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Distinguishing cause from effect – many deficits associated with developmental dyslexia may be a consequence of reduced and suboptimal reading experience



2017): Distinguishing cause from effect – many deficits associated with developmental dyslexia may be a consequence of reduced and suboptimal reading experience, Language, Cognition and Neuroscience, ABSTRACT The cause of developmental dyslexia is still unknown despite decades of intense research. Many causal explanations have been proposed, based on the range of impairments displayed by affected individuals. Here we draw attention to the fact that many of these impairments are also shown by illiterate individuals who have not received any or very little reading instruction. We suggest that this fact may not be coincidental and that the performance differences of both illiterates and individuals with dyslexia compared to literate controls are, to a substantial extent, secondary consequences of either reduced or suboptimal reading experience or a combination of both. The search for the primary causes of reading impairments will make progress if the consequences of quantitative and qualitative differences in reading experience are better taken into account and not mistaken for the causes of reading disorders. We close by providing four recommendations for future research. ARTICLE HISTORY
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Distinguishing cause from effect – many deficits
associated with developmental dyslexia may be a
consequence of reduced and suboptimal reading
Falk Huettig, Thomas Lachmann, Alexandra Reis & Karl Magnus Petersson
To cite this article: Falk Huettig, Thomas Lachmann, Alexandra Reis & Karl Magnus Petersson
(2017): Distinguishing cause from effect – many deficits associated with developmental dyslexia
may be a consequence of reduced and suboptimal reading experience, Language, Cognition and
Neuroscience, DOI: 10.1080/23273798.2017.1348528
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Distinguishing cause from effect many deficits associated with developmental
dyslexia may be a consequence of reduced and suboptimal reading experience
Falk Huettig
, Thomas Lachmann
, Alexandra Reis
and Karl Magnus Petersson
Max Planck Institute for Psycholinguistics, Nijmegen, Netherlands;
Donders Institute for Brain, Cognition and Behaviour, Radboud University
Nijmegen, Netherlands;
Center for Cognitive Science, University of Kaiserslautern, Kaiserslautern, Germany;
Centre for Biomedical Research
(CBMR), University of Algarve, Faro, Portugal
The cause of developmental dyslexia is still unknown despite decades of intense research. Many causal
explanations have been proposed, based on the range of impairments displayed by affected
individuals. Here we draw attention to the fact that many of these impairments are also shown by
illiterate individuals who have not received any or very little reading instruction. We suggest that this
fact may not be coincidental and that the performance differences of both illiterates and individuals
with dyslexia compared to literate controls are, to a substantial extent, secondary consequences of
either reduced or suboptimal reading experience or a combination of both. The search for the
primary causes of reading impairments will make progress if the consequences of quantitative and
qualitative differences in reading experience are better taken into account and not mistaken for the
causes of reading disorders. We close by providing four recommendations for future research.
Received 2 November 2016
Accepted 13 June 2017
Dyslexia; illiterates; literacy;
phonological awareness;
reading experience
1. Introduction
The specific reading disability known as developmental
dyslexia is a burgeoning field of scientific inquiry.
About 8500 studies in the Web of Science over the last
two decades include the key word dyslexia. Yet,
despite frequent claims to the contrary, establishing a
cause of developmental dyslexia remains as elusive as
ever. This is evident from articles in high impact journals,
which not infrequently arrive at contrary conclusions. To
mention just a few, it has been argued that it is widely
agreed that developmental dyslexia is caused by a pho-
nological core deficit’” (e.g. Goswami, 2003; and many
others, see also Blomert, 2011; Olulade, Napoliello, &
Eden, 2013; Saksida et al., 2016; Vellutino, Fletcher,
Snowling, & Scanlon, 2004; Wimmer & Schurz, 2010, for
recent discussions); that dyslexia is a deficit in visuo-
spatial attention, not in phonological processing(Vidya-
sagar & Pammer, 2010); that it is illogical to conclude
that absence of evidence for some aspects of a magno-
cellular deficit in some dyslexics is evidence of its
absence in all(Stein, Talcott, & Walsh, 2000); that phono-
logical and magnocellular deficit accounts both fail to
account for the full range of deficits established for dys-
lexic children the full range of deficits might be
accounted for in terms of a cerebellar deficit(Nicolson,
Fawcett, & Dean, 2001); and that the cerebellum might
stand unfairly accused, an innocent bystander in the pro-
cesses responsible for disordered motor control in devel-
opmental dyslexia the cerebellarsigns and
symptoms associated with developmental dyslexia
reflect a remote effect of neocortical perisylvian
damage on cerebellar function(Zeffiro & Eden, 2001).
It has even been suggested that developmental dyslexia
and specific language impairment are points on a conti-
nuum of learning disorders rather than distinct disabil-
ities (Kamhi & Catts, 1986; cf. Tallal, Allard, Miller, &
Curtiss, 1997; but see Bishop & Snowling, 2004;
Norbury, 2014; Reilly, Bishop, & Tomblin, 2014).
In this opinion article, we conjecture that deficiencies
in reading experience in individuals with dyslexia are a
major reason for the disagreement among researchers
about the causes of dyslexia and therefore the arguable
lack of progress towards understanding the fundamental
causes of the condition. First, we draw attention to the fact
that most known deficits associated with these reading
impairments also occur in normalilliterate or low literate
adults who have received no reading instruction or very
little. We argue that in a substantial number of individuals
with dyslexia, many deficiencies are secondary conse-
quences of a lack of (adequate) reading experience.
There are of course many people with dyslexia who
read as much or even more than people with no
© 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
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any way.
CONTACT Falk Huettig
reading disorders. We suggest that in many of these cases
the observed impairments are partly secondary conse-
quences of suboptimal reading experience. In short, we
suggest that many performance differences in individuals
with dyslexia are effects of quantitatively and/or qualitat-
ively different reading experience to an extent that has
not been fully appreciated by the research community.
In conclusion we argue that the search for the origins of
reading impairments will make progress if the conse-
quences of reading impairments are better taken into
account and not mistaken for their causes. Finally, we
provide four recommendations for future research.
2. A substantial number of dyslexia research
findings are a proxy for a lack of (adequate)
reading experience
Many experimental studies of the effect of literacy on
cognitive processing have been carried out with illiterate
and low literate participants across the world since the
seminal study of deficits in phonemic awareness in illiter-
ate adults by Morais and colleagues (Morais, Cary, Alegria,
& Bertelson, 1979). Here, we review the most important
research findings with regard to their relevance for dys-
lexia research. We will draw on evidence from studies
with illiterate and low literate participants (see
Dehaene, Cohen, Morais, & Kolinsky, 2015; Dehaene
et al., 2010; Demoulin & Kolinsky, 2016; Huettig &
Mishra, 2014), studies with pre-literate children, literate
children, and adults as well as evidence from children
and adults with dyslexia. We discuss experimental evi-
dence from behavioural studies involving categorical per-
ception, phonological awareness, verbal short-term
memory, pseudoword repetition, rapid automatised
naming (RAN), prediction in spoken language processing,
and mirror invariance. We then turn to studies that have
used structural and functional brain imaging techniques.
