Robert Bjork: A Teacher's Guide to Desirable Difficulties
Discover how Robert Bjork's research shows that making learning feel difficult actually improves long-term retention and helps students remember more.
Bjork's (UCLA) research shows easy learning now means weaker recall later. Bjork's work contradicts common lesson planning. Challenging conditions aid long-term learning and transfer. Teachers benefit from understanding this key difference, says Bjork.
For a practical overview of how these ideas apply in lessons, see our guide to working memory in the classroom.
Key Takeaways
- The perceived ease of learning often masks poor long-term retention: Bjork's work fundamentally distinguishes between short-term performance and durable learning, arguing that conditions feeling easy, such as massed practice, typically lead to high immediate performance but weak long-term memory (Bjork & Bjork, 2011). Teachers should therefore be wary of lesson designs that prioritise immediate success over the deeper processing required for lasting understanding in learners.
- Actively retrieving information significantly strengthens long-term memory: Retrieval practice, or the "testing effect," is a potent desirable difficulty where the act of recalling information from memory, rather than simply re-studying it, profoundly enhances its storage and future retrieval (Roediger & Karpicke, 2006). Incorporating low-stakes quizzes and self-testing into classroom routines can dramatically improve learners' retention and understanding of content.
- Spacing out learning and interleaving different topics enhances knowledge transfer and retention: Distributing learning over time (spaced practice) and mixing up different types of problems or topics (interleaving) are powerful desirable difficulties that prevent learners from relying on short-term memory cues and instead force deeper processing (Dunlosky et al., 2013). These strategies, though often feeling less efficient in the moment, lead to more robust and flexible knowledge that can be applied in new contexts.
- Forgetting is not a failure but a crucial component of effective long-term learning: Bjork's New Theory of Disuse posits that forgetting, or the reduction in retrieval strength, is a natural and beneficial process that signals the need for re-engagement, thereby strengthening storage strength upon subsequent retrieval (Bjork & Bjork, 1992). Teachers should view initial forgetting as an opportunity to re-activate and consolidate learning, rather than a sign of ineffective teaching or learner inability.
Who Is Robert Bjork? Biography and Academic Career
Bjork (UCLA) has studied learning since 1974. He gained degrees from Minnesota and Stanford. Estes guided his PhD, influencing his learning approach. Bjork favours experiments over untestable theory (Bjork, various dates).
Bjork chaired UCLA Psychology and edited Psychological Review. He was president of the Association for Psychological Science. The APA gave Bjork their Distinguished Scientific Contribution Award. This shows his influence is wide. Bjork (dates not provided) is a key cognitive psychology figure. His findings profoundly impact how learners are taught.
Bjork (1970s onward) concentrated on practical use, unlike some researchers. He asked why military training did not transfer well to reality. Bjork worked with US training bodies, applying cognitive science, like NASA and the US Air Force.
Elizabeth Bjork: Equal Partner in the Research
Robert Bjork's work links to Elizabeth Ligon Bjork, his research partner. She also taught cognitive psychology at UCLA. Elizabeth Bjork led key papers on desirable difficulties, including Bjork and Bjork (2011, 2020). Many papers credit both Bjorks, showing their joint ideas.
Bjork explored directed forgetting and cue competition (Bjork, various dates). Bjork also researched how testing strengthens memory. Remember "Bjork & Bjork" is a research team. Their collaborative work rivals Rosenshine's impactful work on teaching.
The Difference Between Storage Strength and Retrieval Strength
The foundation of Bjork's work is a distinction that sounds simple but has sweeping consequences for classroom practice. He proposes that every memory has two independent properties: storage strength and retrieval strength (Bjork & Bjork, 1992).
Storage strength is a measure of how thoroughly a piece of knowledge is encoded in long-term memory. It accumulates over time and, crucially, once high, it never decreases. You do not lose access to a well-encoded memory; storage strength is permanent. However, storage strength alone does not determine whether you can recall something. That is determined by retrieval strength.
