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The Swedish government recently announced it was moving from the classroom use of digital devices back to physical books. It cited concerns over declining test scores and increasing screen time.

Are these concerns well founded? And what does the science of reading say about the possible consequences of reading on digital devices versus books?

To address these questions, it’s worth remembering that, although reading might appear to be an easy task, this impression is false. Reading is arguably the most difficult task one must learn – one that requires years of formal education and practice to master. In contrast to spoken language, it is a skill we are not biologically predisposed to learn.

Millions of Australians, both children and adults, struggle with literacy.

In this series, we explore the challenges of reading in an age of smartphones and social media – and ask experts how we can become better readers.

Why is reading so difficult?

To understand why reading is difficult, one must first understand the physiology of reading.

As you are reading this sentence, your eyes are making a series of rapid movements, called saccades, from one word to the next. During these saccades, the processing of visual information is suppressed and is only available during brief intervals, called fixations, when the eyes are stationary.

Experiments that measure readers’ eye movements have shown we fixate most words because our capacity to extract visual information during each fixation is extremely limited.

In languages like English that are read from left to right, our capacity to perceive the features that distinguish letters is limited to a small region of the visual field called the perceptual span. This span extends from 2-3 letter spaces to the left of fixation to 8-12 letter spaces to the right of fixation.

The span’s asymmetry reflects the movement of attention through the text. It extends to the left in languages like Arabic, which are read from right to left. The size of the span is smaller for dense writing systems, such as Chinese.

We also know from eye-tracking and brain-imaging experiments that words require time to identify. Our best estimates suggest visual information requires 60 milliseconds to propagate from the eyes to the brain and words then require an additional 100-300 milliseconds to identify. (A millsecond is one-thousandth of a second) .

These constraints limit the maximum rate of reading to 300-400 words per minute, depending on the difficulty of the text and one’s level of comprehension.

Speed-reading advocates, who falsely promise faster reading speeds, teach you how to skim a text. Comprehension declines at a rate inversely proportional to the gain in speed.

Importantly, the upper limit for reading speed requires years of practice to attain, because it requires the brain systems that support vision, attention, word identification, language processing and eye movements to operate in a highly coordinated manner. Anything that prevents this coordination will therefore reduce comprehension.

Consequences of digital reading

So what are the likely consequences of digital reading?

With some devices, such as e-readers, there is little reason to suspect digital reading differs from the reading of books, because both formats support the mental processes required for skilled reading.

The more questionable devices are those introducing distractions (such as news websites interspersed with ads) or which have suboptimal formatting, such as centre-justified text with large or unequal-sized gaps between words. The latter is rarely a feature of paper-based texts.

Although the consequences of these two factors are under-researched, enough has been learned about human cognition to make informed predictions.

For example, images and audio unrelated to a text such as pop-up ads can capture attention. Although most adults have developed a level of executive control sufficient to ignore such distractions, young children have not.

The implications for a child who is struggling to understand the meaning of a text are obvious. Their comprehension will suffer to the extent that additional effort is required to ignore distractions, or if they do not yet have the mental coordination to understand the text has been disrupted.

There is also evidence from eye-tracking experiments that many digital environments, such as webpages, can induce specific reading strategies, such as skimming for gist or searching for information.

Although such strategies might be adaptive in some contexts, they reduce overall comprehension. This possibility should be especially concerning for children, because years of practice are needed to coordinate the mental systems that support adult levels of reading skill.

Such concerns have recently drawn more attention, because the onset of the COVID-19 pandemic caused a shift to online education and a marked increase in digital reading. Although these changes were motivated by practical necessity, their long-term consequences remain unclear.

So far, eye-tracking research has been carried out on computer screens. New technology is becoming available which will allow us to directly compare eye movements and comprehension between digital devices and paper. This should give us more clarity about the benefits versus costs of digital devices.

Given reading ability is predictive of one’s education, socioeconomic status and wellbeing, the importance of assessing the long-term consequences of digital reading cannot be overstated.

Erik D Reichle has received funding from the US National Institute of Health, US Institute of Education Sciences, UK Economic and Social Research Council, and Australian Research Council.

Lili Yu does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.