We will illustrate how illiterate and low literate people
produce the same or very similar experimental results
as individuals with dyslexia in these tasks. We suggest
that many performance similarities of illiterates and
people with dyslexia are not a coincidence but likely
reflect (at least in part) a common lack of adequate
reading experience. Many conclusions from dyslexia
research might therefore be misleading with respect to
the underlying causes of dyslexia. Before we turn to the
experimental evidence we will discuss what we mean
by reduced reading experience.
2.1. What do we mean by a lack of (adequate)
reading experience?
We consider reduced reading experience to be any
reading experience that is below the average of a
reader of the same age and educational background.
Reading experience is thus operationalised as any experi-
ence with any reading materials including writing on
social media and television programmes. Reading experi-
ence therefore also includes reading exposure in non-
school settings such as home settings or exposure in
pre-school nurseries. An important issue is that it is
very difficult to quantify reading experience, especially
reading exposure in non-school settings as well as
exposure/training in the pre-cursors of reading (e.g.
rhyme judgments and other phonological awareness
tasks) in pre-school nurseries and home settings (but
see Lefly & Pennington, 2000). We will return to this
issue in the conclusion section of this article. Note that
many individuals with dyslexia have (almost by defi-
nition) qualitatively different reading experience. As we
draw attention to relevant research with illiterate individ-
uals here in Section 2, we will primarily focus on quanti-
tatively different reading experience before discussing
qualitatively different experience in Sections 3 and 4. It
is, however, important to keep in mind the strong inter-
relation between quantitative and qualitative reading
2.2. Similarities in performance in behavioural
2.2.1. Categorical perception
In line with findings that individuals with dyslexia some-
times show less sharp category boundaries in the categ-
orical perception task than individuals with no reading
impairments, it has been suggested that a cause of dys-
lexia may be a low-level speech perception impairment
(Godfrey, Syrdal-Lasky, Millay, & Knox, 1981; Hurford &
Sanders, 1990; Mody, Studdert-Kennedy, & Brady, 1997;
Reed, 1989; Tallal, 1980). In the categorical perception
task participants have to judge CV syllables from a conti-
nuum of speech sounds (e.g. /da/ /ta/) and are asked
to indicate which stimulus they heard. Typically, partici-
pants judge stimuli to be consistently either /da/ or /ta/
leading to a steep slope at the category boundary in
identification curves (hence referred to as categorical
perception). Serniclaes, Ventura, Morais, and Kolinsky
(2005) conducted a study investigating categorical per-
ception using this task in Portuguese illiterate partici-
pants. They found overall similar performances of
illiterates and literates on /ba/-/da/ contrasts with this
task but observed that illiterates (like participants with
dyslexia) had a less sharp categorical boundary.
A difficult issue with research on categorical percep-
tion is that most of the studies (both with illiterates
and individuals with dyslexia) have used synthetic
speech. This has been criticised (Blomert & Mitterer,
2004) because there is evidence that performance on
synthetic continua only weakly predicts comprehension
of natural speech. Blomert and Mitterer (2004) demon-
strated that when a stimulus continuum based on
natural speech is used no perception deficits in partici-
pants with dyslexia are observed. Moreover, and
perhaps more importantly, gating studies on speech per-
ception have revealed that low-level speech perception
is not (or only very slightly) impaired, both in individuals
with dyslexia (Griffiths & Snowling, 2001; Metsala, 1997)
and in illiterates (Ventura, Kolinsky, Fernandes, Querido,
& Morais, 2007). Mendonça et al. (2003) assessed phono-
logical sensitivity to different sub-lexical and lexical units
in a syllable comparison task, where the sound difference
between consonants was manipulated (i.e. voicing,
place, and manner of articulation); and a minimal pair
comparison task, where the consonants in stressed and
unstressed syllables were manipulated. A comparison
of results from 16 illiterates and 16 literates revealed
small benefits of literacy for the discrimination of
similar phonemes (consonant pairs differing in voicing),
particularly when these phonemes appeared on
unstressed syllables characterised by less prosodic pro-
minence. Overall however, their results largely confirmed
that low-level speech perception is hardly modulated by
literacy. These findings, we suggest, are most compatible
with the notion that extensive reading practice has little
impact on fine discrimination of perceptual categories
but may result in more fine grained phonological proces-
sing of spoken words (Smith, Monaghan, & Huettig, 2014;
cf. Ziegler & Goswami, 2005), which we will discuss
further in the next sections.
2.2.2. Phonological awareness
A dominant explanation of developmental dyslexia is
that it reflects a phonological core deficit (e.g.
Goswami, 2003; Stanovich, 1988). Indeed, one of the
most consistent findings of research on both illiterates
(with no known socio-cultural/economic, developmental,
or acquired impairments; see Reis & Petersson, 2003;
Reis, Guerreiro, & Petersson, 2003) and individuals with
dyslexia is that they show poor awareness of the decom-
positional nature of speech units. Successful reading
requires awareness that words can be decomposed
into smaller segments because arbitrary characters
must be mapped to corresponding units of spoken
language. Without awareness that words (in alphabetic
writing systems) can be decomposed, a reader cannot
efficiently decode the orthography of her language. Pho-
nemic awareness develops from larger to smaller units
(Morais, Content, Cary, Mehler, & Segui, 1989). Children
are typically-able to recognise and manipulate syllables
before they can recognise and manipulate onsets and
rhymes. Onset and rhyme awareness in turn develops
before children detect and manipulate phonemes (see
Anthony & Francis, 2005; Anthony, Lonigan, Driscoll, Phil-
lips, & Burgess, 2003, for review). Early forms of phonolo-
gical knowledge and segmental awareness (e.g. syllable,
onset, and rhyme awareness) develop without teaching
and before reading instruction. Such pre-literate devel-
opment of phonological awareness is influenced by the
properties of the particular language that is acquired
(e.g. the salience of syllables and complexity of onsets;
Caravolas & Bruck, 1993; Cossu, Shankweiler, Liberman,
Katz, & Tola, 1988; Demont & Gombert, 1996; Durguno-
ğlu & Öney, 1999).