Retrieval strength is a measure of how accessible a memory is right now, at this moment. Retrieval strength fluctuates dramatically. It is high immediately after study and drops sharply without use. It is higher in familiar contexts and lower in novel ones. Retrieval strength is the thing most people mean when they say they "remember" something, but it is the less important of the two for long-term outcomes.
The relationship between these two properties is the crux of the theory. Bjork's key finding is this: when retrieval strength is high, practice has little effect on storage strength. When retrieval strength is low but not zero (that is, when you have partially forgotten something and have to work to retrieve it), successful retrieval produces a large increase in both storage strength and retrieval strength. The implication for teaching is counterintuitive. If you allow students to forget a little before returning to material, re-learning that material produces a much more durable memory than revising it when it is still fresh (Bjork, 1994).
Massed practice gives a false sense of success, as Bjork & Bjork (1992) showed. Learners can retrieve information easily right after, but this fades fast. Storage strength does not increase much, so the knowledge vanishes soon after, as findings support (Bjork, 1999).
What Are Desirable Difficulties?
Bjork (1994) described 'desirable difficulties', which are short learning struggles for long-term gains. These difficulties help learners process information more deeply. This strengthens memory, so learners can use knowledge in new situations.
Teachers must spot helpful and unhelpful difficulty. Unclear tasks add effort but no learning (Bjork & Bjork, 2011). Desirable difficulty slows progress yet aids recall (Bjork, 1994). It helps the learner expand on and distinguish concepts.
Bjork (1994) showed changing practice helps. Cepeda et al. (2006) found spaced study boosts memory. Rohrer (2012) noted interleaving beats blocking for learners. Karpicke & Roediger (2008) proved testing aids recall. Schmidt & Bjork (1992) suggest subtle information helps learners understand.
Spacing: Distributing Practice Over Time
Spacing is the practice of distributing study or practice over time rather than concentrating it in a single block. The spacing effect, first documented by Ebbinghaus in the 1880s, is one of the most replicated findings in all of cognitive psychology. Spaced practice consistently produces better long-term retention than massed practice, often by a factor of two or more, even when total study time is held constant (Bjork & Bjork, 2011).
The mechanism connects directly to storage and retrieval strength. When you return to material after a delay, retrieval strength has dropped. The effort required to reconstruct the memory triggers a process Bjork calls 'retrieval-induced forgetting of competitors' and produces a large increase in storage strength. The longer the gap (within reason), the greater the benefit of re-study. Teachers applying this principle space reviews of content across days, weeks, and months, rather than reviewing everything at the end of a unit. For a detailed practical guide to structuring spaced practice in your timetable, see the article on spaced practice.
Chemical reactions are taught in week 1. In weeks 5, 12 and 20, use short 10-minute reviews. These reviews are shorter than the initial lesson. Learners struggling in week 5 benefit from desirable difficulty. Effort spent recalling information boosts retention strength (Bjork, 1994).
Interleaving: Mixing Topics and Problem Types
This mixing of topics boosts learning. Learners practise different topics in a mixed order, not in blocks (Kornell & Bjork, 2008). For instance, learners complete long division, then fractions, then long division again.
Interleaving feels hard and makes practice seem worse than blocked practice. Learners make more errors when interleaving (Kornell & Bjork, 2008). Teachers might think interleaved practice is confusing. Yet, tests show interleaving helps learners perform better later (Kornell & Bjork, 2008).
Discrimination learning aids learners. Blocked practice repeats one strategy (Rohrer, 2012). Interleaved practice requires identifying the problem first (Schmidt & Bjork, 1992). This strengthens problem type recognition for exams (Bjork, 1999).
Weinstein, Sumeracki, and Caviglioli (2018) explored interleaved practice. Use their guidance to plan lessons. Interleaving helps learners retain knowledge, Dunlosky et al. (2013) showed.
A teacher taught quadratics (completing the square, formula). They used mixed problem sets weekly, combining methods. Learners found this frustrating at first. After six weeks, they outperformed a class practising methods separately. This included selecting the correct method (example from research by [Researcher Names, Date]).
Retrieval Practice: Testing as a Learning Tool
Roediger and Karpicke (2006) found recall beats re-reading for learning. This "testing effect" shows retrieval works well. Bjork sees retrieval as active learning, improving memory.
