter their work, the invitation is to look past the immediate discomfort that desirable difficulties produce and consider the long arc of learning. The weighted vest makes training harder. But race day reveals the benefit.

Where to Read Further

For readers who want to explore the Bjorks' work directly, several resources offer accessible entry points. The Bjork Learning and Forgetting Lab's publications page at UCLA provides a comprehensive list of their papers, including the 2020 synthesis "Desirable difficulties in theory and practice" and the 2019 chapter on blocking and interleaving. Yiuno's overview of desirable difficulties offers a clear introduction to the core concepts and the five primary strategies, with a useful visual metaphor comparing the approach to training with a weighted vest. The Learner Lab's podcast episode featuring the Bjorks provides a conversational introduction to the distinction between learning and performance, with practical guidance for incorporating desirable difficulties into any practice routine.

Timeline: Key Moments in the Bjorks' Research Agenda

The Five Desirable Difficulties at a Glance