Morais and colleagues (Morais et al., 1979) asked illit-
erate Portuguese speakers to take part in a study to
investigate whether phonemic awareness is acquired
spontaneously during development or whether its occur-
rence requires some specific training. Thirty illiterates
(aged 3860 years) and 30 late literates (aged 2660
years; individuals who had taken part in adult literacy
programmes after the age of 15 years) were asked to
add or delete one phoneme (e.g. p) of an utterance
(becoming another word or a nonword) produced by
the experimenter (e.g. alhaçobecame palhaçoor
pursobecame urso). Note that in word trials the
correct response could be found by searching memory
for a similar sounding word and thus only nonword
trials provide information about participantssegmenta-
tion abilities and phonological awareness. Mean correct
responses on nonword trials were 19% for illiterates
but 72% for late literates. Subsequent studies have repli-
cated these results (e.g. Morais, Bertelson, Cary, & Alegria,
1986) but have shown that illiterates perform much
better when tested on other meta-phonological abilities
such as syllable detection (Morais et al., 1989) or rhyme
awareness (Adrián, Alegria, & Morais, 1995; Morais
et al., 1986). Read, Yun-Fei, Hong-Yin, and Bao-Qing
(1986) demonstrated that it is not literacy per se (i.e.
the ability to read and write) but alphabetic literacy (i.e.
the knowledge of an alphabetic script), which causes
differential performance on phonemic awareness tasks.
Read and colleagues found that phonemic awareness
of Chinese readers who had no alphabetic knowledge
was similar to illiterates but phonemic awareness of
Chinese readers who had alphabetic knowledge was
similar to those of late literates.
Many studies have found that phonological aware-
ness in pre-school children is an important predictor of
early reading skills after schooling has started (e.g.
Adams, 1990; Bryant, MacLean, & Bradley, 1990; Bryant,
Bradley, Maclean, & Crossland, 1989; Byrne & Fielding-
Barnsley, 1991; Cronin & Carver, 1998; Schatschneider,
Fletcher, Francis, Carlson, & Foorman, 2004; Stanovich,
1992; Tunmer, Herriman, & Nesdale, 1988; Vellutino &
Scanlon, 2001; Wagner & Torgesen, 1987; and many
others). Moreover, training pre-school children in phono-
logical awareness before reading instruction has been
found to facilitate subsequent reading acquisition
(Lundberg, Frost, & Petersen, 1988; see also Ball & Blach-
man, 1991; Bradley, 1988; Bradley & Bryant, 1983;
McGuinness, McGuinness, & Donohue, 1995). These
studies show that phonological awareness is important
for early reading acquisition in alphabetic writing
systems. Indeed, training in phonological awareness
tasks in the early school years also results in more
rapid reading acquisition in children with no detectable
signs of reading impairment.
Do these studies thus suggest that the phonological
awareness deficits in individuals with dyslexia are a
likely cause of developmental dyslexia? As for adult illit-
erates, there is evidence that many children need instruc-
tion for phonemic awareness (Ehri & Robbins, 1992;
Masonheimer, Drum, & Ehri, 1984). Moreover, exposure
to pre-school phonological awareness training (e.g. kin-
dergarten or parental teaching) differs greatly and is
often influenced by factors such as nursery access and
the socioeconomic status (SES) of the children. Some
children get a head start in phonological awareness
training prior to school whereas others have a lot of
catching up to do.
It is pertinent at this junction to further discuss the
issue of SES. There is no doubt that dyslexia is a real
phenomenon (see Stein, 2017, for further discussion)
and has a partly biological origin. Twin studies estimate
the heritability of the disorder to be between 0.50 and
0.60 (e.g. Grigorenko, 2001; see also Carrion-Castillo,
Franke, & Fisher, 2013; Carrion-Castillo et al., 2017;
Ramus, 2013). It has also been convincingly demon-
strated that environmental factors, in particular SES,
play a large role in reading skills (Bowey, 1995; Fluss
et al., 2009). Indeed, SES has been found to systematically
mediate the relationship between phonological aware-
ness and reading ability (Noble, Farah, & McCandliss,
2006; Noble, Wolmetz, Ochs, Farah, & McCandliss,
2006). The study by Fluss et al. (2009) particularly demon-
strates the influence of SES. They tested 1062 elementary
school children from Paris and observed that children
from areas with low neighbourhood SES were almost
10 times more likely to have a reading disorder than chil-
dren from areas with high neighbourhood SES. Level of
maternal education and father unemployment were
the largest SES factors associated with poor phonological
awareness and poor decoding skills. In short, differences
in reading-relevantlanguage experience emerge very
early on and have a large influence on whether children
develop dyslexia or not.
Finally, it is often overlooked that phonological aware-
ness greatly supports vocabulary acquisition. There is
much work that has demonstrated the reciprocal
relationship between vocabulary acquisition and
reading skills (e.g. Bishop & Adams, 1990; Butler, Marsh,
Sheppard, & Sheppard, 1985; Catts, Fey, Zhang, &
Tomblin, 1999; Chaney, 1998; Dickinson, McCabe, Ana-
stasopoulos, Peisner-Feinberg, & Poe, 2003; Lonigan,
Burgess, & Anthony, 2000;Scarborough, 1989; Shankwei-
ler et al., 1999; Share, Jorm, Maclean, & Matthews, 1984;
Stahl & Fairbanks, 1986; Tunmer et al., 1988; Vellutino &
Scanlon, 2001; and many others). For example, children
with good phonological awareness can draw on these
abilities to decode words more rapidly and thus get a
head start in figuring out the meaning of new words
(cf. Beck, Perfetti, & McKeown, 1982) which in turn
further increases their phonological awareness.
We do not deny the possibility that severe deficiencies
in the acquisition of phonemic awareness may be cau-
sally related to the reading impairments in some individ-
uals with dyslexia. We do however note that this has not
been convincingly demonstrated to date. Indeed,
deficiencies in phonological awareness are entirely con-
sistent with the assumption that dyslexia is linked to sub-
optimal reading experience (which we will discuss later
on in this article). Moreover, it is also feasible that
deficiencies in the acquisition of phonemic awareness
reflect comorbidity with the actual underlying cause of
dyslexia (cf. Williams & Lind, 2013). In any case, there is
no doubt that it is experience with alphabetic writing
systems that radically improves phonological awareness.