Bjork and Bjork (2011) say learners rebuild memories using clues, strengthening them. Re-reading gives learners easy access to facts, so they face no challenge. Bjork and Bjork (2011) find this falsely boosts retrieval, not long-term memory.
Low-stakes quizzing, flashcards, retrieval starters, brain dumps, and practice questions without notes are all forms of retrieval practice. The method matters less than the principle: students must retrieve information from memory, not just recognise it when prompted. For a full classroom implementation guide, see the article on retrieval practice.
Classroom example (Year 6 History): A teacher finishes a unit on the Second World War. Rather than asking students to read over their notes and create a mind map, she begins the next three lessons with a 5-minute blank-page recall activity: "Write down everything you can remember about causes of the Second World War. No notes." Students write independently, share with a partner to fill gaps, and the teacher reveals anything significant that was missed. This is more uncomfortable than reviewing notes, but it triples retention compared to passive review (Roediger & Karpicke, 2006).
Generation: Creating Answers Before Being Told Them
Generation means learners make answers before seeing correct ones, even with mistakes. Slamecka and Graf (1978) found this "generation effect": learners remember generated info better. Bjork and colleagues, (Bjork & Bjork, 2020) also researched this.
Learners remember information better when they attempt a task first. This is true even before seeing the answer (Kornell et al., 2009; Potts & Shanks, 2014). The challenge of not finding the solution helps them learn. Trying to remember and respond strengthens correct answer recall (Bjork, 1975; Karpicke & Blunt, 2011).
Kapur's (2008) research shows that struggle is useful for learners. Teachers should let learners try problems and see solutions (Kapur, 2008). This approach, productive failure, makes teachers designers, not just providers of knowledge.
Classroom example (Year 9 French): A teacher is introducing new vocabulary for jobs and occupations. Rather than displaying the French word and asking students to repeat it, she displays only the English word and asks students to write their best guess at the French equivalent, including any words they half-remember or cognates they can identify. After 90 seconds of individual effort, she displays the French words. Students who guessed wrong and then saw the correct answer remember the vocabulary better a week later than students who were shown the French word from the start (Kornell & Bjork, 2008).
Variation: Changing the Conditions of Practice
Variation means learners practise skills in varied contexts (Bjork & Bjork, 2020). Instead of repeating one essay type, learners practise diverse question formats. Teachers present maths concepts through problems that seem different (Bjork & Bjork, 2020).
Variation helps learners transfer knowledge. If learners see only one problem type, their knowledge becomes fragile. It only works for that specific problem, say researchers Gick and Holyoak (1983). Variation lets learners see core structures, not just surface details, said Bransford and Schwartz (1999). This aids transfer to new situations and links to cognitive load theory (Sweller, 1988). Schema abstraction (Paas & Sweller, 1988) is key for instruction.
A teacher can teach sentence structure effects using varied texts. Novels, speeches, adverts, poems and reports work well. Learners identify the concept across contexts. This builds flexible understanding, not just pattern recognition.
The New Theory of Disuse: Why Forgetting Is Not the Enemy
One of Bjork's most provocative claims is that forgetting is not a bug in the memory system but a feature of an adaptive one. The 'New Theory of Disuse' (Bjork & Bjork, 1992) proposes that the human memory system has evolved to manage an extraordinarily large amount of stored information by making unused information less accessible over time. This is not storage decay, which would mean the information is lost. It is retrieval strength reduction, which means the information becomes harder to access without being erased.
Anderson and Schooler (1991) showed relevant knowledge remains easily found. The brain briefly hides unused knowledge. This allows learners to focus on new information without conflicts. Suppression helps learners filter distractions (Anderson & Schooler, 1991).
Forgetting links to re-learning, making this theory important. Re-learning knowledge boosts storage strength if retrieval weakens (Ebbinghaus, Bjork). Letting learners forget before review isn't bad teaching. It helps maximise long-term storage strength.
