Avoiding 'lethal mutations'

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Date published 19 December 2018

When is best-practice not best-practice?

"Lithium: After a moment it blazed as hellfire, a passionate retort which sent it cavorting across the waters like a crazed insect."

Good ideas – even when they are well-rooted in evidence-based research – can be implemented in ways which render them no longer effective, or even counter-productive; becoming examples of what Dylan Wiliam (2011) and others have dubbed ‘lethal mutations’.

One of my perennial frustrations as a teacher was when a strategy – apparently working successfully in one subject area – was identified as ‘best practice’ and imposed on different subjects. An example that sticks in my head was a literacy strategy, introduced in a school I once worked in, which encouraged pupils to use ‘exciting adjectives’ and make greater use of metaphors and similes in their writing. Whilst these might be desirable in the context of English, it doesn’t necessarily translate well when applied to pupils recording observations of alkali metals reacting with water in science – as the invented example above is intended to light-heartedly illustrate.

This is true even when trying to apply generally reliable principles of learning like retrieval practice or spaced learning (Pashler et al, 2007). It’s vital that teachers are able to successfully adapt them to the context of their subject but still maintain fidelity to the central core of these ideas. Where the central ideas are not well understood or where the adaptation to subjects is not treated with care, even really great ideas can mutate into monsters.

Here are a couple of examples:

Spaced practice

The idea of spacing out learning comes from the evidence that long-term learning appears advantaged when we have the opportunity to forget (a bit) before successfully retrieving learned material (Kang, 2016). However, this idea can give rise to the potential ‘lethal mutation’ that it’s best to chop-and-change different topics from lesson to lesson in order to achieve this spacing.

For example, if I’m teaching evolution through natural selection in biology, I might want to carefully sequence the teaching of this complex idea over several lessons – for instance, examining inheritance, variation and selection before bringing the parts together as a whole – perhaps, as Darwin did in ‘The Origin of Species’, first looking at artificial selection, then applying the same logic to natural selection. I’m not convinced it would be helpful to disrupt this sequence with completely different topics (e.g. interspersing lessons on the particle model, the weather cycle or balanced and unbalanced forces). If spacing of the teaching breaks up this structure, then there’s a danger that the curriculum becomes a noise of disjointed and unconnected ideas.

This kind of ‘radical’ spacing almost certainly isn’t necessary – pupils will forget quite a lot from one lesson to the next anyway! Better to think about the spacing of practice: To consider how to space out the opportunities to retrieve and review previously covered sequences of learning. The advice on this is relatively straightforward and appears to work well even where the material is complex (e.g. Karpicke & Aue, 2015).

Redundancy effect

Another issue arises from the finding that when text is provided in support of verbally presented material, it creates an additional strain on working memory – increasing ‘cognitive load’ (CESE, 2017). The reason for this is that reading and processing verbal speech utilise the same component of our working memory and – as people might experience when trying to read with the TV or radio on – it’s extremely difficult to split our attention in this way. However, does this mean that teachers should never read aloud – or ask a student to read a passage to the class?

It’s true that research has found that providing text in addition to a verbal presentation can create additional cognitive load (e.g. Kalyuga et al, 1999) – and therefore reading aloud over your power point presentations is probably best avoided (Ashman, 2018). However, there may be many educationally valid reasons for taking a small ‘hit’ on attention splitting to achieve other kinds of important goals. Reading aloud might be extremely useful – for example to provide information about the pronunciation of unfamiliar words, or the cadence and rhythm of a poem in English, or provide pupils with important opportunities to practice speaking in French.

"Another issue arises from the finding that when text is provided in support of verbally presented material, it creates an additional strain on working memory – increasing cognitive load"

- CESE, 2017

We should consider the complexity of the materials when assessing how problematic these kinds of split-attention effects might be, as we may tolerate a bit of additional load when tasks are relatively simple, or – if it’s more complex – we might ameliorate the issue by breaking large sections of text into more manageable chunks (CESE, 2018).

Great ideas – however evidence-based – are never ‘plug and play’ in teaching. When designing our Masters in Expert Teaching, avoiding ‘lethal mutations’ has been a particular focus. As well as helping teachers to get ‘under the hood’ of the research – identifying the ‘active ingredients’ of an intervention or strategy – the coaching sessions support teachers to translate and apply the evidence with greater fidelity to their practice. Like many teachers, we’ve too often seen ‘best practice’ create more problems than it solves to leave the implementation of new strategies to chance.

This article originally appeared on the website of the Institute for Teaching. In March 2019 the Institute for Teaching merged with Ambition School Leadership to form Ambition Institute.


  • Ashman, G. (2018) Battling the bandwidth of your brain. researchED Magazine.https://researched.org.uk/battling-the-bandwidth-of-your-brain/
  • CESE (2017) Cognitive Load Theory: Research that teachers really need to understand. NSW. Dept of Education. [online] https://www.cese.nsw.gov.au/publications-filter/cognitive-load-theory-research-that-teachers-really-need-to-understand [retrieved 13 November 2018]
  • CESE (2018) Cognitive load theory in practice: Examples for the classroom. Center for Education Statistics and Evaluation. [online] https://www.cese.nsw.gov.au//images/stories/PDF/Cognitive_load_theory_practice_guide_AA.pdf
    [retrieved 13 November 2018]
  • Kalyuga, S., Chandler, P., & Sweller, J. (1999). Managing split‐attention and redundancy in multimedia instruction. Applied Cognitive Psychology: The Official Journal of the Society for Applied Research in Memory and Cognition, 13(4), 351-371.
  • Kang, S. H. (2016). Spaced repetition promotes efficient and effective learning: Policy implications for instruction. Policy Insights from the Behavioral and Brain Sciences, 3, 12-19.
  • Karpicke, J. D., & Aue, W. R. (2015). The testing effect is alive and well with complex materials. Educational Psychology Review, 27(2), 317-326.
  • Pashler, H., Bain, P. M., Bottge, B. A., Graesser, A., Koedinger, K., McDaniel, M., & Metcalfe, J. (2007). Organizing Instruction and Study to Improve Student Learning. IES Practice Guide. NCER 2007-2004. National Center for Education Research.
  • Wiliam, D. (2011). Embedded formative assessment. Solution Tree Press.

Nick Rose
Nick Rose
Fellow, Learning Design

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