It is important to note that it depends on the proper-
ties of the orthography acquired as to how quickly
reading instruction improves phonological awareness
(Ziegler et al., 2010). A useful way to describe the ortho-
graphic code of a particular language is in terms of trans-
parency and granularity (Wydell & Butterworth, 1999;
Ziegler & Goswami, 2005). Children learning to read a
transparent (i.e. consistent print-to-sound mapping)
orthography (e.g. Italian) learn to recode letter strings
with 90% accuracy within a few months (Goswami, Por-
podas, & Wheelwright, 1997; Seymour, Aro, & Erskine,
2003). Children learning to read a non-transparent ortho-
graphy (e.g. English) acquire the same level of accuracy
only after 34 years of reading instruction (Goswami
et al., 1997). Moreover, there are suggestions that inci-
dences of dyslexia in opaque languages (e.g. English)
are considerably higher than in languages with more
transparent print-to-sound mapping (e.g. Italian or
Spanish; Castles & Coltheart, 1993; Jiménez & Ramírez,
2002) though there have been few systematic assess-
ments of reported incidences of dyslexia across
languages. What is clear is that the profile of individuals
with dyslexia in transparent orthographies can be differ-
ent. Dyslexia in opaque languages manifests itself in
reading accuracy measures whereas in transparent
languages is often only revealed by reading speed
measures. This means that it is much easier to predict
and assess dyslexia in opaque than in transparent
languages (Moll et al., 2014). Relatedly, dyslexia
appears to occur much less in orthographies with a
coarse-grained granularity (e.g. Mandarin; Ho, Chan,
Chung, Lee, & Tsang, 2007). It is therefore likely that
lack of sufficient experience with consistent print-to-
sound mapping is a main determinant of the phonologi-
cal awareness deficits in people with dyslexia, and this is
of course obviously the case for illiterates. This con-
clusion is also suggested by a study that showed that
the phonological awareness deficits of children with dys-
lexia tend to disappear if they learn a consistent ortho-
graphy (Dutch) but not if they learn an inconsistent
orthography such as English (De Jong & van der Leij,
In summary, as adult illiterates learn to read a trans-
parent and fine-grained orthography, their phonological
awareness skills progressively reach close to normal
levels. Similarly, as children get exposed to written
language, their phonological awareness increases
rapidly. Many children with dyslexia who learn a consist-
ent orthography eventually develop normal levels of
phonological awareness. There is little doubt that
people struggling to acquire a new skill typically, and
quickly, receive comparatively less experience in apply-
ing the skill. Thus, it is likely that children who struggle
with reading acquisition quickly get a lot less adequate
reading experience than children who excel. We there-
fore conjecture that phonological awareness deficits
are largely a consequence of a lack of adequate experi-
ence with consistent print-to-sound mapping, and not
a cause of dyslexia.
There is much experimental evidence that is consist-
ent with the notion that developmental dyslexia involves
a phonological core deficit (e.g. Goswami, 2003; Ramus,
2014; Stanovich, 1988). It is important to stress here
that we do not argue that lack of reading experience
explains all of the phonological deficits observed in
people with dyslexia nor do we aim to provide a compre-
hensive discussion of phonological theories of dyslexia
here (see Ahissar, 2007; Ramus, Marshall, Rosen, & van
der Lely, 2013; Shaywitz & Shaywitz, 2008 for further dis-
cussion). What we do stress is that experimental studies
need to tightly control the reading experience of partici-
pants. Studies with illiterate and ex-illiterate control
groups offer one particularly useful way to achieve this.
A recent study by Fernandes, Vale, Martins, Morais, and
Kolinsky (2014) illustrates this point nicely. Using a
paradigm introduced by Van Leeuwen and Lachmann
(2004) they examined the influence of surrounding con-
tours on letter and pseudo-letter processing by children
with dyslexia and adult illiterates and ex-illiterates. All
groups showed a congruency effect for pseudo-letters,
namely better performance for targets surrounded by
congruent shapes than for targets surrounded by incon-
gruent shapes but only the children with dyslexia (but
not the adult illiterates and ex-illiterates) showed a con-
gruency effect for letters. Interestingly, the letter (but not
the pseudo-letter) congruency effect of the children with
dyslexia correlated strongly with measures assessing
their phonological recoding abilities, consistent with
the hypothesis that developmental dyslexia involves a
phonological deficit.
2.2.3. Verbal short-term memory
Several studies have reported deficits of participants with
dyslexia in verbal memory tasks including story recall
(Felton, Wood, Brown, Campbell, & Harter, 1987;ONeill
& Douglas, 1991), list learning (Douglas & Benezra,
1990; Felton et al., 1987; Kinsbourne, Rufo, Gamzu,
Palmer, & Berliner, 1991; McGee, Williams, Moffitt, &
Anderson, 1989; Michaels, Lazar, & Risucci, 1997; Rudel
& Helfgott, 1984), and paired associate learning (Helfgott,
Rudel, & Kairam, 1986; Vellutino, Steger, Harding, & Phil-
lips, 1975), suggesting that verbal memory deficits might
be causally related to dyslexia. Note, however, that par-
ticipants with dyslexia show comparable performance
to controls on spatial/visual memory tasks (Fletcher,
1985; Liberman, Mann, Shankweiler, & Werfelman,
1982; Nelson & Warrington, 1980): the short-term
memory deficit appears largely specific to verbal short-
term memory.
Similar to people with dyslexia, memory deficits in illit-
erates appear to be related to verbal short-term memory
rather than visual/spatial memory. A number of studies
observed a difference on digit span tasks between illiter-
ate and literate participants (Ardila, Rosselli, & Rosas,
1989; Reis, Guerreiro, Garcia, & Castro-Caldas, 1995; Reis
et al., 2003). In digit span tasks participants are required
to verbally repeat a list of digits forwards (or sometimes
backwards). Reis et al. (2003) observed that number of
years of formal education improved performance
(mean digit span for illiterates was 4.1, for low literates
with 4 years of education it was 5.2, and for high literates
with 9 years of education: 7.0). Improvement contingent
upon formal schooling thus appears to be graded, which
suggests that performance on digit span tasks (and prob-
ably short-term memory tests in general) is at least partly
a function of schooling/literacy. Petersson, Ingvar, and
Reis (2009) found no significant difference between illit-
erates and low-level literates on the Wechsler spatial
span task but replicated the significant difference on the
Wechsler digit span (see also Silva, Faísca, Ingvar, Peters-
son, & Reis, 2012). These results are most compatible with
the notion that verbal memory deficits both in illiterates
and individuals with dyslexia are at least partly a second-
ary consequence of reading practice leading to the
development of more fine-grained phonological rep-
resentations (cf. Demoulin & Kolinsky, 2016;Smith
et al., 2014; Ziegler & Goswami, 2005). Moreover, it is
also conceivable that reading practice trains our short-
term memory for verbal material (a notion that has
been little explored experimentally so far).
2.2.4. Pseudoword repetition
A task that partly involves verbal short-term memory is
pseudoword repetition. Many studies have reported
that individuals with dyslexia perform more poorly on
pseudoword repetition than control participants (e.g.