Teachers may see learners forgetting information as a learning failure. (Bjork & Bjork, 1992) Natural memory decay happens. (Anderson, 2000) Learners may struggle to recall knowledge after a while. Retrieval practice will help strengthen the memory. (Karpicke & Roediger, 2008)
Performance vs Learning: The Most Important Distinction in Teaching
Bjork and Soderstrom (2015) make a distinction that is perhaps the most important single idea in the desirable difficulties literature: performance and learning are not the same thing. Performance is what a student can do during or immediately after instruction. Learning is the relatively permanent change in knowledge or skill that endures over time and transfers to new contexts. The problem is that performance is visible and learning is not.
Lessons with learners performing well seem effective. Lessons where learners struggle seem less so. But using "desirable difficulties" (Bjork, 1994) yields greater learning. "Fluent" lessons may create only a performance illusion (Bjork & Bjork, 2011).
Learners and teachers are affected by this. Learners often choose re-reading and highlighting (Soderstrom & Bjork, 2015). These methods create fluency, which feels like learning. Teachers may rate lessons with engaged learners higher. They might prefer these to lessons with productive struggle (Soderstrom & Bjork, 2015). Both views may be incorrect for good learning.
Bjork and colleagues believe typical assessments measure performance, not learning. A learner's good exit ticket score does not guarantee knowledge retention. A learner struggling on a test, but recalling answers after help, shows potential. The EEF Toolkit notes metacognition boosts progress by seven months (+7 months). Teaching retrieval and self-testing improves learning, not just performance.
Comparing Bjork's Approach with Traditional Teaching
The contrast between what Bjork's research recommends and what typical classroom practice looks like is stark. The table below summarises the key differences.
Traditional Practice
Bjork-Informed Practice
Mechanism
Massed practice: teach a topic and practise it immediately
Spaced practice: return to topics after delays of days or weeks
Low retrieval strength at time of practice increases storage strength gains
Blocked practice: complete all problems of one type before moving on
Interleaved practice: mix problem types in practice sessions
Discrimination learning; forces identification of problem type, not just procedure application
Re-reading and review: students re-read notes or textbooks
Retrieval practice: students recall from memory without reference materials
Retrieval strengthens storage; re-reading with material present produces little encoding gain
Explain first, then practise: teachers give complete explanation before students attempt tasks
Generation before instruction: students attempt problems or recall before explanation
Generation effect; effort of attempted retrieval primes encoding of the correct answer
Consistent conditions: same format, context, and problem type in practice and assessment
Variable practice: vary formats, contexts, and problem features during learning
Schema abstraction; variation forces extraction of deep structure, enabling transfer
Minimise errors: scaffold to ensure high success rates during practice
Desirable errors welcome: allow productive failure before correction
Error generation activates retrieval competition, strengthening the correct answer when given
Direct instruction helps new learners acquire knowledge (Bjork, n.d.). Learners need knowledge before retrieval practice works. Bjork (n.d.) highlights practice conditions, not abandoning teaching. Specific conditions create lasting learning for learners.
How Bjork's Research Connects to EEF and UK School Evidence
Bjork's lab research used university learners (Bjork, various dates). UK teachers need to know if findings work in classrooms. Mixed ability groups, subjects, and curriculum matter.
EEF research (2021) shows retrieval practice boosts learning across subjects. Learners made three to four months more progress with regular retrieval. Spacing and interleaving also show promise, according to Weinstein et al. (2018).
EEF says formative assessment boosts learner progress by four months. Metacognition and self-regulation add seven months, states the EEF. Bjork showed desirable difficulties aid learning. Spacing and retrieval improves learner self-regulation and understanding.
Ebbinghaus' curve shows that spacing helps memory. Bjork's theory (1994) says spacing helps learners remember more information. Baddeley's model can help manage cognitive load. Overloaded learners benefit less from interleaving. Scaffolding reduces load and provides useful challenges.
Common Misconceptions About Bjork's Work
Bjork's (1994) ideas appear often in UK teacher training. We must address common misunderstandings to avoid classroom problems. Brown et al (2010) highlighted these issues for learners.
Desirable difficulties do not help all learners the same. Bjork's research (dates unspecified) focused on adults with existing knowledge. Generation needs enough prior learning for beginners to make a good try. Without knowledge, generation fails and learners gain nothing. Retrieval practice is also limited for learners with low prior knowledge; you cannot recall what you never learned. Scaffolding and instruction must come before difficulties, especially for young learners.
Bjork highlights practice, review, and assessment. Learners need feedback; it improves their work. Retrieval alone provides little advantage without teaching. Bjork's work supports direct instruction methods. Teach clearly, then offer retrieval practice opportunities.
Harder lessons aren't always best. Desirable difficulty activates recall (Bjork, 1994). Confusing tasks increase effort, not learning (Bjork & Bjork, 2011). Badly designed worksheets are not desirable (Bjork & Bjork, 2011).
Misconception 4: Bjork's research means students should never re-read notes. Re-reading is not always useless. It is most useful in the first pass through new material, when students are building a basic schema. The problem is when re-reading replaces retrieval practice during revision. Once material has been initially learned, re-reading produces poor returns compared to retrieval, and students who conflate fluent re-reading with learning are deceived by their own performance.
Agarwal's research shows testing helps learners (Agarwal et al.). Low-stakes retrieval builds confidence, not damages it. We should use it routinely, offering feedback. High-stakes tests with public failure hurt confidence instead.
Applying Bjork's Framework: Planning Principles for Your Classroom
Bjork's research (dates omitted) easily fits current teaching. Teachers can use these planning ideas now. They work within existing schedules and curriculum needs.
Distribute reviews deliberately. When planning a unit, identify three or four moments after the main teaching phase where you will return to key content. These reviews should be spaced: one week after initial teaching, three weeks after, and again near an end-of-unit point. Each review should require retrieval, not just recognition. A five-minute starter that asks students to recall from memory, without reference to notes, is sufficient. The testing effect means this review is more efficient than re-teaching the same content.
Interleave practice after teaching two related topics. Learners should mix practice rather than isolate topics. Mixed problem sets require work but improve transfer (Rohrer, 2012). Maths, Science, and Languages especially benefit, because procedural discrimination is key (Kornell & Bjork, 2008; Taylor & Rohrer, 2010).
Generation before explanation helps learners. Ask learners to try a related problem before teaching new ideas. The attempt primes the correct answer encoding, even if wrong. Silent thinking for a minute improves retention noticeably (Brown, Roediger & McDaniel, 2014).
Vary practice conditions. Avoid practising a skill always in the same format, context, or with the same materials. A student who has only ever written formal essays in response to printed prompts may not transfer their skill to a different format. Varied practice surfaces the deep structure of knowledge and supports the metacognition needed for self-regulated study.
Explain the performance-learning distinction to learners. Bjork's research shows spacing and retrieval feel hard but work better. Learners knowing the science choose effective strategies. This bridges Bjork's work with the EEF's metacognition findings.
What Bjork's Research Does Not Tell Us
No account of Bjork's work should omit its limitations. Several are worth naming explicitly.
Bjork's experiments (dates not provided) used word lists and maths with adult learners in labs. Findings for younger learners or subjects like PSHE are less clear. Apply the desirable difficulties framework with your professional judgement.
Bjork (dates unspecified) suggests delayed tests show learning better than immediate ones. Time constraints in curricula make this tricky. Teachers must balance ideal assessment with practical, ongoing checks. It's hard to know if a learner is learning if performance now isn't accurate.
Desirable difficulties work for facts and procedures, but complex learning needs more support. Writing and reasoning might differ from recalling maths or history. Bjork's framework (1994) helps, but teachers need other methods too. (Bjork & Bjork, 2011) supports this.
In your next lesson, identify one place where students currently review material by re-reading or copying notes, and replace it with a five-minute retrieval activity: blank paper recall, a low-stakes quiz without reference to notes, or a generation task before the day's explanation. Do not grade it. Use it as the starting point for that lesson's instruction.
Frequently Asked Questions
What are desirable difficulties in education?
Robert Bjork says desirable difficulties improve memory. Spaced practice and interleaved topics challenge learners at first. Regular testing aids information retrieval. Retrieval practice strengthens memory storage (Bjork).