Brady, Poggie, & Rapala, 1989; Brady, Shankweiler, &
Mann, 1983; Hulme & Snowling, 1992; Kamhi & Catts,
1986; Snowling, 1981; Snowling, Goulandris, Bowlby, &
Howell, 1986; Van Bon & Van Der Pijl, 1997; Van Daal &
van der Leij, 1999), compatible with the notion that a
phonological core deficit provides a causal explanation
for dyslexia. But note that Reis and Castro-Caldas
(1997) found that illiterates also performed much
worse than literates on pseudoword repetition (repli-
cated in Castro-Caldas, Petersson, Reis, Stone-Elander, &
Ingvar, 1998). Petersson and colleagues have argued
that this is related to an inability to handle certain
aspects of sub-lexical-phonological structure (Petersson,
Reis, Askelöf, Castro-Caldas, & Ingvar, 2000) and
suggest that phonological representations, or the pro-
cessing of these representations, are differently devel-
oped in literates and illiterates (Petersson et al., 2000,
2001; cf. Smith et al., 2014). Consistent with this interpret-
ation, Boada and Pennington (2006) provided evidence
that reading-impaired children have less fine-grained
phonological representation, linked to poorer perform-
ance in pseudoword repetition tasks (see Rispens &
Baker, 2012, for further discussion).
Again, it is noteworthy that there are some reports that
the accuracy of pseudoword repetition in pre-literate chil-
dren predicts subsequent literacy levels (e.g. Gathercole &
Baddeley, 1993). But as for the studies of phonological
awareness as a predictor of later reading abilities,
although these show that phonological processing abil-
ities are important for reading acquisition, they do not
show that pre-literate differences in pseudoword rep-
etition are caused by an underlying phonological deficit.
We conjecture that an important mediator of perform-
ance is the same for illiterates and people with dyslexia;
namely, a lack of adequate experience, resulting in less
fine-grained, multiplexed phonological representations.
2.2.5. Rapid automatised naming
A great number of studies show slowed RAN in partici-
pants with dyslexia (see Araújo, Reis, Petersson, &
Faísca, 2015, for a recent meta-analysis). RAN refers to
the time required for an individual to quickly and accu-
rately name an array of well-known visual stimuli (e.g.
letters, digits, objects, or colours). So far no RAN study
has been carried out with illiterate participants. But yet
again, RAN performance correlates positively with
reading skills in readers with no history of reading
impairments (Araújo et al., 2015). Araújo et al. (2015)
also showed that there is an independent association
between RAN and reading competence in individuals
with dyslexia when the effect of phonological skills is
The RAN deficits in participants with dyslexia are inter-
esting because it has been argued that picture naming
involves many perceptual and cognitive stagesinclud-
ing visual object recognition, conceptual preparation,
lexical selection, morpho-phonological code retrieval,
phonological encoding, syllabification, phonetic encod-
ing, and articulation (Indefrey & Levelt, 2004; Levelt
et al., 1991). Araujo, Huettig, and Meyer (2016) recently
tested skilled adult readers and readers with dyslexia on
a serial object-naming RAN task, independently manipu-
lating word frequency and phonological neighbourhood
density. Eye-movement measures revealed that both
lexical frequency and neighbourhood density effects
occur at early stages of lexical-phonological encoding
and final stages of word production in both groups.
Importantly, the effect of reading competence was
always in the direction of less efficient processing for
reading impaired than control readers, thus suggesting
that the deficit spans all stages of naming. This finding
is consistent with the notion that many aspects of RAN
deficits in individuals with dyslexia are a consequence
of reduced reading experience rather than a deficit at
one specific stage of the naming process (though this
account remains to be tested in illiterate individuals).
2.2.6. Anticipatory eye movements
Huettig and Brouwer (2015) used an eye-tracking
method and observed that adults with dyslexia antici-
pate visually presented target objects that are about to
be mentioned in a concurrent spoken sentence later
than control participants. In addition, participantsword
reading scores correlated positively with their anticipat-
ory eye movements, providing a possible link between
reading experience and anticipatory spoken language
Similar findings have also been observed with illiter-
ates. Mishra, Singh, Pandey, and Huettig (2012) pre-
sented low and high literates in India with simple
every-day spoken sentences which included a target
word (e.g. door). As participants listened to the sen-
tences they were asked to look at a visual display of
four objects (a target, i.e. the door, and three visual dis-
tractor objects). The spoken sentences were created to
encourage predictive eye gaze to the visual target
objects. The high literacy group started to shift their
eye gaze to the target object well before the onset of
the spoken target word. The low literates on the other
hand did not anticipate the targets and looked at the
targets only once they heard the target mentioned in
the spoken sentence more than a second later. This
suggests that reading experience modulates predictive
spoken language processing. Note that there was no
hint of a delay in non-anticipatory language-mediated
eye movements in the dyslexic group in Huettig and
Brouwer (2015) and only small delays in the low literate
group in Mishra et al. (2012). This strongly suggests that
the general word-object mapping in adults with dyslexia
and low literates is similarly fast and efficient as that of
literate adults with no reading disorders. It means that
the prediction differences are unlikely to be mere deficits
in visual scene processing (as reading is a highly trained
visual skill and correlates with the skill of interpreting 2D
visual representations such as line drawings, cf. Bramão
et al., 2007; Reis, Faísca, Ingvar, & Petersson, 2006; Reis,
Petersson, Castro-Caldas, & Ingvar, 2001). Mani and
Huettig (2014) provided converging evidence for the
role of literacy in listenersanticipation of upcoming
spoken language. They tested 8-year-old German chil-
dren at the beginning of literacy acquisition and found
a robust positive correlation between childrens word
reading and their prediction skills in a similar eye-track-
ing task. Furthermore, James and Watson (2013) found
that reading experience as measured by performance
in the Comparative Reading Habits questionnaire
(Acheson, Wells, & MacDonald, 2008) and the American
Adult Reading Test (Blair & Spreen, 1989) is linked to pre-
dictive spoken language processing even among Amer-
ican college students. To sum up, illiterates and people
with dyslexia show similar language prediction deficits,
with evidence consistent with the notion that this at
least partly reflects their common reduced reading
exposure rather than a causal impairment due to a
reading disorder.