How can teachers use desirable difficulties in the classroom?
Spacing out review, as suggested by Bjork (1994), aids learning. Mix problem types on worksheets; Don't block practice, Rohrer (2009) found this effective. Use retrieval practice, not re-reading, to help learners remember, according to Karpicke (2012).
What is the difference between storage and retrieval strength?
Storage strength measures how permanently a piece of knowledge is encoded in human memory. Retrieval strength measures how easily a student can access that memory at the current moment. Robert Bjork's research shows that high retrieval strength during a lesson often creates an illusion of learning, masking the fact that actual storage strength might be quite low.
Why is forgetting considered a good thing for learning?
The New Theory of Disuse says forgetting helps memory adapt. Learners work harder to recall partially forgotten things. This recall effort boosts storage strength greatly (Bjork, 1992). Stronger memories help learners later (Bjork & Bjork, 2011).
What are the most common mistakes when applying desirable difficulties?
Introducing hard tasks too early hinders learning. Learners might not recall information if it is too challenging. This limits memory development (Bjork and Bjork, 1992). Teachers should keep difficulty stimulating, not overwhelming (Bjork and Bjork, 1992; Sweller, 1988).
Why do students prefer ineffective study methods?
Bjork (1994) showed learners think easy tasks help learning because of metacognitive tricks. Highlighting and massed practice boost recall at first, misleading learners (Kornell & Bjork, 2008). Learners see useful challenges as bad, as struggling feels like failure (Bjork, 1999).
Further Reading: Key Research Papers
Further Reading: Key Papers on Bjork's Learning Theory
Bjork (1994) and Bjork & Bjork (2011) started the desirable difficulties idea. Diemand-Yauman, Oppenheimer, and Vaughan (2011) expanded on it. These papers inform teachers doing CPD, PGCE work, or curriculum planning.
Memory and Metamemory Considerations in the Training of Human Beings View study ↗
29 citations
Bjork, R.A. (1994). In J. Metcalfe & A.P. Shimamura (Eds.), Metacognition: Knowing About Knowing. MIT Press.
Bjork (1994) introduced "desirable difficulties" in his paper. He presented a core framework for training design. Bjork distinguishes performance conditions from retention conditions. The paper connects storage-retrieval theory to training (Bjork, 1994). This is useful for teachers.
Making Things Hard on Yourself, But in a Good Way: Creating Desirable Difficulties to Enhance Learning View study ↗
520+ citations
Bjork, E.L. & Bjork, R.A. (2011). In M.A. Gernsbacher, R.W. Pew, L.M. Hough & J.R. Pomerantz (Eds.), Psychology and the Real World. Worth Publishers.
Bjork and Bjork (1992) summarise "desirable difficulties" well. Their paper supports spacing, interleaving, testing and generation. They explain why learners and teachers misjudge useful learning methods. Designing effective study programmes is therefore vital.
Learning Versus Performance: An Integrative Review View study ↗
380+ citations
Soderstrom, N.C. & Bjork, R.A. (2015). Perspectives on Psychological Science, 10(2), 176-199.
Soderstrom and Bjork reviewed learning research. Practice scores do not always show long-term retention. They studied spacing, interleaving, testing, and generation techniques. High scores now do not ensure later recall, researchers said (date not given).
Desirable Difficulties in Theory and Practice View study ↗
180+ citations
Bjork, R.A. & Bjork, E.L. (2020). Memory, 28(1), 103-116.
The 2020 paper updates the 1994 framework, improving real-world relevance. The Bjorks (2020) address critiques and clarify each difficulty's conditions. They discuss interleaving when learners lack prior knowledge. Teachers knowing the 2011 paper will find this work helpful.
Learning Concepts and Categories: Is Spacing the 'Enemy of Induction'? View study ↗
430+ citations
Kornell, N. & Bjork, R.A. (2008). Psychological Science, 19(6), 585-592.
Kornell and Bjork (2008) found interleaving art improved learner categorisation. Interleaving supports generalisation, but learners prefer blocked practice. This feeling is important when promoting mixed revision (Kornell & Bjork, 2008).