2.2.7. Mirror invariance
The similarities in performance between illiterates/low
literates and individuals with dyslexia are not only appar-
ent in tasks involving spoken language but are also
evident in tasks involving visual processing (e.g. Lach-
mann, Khera, Srinivasan, & van Leeuwen, 2012; Lach-
mann & van Leeuwen, 2008). Note that the visual
routines supporting object recognition that humans
have acquired during evolution can actually impede
reading acquisition. Mirror invariance (Pegado, Naka-
mura, & Hannagan, 2014) or symmetry generalisation
(Lachmann, 2002), for instance, is the ability to quickly
identify visual stimuli irrespective of their (e.g. left-
right) orientation. This ability is well investigated in
monkeys (Logothetis, Pauls, & Poggio, 1995; Noble,
1966; Rollenhagen & Olson, 2000), and humans (Bieder-
man & Cooper, 1991; Bornstein, Gross, & Wolf, 1978; Stan-
kiewicz, Hummel, & Cooper, 1998). In order to learn to
read, however, mirror invariance must be suppressed,
in particular in the alphabetic phase (cf. Frith, 1986)of
reading acquisition. In this phase the letters of a word,
i.e. the graphemes, are decoded into the corresponding
sound one by one, and the sounds are merged together
into syllables and words. Fine details of each individual
grapheme, its orientation (e.g. band d) and the order
of graphemes in the configuration are crucial. Alphabetic
reading remains, however, a strategy of skilled reading
(Coltheart, 1978). Pegado et al. (2014) demonstrated
that literates, but not illiterates, displayed substantial
mirror costswhen judging whether mirror pairs (e.g.
iblo oldi) were the sameor not. Literacy also resulted
in mirror costs for false fonts and pictures. Importantly,
Pegado et al. (2014) found that the effect is related to
reading experience: participants who had learned to
read late in life also showed mirror costs (see also
Pegado, Nakamura, Cohen, & Dehaene, 2011).
Here again, similar results can be observed in individ-
uals with dyslexia. Lachmann and Van Leeuwen (2007)
showed that mirror discrimination abilities develop
more slowly in children with dyslexia and that they
perform better than individuals with no reading impair-
ments on mirror invariance (and related) tasks. Conver-
ging evidence comes also from beginning readers.
Cornell (1985), for instance, found that beginning
readers find it difficult to discriminate mirror pairs (e.g.
band d). Cornell (1985) estimates that it takes about
two years of reading experience for mirror confusion
to disappear. In short, mirror confusion in individuals
with dyslexia is not necessarily a cause of their reading
impairments but it is conceivable that it is linked to a
lack of adequate reading experience.
2.3. Similarities in brain structures and fMRI
Structural (Vandermosten et al., 2012, for review) and
functional (Vandermosten, Hoeft, & Norton, 2016,for
review) imaging studies have revealed abnormalities in
people diagnosed with developmental dyslexia (for a
comprehensive discussion of brain networks involved
in normal reading see Dehaene & Cohen, 2011;Naka-
mura et al., 2012; Price, 2012; Pugh et al., 1996;
Rueckl et al., 2015). There are once again some note-
worthy similarities between illiterates/low literates
and individuals with dyslexia, which we will now
2.3.1. Structural imaging
Several studies have reported differences in the structure
of the corpus callosum (i.e. white matter tracts connect-
ing the two hemispheres of the brain) between illiterates
and literates. Castro-Caldas et al. (1999) showed that the
posterior mid-body region of the corpus callosum is sig-
nificantly thinner in Portuguese illiterates than literates.
This finding was replicated by Petersson, Silva, Castro-
Caldas, Ingvar, and Reis (2007) who used voxel-based
morphometry on structural MRI data and observed
greater white matter density in the posterior third of
the mid-body region of the corpus callosum in literates
(4 years of school attendance between 6 and 10 years
of age) than in illiterates. It is interesting to note that a
rostral to caudal myelination process of the corpus callo-
sum during childhood and early adulthood has been
characterised (Thompson et al., 2000). In particular, the
white matter fibres that cross over in the posterior
mid-body region of corpus callosum interconnect the
parieto-temporal regions and undergo extensive myeli-
nation during the typical period of reading acquisition
(i.e. 610 years of age). This suggests that acquiring
reading and writing skills at the appropriate age
shapes not only the morphology of the corpus callosum
and the corresponding interhemispheric connectivity
but also the pattern of interaction between the intercon-
nected inferior parietal regions. Carreiras et al. (2009)
compared illiterates with individuals who acquired
reading late in life and also observed a greater amount
of white matter in the splenium of the corpus callosum.
These results strongly suggest that reading experience
strengthens white matter pathways in the corpus
Very similar structural differences in the corpus callo-
sum have been reported in developmental dyslexia.
Dougherty et al. (2007), Robichon and Habib (1998),
and Rumsey et al. (1996) all reported reduced white
matter in the corpus callosum. Carreiras et al. (2009) con-
cluded in their paper that white matter differences in the
corpus callosum are likely to be a consequence of differ-
ences in reading experience rather than the cause of
reading difficulties. We agree.
2.3.2. Functional neuroimaging
A number of cross-sectional MRI studies with adults and
children have suggested that functional deficits and
structural disruptions of the thalamus are a cause of
developmental dyslexia (Diaz, Hintz, Kiebel, & von Krieg-
stein, 2012; Jednoróg et al., 2015; Livingstone, Rosen, Dri-
slane, & Galaburda, 1991; Preston et al., 2010). Skeide
et al. (2017) recently combined a controlled longitudinal
reading intervention with resting-state fMRI in a sample
of 30 illiterate Indian adults. Evidence for plasticity was
found in the intrinsic functional connectivity of the
right superior colliculus and the bilateral pulvinar nuclei
of the thalamus. Moreover, the coupling between these
regions and the right occipital cortex increased and the
individual levels of coupling were correlated with indi-
vidual gains in decoding skill. These results suggest a
reading-related, functional reconfiguration upstream to
the visual cortex. Thus, faulty subcortical activity
reported in developmental dyslexia might be conse-
quential and not causal.
Finally, it has long been observed that written
language processing is consistently left-lateralised in
most readers. Petersson et al. (2007) used PET with illiter-
ates and low literates. Their analysis revealed a positive
functional left-right difference in the literate participants.
The illiterate participants in contrast showed a negative
left-right difference in the inferior parietal region. Peters-
son et al. (2007) concluded that literates are relatively
left-lateralised and that literacy influences the functional
balance between the left and right inferior parietal
regions. This is consistent with functional connectivity
findings related to the inferior parietal region and differ-
ences between controls and participants with dyslexia
(Horwitz, Rumsey, & Donohue, 1998). In addition, many
studies have found that increased bilateral processing
is associated with reading disorders such as develop-
mental dyslexia (e.g. Shaywitz et al., 2007). Eisner et al.
(2016) investigated the effects of literacy cross-section-
ally across groups in India before training (N= 91) as
well as longitudinally (a group of 29 illiterates was
trained in reading and writing for 6 months). Reading
ability was correlated with left-lateralised processing of
written materials (but not other visual categories) in
various regions along the dorsal and ventral streams.
Importantly, training-related changes in the lateralisation
of responses to written stimuli (e.g. posterior fusiform
gyrus) in their study suggest a causal relationship
between the degree of hemispheric asymmetry and
reading ability. These results also suggest that bilateral
processing of written materials is related to the relative
lack of reading experience and not necessarily diagnostic
of a reading disorder such as dyslexia.
3. Potential counter-arguments
There are a number of counter-arguments one may raise
against the view that the performance differences
between individuals with dyslexia and literate controls
are, to a non-negligible extent, direct consequences of
reduced reading experience. In this section we consider
three counter-arguments.
3.1. Some people with dyslexia read as much or
even more than some people with no reading
We have argued so far that children who struggle with
reading acquisition quickly get a lot less reading experi-
ence than children who excel at reading. This can be
compared to a snowball effect, that is, a situation
where something relatively small and insignificant
grows exponentially at a rapid pace. Reading experience
accumulates not only in the size of reading-related
knowledge but also in the speed of reading acquisition.
We have argued that this fact has been underappre-
ciated by the research community. Clearly however,
this does not hold for all individuals with dyslexia.
Parents and educational institutions often spot reading
difficulties early and provide opportunities for additional
reading training. We do acknowledge that there are
many reading-impaired people with approximately
equal or sometimes even more reading experience
than individuals with no reading impairments.
However, another factor we believe has been underap-
preciated is that often, (perhaps small) impairments in
people with dyslexia lead to substantial suboptimal
reading experience.
3.2. So-called compensated dyslexics
A similar argument concerns the fact that many individ-
uals with dyslexia are able to enter as well as successfully
complete higher education programmes (e.g. university
courses). University students with dyslexia, often referred
to as compensated dyslexics, presumably have enough
reading experience to be able to succeed at the highest
educational level. Although reading-impaired individuals
often receive special support in educational settings (e.g.
an increased time to answer exam questions) there is no
doubt that many of these people who have gone
through higher education, for instance, have read
much more than many non-reading-impaired individuals
who have not entered higher education programmes.
Nevertheless, we believe that the characterisation of
their reading experience as suboptimal (and examining
the consequences of suboptimal experience) is
3.3. Reading level control groups
A final counter-argument concerns the fact that many
studies involving participants with dyslexia attempt to
account for differences in reading experience by
testing reading-matched samples. For example, if
affected individuals in Grade 5 are matched to children
with the same reading level, for example in Grade 3,
but still show differences in phonological tasks or
tests probing other functions, one may argue that it
cannot be concluded that reading experience is an
important factor. Note that reading level control
groups are argued to be inappropriate (as they fail to
account for many confounding variables such as age,
see Van den Broeck & Geudens, 2012, for detailed dis-
cussion of this argument). However one may still
want to question how we can suggest that it is
reading experience that plays a crucial role if partici-
pants have equivalent reading skills but show differ-
ences on tasks assumed to tap certain reading-
relevant core skills. Again, we believe that the answer
to these questions is that many of these reading-experi-
enced, compensated, and reading level-matched indi-
viduals with dyslexia have had substantial suboptimal
reading experience and that suboptimal reading pro-
cedures/strategies often become automatised in
affected individuals.
4. A substantial number of dyslexia research
findings are a proxy for suboptimal reading
4.1. What do we mean by suboptimal reading
It is important at this stage to point out that reading
acquisition is a form of procedural learning (Lachmann
& van Leeuwen, 2014; Nicolson, Fawcett, Brookes, &
Needle, 2010). There are many human activities that
become automatised by learning. When we cycle or
drive a car, most of the time we are not aware of the
sequence of movements we are initiating: a chain of
motor commands has become automatised. Reading
and writing are particularly pertinent examples of an
overlearned behaviour arrived at through extensive prac-
tice (cf. Logan, 1988). Many of us have had the experi-
ence of the mind wandering and thinking about
something else when reading a novel, yet we may con-
tinue to read for several pages before noticing that we
are no longer taking in any meaningful information from
the book. Writing is typically preceded by a conscious
decision to write but people are usually not aware of
their detailed finger movements. Reading and writing
require the recruitment of pre-existing visual and audi-
tory processing skills, which have to be fine-tuned for
the new purpose. For instance, the newly acquired pro-
cedures have to be optimised for word reading and
reading longer texts, and coordinated to allow for effi-
cient grapheme-phoneme mapping (to read alphabetic
scripts) and phoneme-grapheme mapping (to write
alphabetic scripts). Finally, these new coordination
skills become automatised (Lachmann & van Leeuwen,
Note however that automatisation is far from instan-
taneous and typically requires years of practice (e.g.
Lachmann & van Leeuwen, 2008). As discussed above,
a consistent print-to-sound mapping is very helpful in
this regard and is the reason why Italian children, for
instance, learn to recode letter strings with 90% accu-
racy within a few months (Goswami et al., 1997;
Seymour et al., 2003) in contrast to children learning
to read English, who acquire the same level of accuracy
only after 34 years of reading instruction (Goswami
et al., 1997). There are many other factors that
impede the acquisition of reading routines and make
overlearning challenging. One possibility is that rela-
tively minor deficiencies in low-level auditory (cf.
Ahissar, Protopapas, Reid, & Merzenich, 2000;Christ-
mann, Lachmann, & Steinbrink, 2015; Hämäläinen, Sal-
minen, & Leppänen, 2013; Richardson, Thomson,
Scott, & Goswami, 2004; Talcott & Witton, 2002)and
low-level visual (cf. Becker, Elliott, & Lachmann, 2005;
Slaghuis & Ryan, 1999;Stein,2002;Stein&Talcott,
1999) abilities result in suboptimal reading routines.
Note that we do not claim here that these deficiencies
are the one and only cause of dyslexia. We merely point
out that it is conceivable that such low-level auditory
and visual deficiencies have by themselves very little
behaviourally observable effects but nevertheless can
result in the abnormal coordination of reading routines
and inefficient procedural learning, thereby exerting
large effects on reading acquisition. Another example
is that failures or inefficiencies in the procedural learn-
ing required to process symmetry (Pegado et al., 2011;
Perea, Moret-Tatay, & Panadero, 2011) may lead to inef-
ficient coordination of reading routines and result in
suboptimal reading experience. We do not argue here
that inefficiencies in procedural learning are the
major cause of dyslexia. We believe however that rela-
tive minor impairments can greatly exacerbate the
reading disorder if they result in suboptimal reading
4.2. Consequences of suboptimal reading
strategies and their automatisation
As with reduced reading experience, we believe that sub-
optimal reading experience also has far-reaching and real
consequences for cognitive performance in people with
reading impairments (cf. Kosmidis, 2017). To illustrate
the point we give a few examples here, though the list
should not be considered exhaustive. Inefficient reading
strategies may slow down the phonological restructuring
associated with learning alphabetic orthographic scripts
and thus have secondary (albeit small) effects on perform-
ance in low-level speech perception tasks such as categ-
orical perception (see Section 2.2.1). Deficient reading
strategies slow down the emergence of fully proficient
phonemic awareness when learning alphabetic writing
systems (see Section 2.2.2). Suboptimal reading routines
and processing strategies slow down the development
of more fine-grained phonological representations result-
ing in poorer performance in verbal memory tasks
(Section 2.2.3) and pseudoword repetition (Section
2.2.4). Like reduced reading experience, inefficient
reading strategies impede the development of proficient
general language skills. This becomes apparent in poorer
performance at all stages of RAN (Section 2.2.5) and less
anticipation of upcoming language input (Section 2.2.6).
Suboptimal reading routines such as a failure to efficiently
and automatically suppress symmetry may lead to dis-
tracting active and conscious symmetry suppression.
This has the result that mirror discrimination abilities
develop more slowly in children with dyslexia (Section
2.2.7). Finally, it is easily conceivable that engrained
deficient reading procedures could result in differences
in fMRI bold responses as well as structural brain differ-
ences (Section 2.3).
5. Conclusion
In the present opinion paper we have considered exper-
imental evidence concerning categorical perception,
phonological awareness, verbal short-term memory,
pseudoword repetition, RAN, prediction in spoken
language processing, and mirror invariance, as well as
results from structural and functional brain imaging. The
impairments/differences observed in individuals with
dyslexia are, to a striking degree, also shown by illiterate
individuals who have received no reading instruction or
very little. This is, we propose, no coincidence. Similarities
in performance of course do not necessarily imply similar
underlying causes. However, we conjecture that the dys-
lexia research community should take seriously the possi-
bility that a common factor could be a lack of reading
experience. We suggest that some (though of course
not all) individuals with dyslexia read much less than
people with no reading impairments. And indeed it is
hardly surprising that individuals engage much less in
an activity that they find difficult and frustrating.
We have pointed out throughout this article that there
are many reading-impaired people with approximately
equal or even more reading experience than individuals
with no reading impairments. We have made the case
that not all reading experience is the same. Indeed,
people with dyslexia have by definition altered reading
experience. We conjecture that the reading experience
of many such individuals can be described as suboptimal
reading experience, involving the acquisition and prac-
tice of reading routines and processing strategies that
result in an abnormal functional coordination of
reading. What is underappreciated by the research com-
munity, we believe, is the extent to which people strug-
gling to acquire reading skills quickly fall behind, right
from the beginning of reading acquisition, and typically
never catch up. Reading difficulties have a snowball
effect: small difficulties become larger and larger until
the consequences of suboptimal reading experience
become huge and in a certain sense causing the affected
individuals brain to remain low literate.
All the findings reviewed above suggest at the very
least that the proposed causes of developmental dyslexia
are open to reinterpretation as consequences of quanti-
tative and qualitative differences in reading experience.
It is important to point out here that we do not deny
that reading impairments such as developmental dys-
lexia have an underlying cause (or even several indepen-
dent causes) but we believe it is telling that an impartial
reading of the research literature to date suggests that
a cause has not yet been identified. Some may criticise
us for not providing an opinion on the underlying
cause of dyslexia in this paper. If a substantial number
of the performance differences between reading
impaired and non-reading impaired individuals are a con-
sequence of reduced and/or suboptimal reading experi-
ence, then what is the cause of developmental
dyslexia? One may point out that even if we are right
there must still be a cause for dyslexia. By not suggesting
a cause are we not ducking the crucial question? This has
been intentional. Our aim has been to draw attention to
the fact that many of the impairments observed in
people with developmental dyslexia are also shown by
illiterate individuals who have received no reading
instruction, or very little. Our intention has been to illus-
trate that it is conceivable that a very large proportion
of dyslexia research findings may simply reflect reduced
and suboptimal reading experience. It is up to the dys-
lexia research community to convincingly demonstrate
that particular research findings are not only (or even
partly) a secondary consequence of reduced and subop-
timal reading experience. We do not want to dilute this
message by simultaneously providing an alternative sug-
gestion for a cause of dyslexia.
How can this be achieved? First and most importantly,
to make progress we need more experimental studies
that quantify how much children and adults with dys-
lexia actually read. Second, studies must assess more rig-
orously the quality and type of reading experience that
participants get. Accurate assessments of home literacy
environments will be crucial (cf. Boerma, Mol, & Jolles,
2017; Katzir, Lesaux, & Kim, 2009; Mol & Bus, 2011; Stano-
wich & West, 1989). Third, cross-sectional studies will
continue to be useful but we need to recognise and
take more seriously their limitations. Cross-sectional
comparisons, we suggest, should be treated as explora-
tory and considered at best indicative with regard to
causal explanations. Our review suggests that the validity
of cross-sectional studies of dyslexia is increased if an illit-
erate (illiterate adults) or pre-literate (children) control
group rather than a normalcontrol group of highly lit-
erate individuals is included in the study. Fourth, the gold
standard will have to be tightly controlled large-scale
longitudinal studies. It is indispensable that hypotheses
from cross-sectional studies are validated by longitudinal
studies. Some such investigations have already been
carried out (Bradley, Corwyn, Burchinal, McAdoo, &
Garcia Coll, 2001; Bus, Van IJzendoorn, & Pellegrini,
1995; Linkersdörfer et al., 2015; Lyytinen, Erskine, Hämä-
läinen, Torppa, & Ronimus, 2015; Maurer et al., 2011;
Wimmer, Mayringer, & Landerl, 2000) or are currently
being conducted. It is important that future longitudinal
studies are large-scale enough to include enough indi-
viduals who will develop dyslexia (i.e. enough pre-
readers with and without familial and/or genetic risk of
dyslexia, cf. Hakvoort, van der Leij, Maurits, Maassen, &
van Zuijen, 2015; Saygin et al., 2013; Vandermosten
et al., 2016; Willems, Jansma, Blomert, & Vaessen, 2016),
and that reading experience is monitored meticulously
throughout any study. Conducting such a study is no
mean feat and will require a large, multiple lab effort.
But clearly, if we want to determine the cause(s) of devel-
opmental dyslexia with a sufficient degree of confidence
there is no other way.
We thank Saoradh Favier and three anonymous reviewers for
their comments on a previous version of this paper.
Disclosure statement
No potential conflict of interest was reported by the authors.
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