The specific cause(s) of reading and writing disorders remain unknown. Disorders and difficulties with reading and writing have historically been
associated with a number of external and internal causal factors and may be the result of an interplay of these factors. They are likely multifactoral,
with no single cause that can account for all written language disorders (Silliman & Berninger, 2011). Children with a history of oral language
difficulty are at high risk for difficulty learning to read and write (Kamhi & Catts, 2012).
Most children learn to read without much difficulty; their early literacy experiences support the development of skills needed for learning to read and
write. Other children have more limited literacy experiences but go on to develop written language skills with appropriate high-quality instruction (e.g.,
Justice, Chow, Capellini, Flanigan, & Colton, 2003; Justice, Mashburn, Hamre, & Pianta, 2008; Scanlon & Vellutino, 1996, 1997).
Children with written language disorders struggle with learning to read and write—often despite rich early literacy experiences and adequate learning
opportunities—and they continue to struggle throughout their academic careers. These children with “unexpected” difficulties are the ones
identified as having disorders of reading and writing (Pennington & Bishop, 2009).
External factors are environmental variables that can have a negative impact on the child's reading acquisition. They include the following:
- Limited early literacy experience (e.g., absent or limited early joint reading experiences and insufficient opportunity to draw/scribble with
crayons) that affects exposure to vocabulary and advanced grammar and discourse and can, in turn, lead to further language problems (Stothard, Snowling,
Bishop, Chipchase, & Kaplan, 1998).
- Insufficient and/or inadequate reading and writing instruction (Vellutino et al., 1996).
- Insufficient early oral language experience that is too constrained to support the acquisition of literacy (Hoff, 2013).
- Matthew effects (Duff, Tomblin, & Catts, 2015; Stanovich, 1986)—which are negative consequences associated with low performance in reading and writing (e.g.,
child is in a low-ability group) that can lead to low expectations, poor motivation, and limited practice, which, in turn, affect written language
Internal factors are those intrinsic to the child. They include genetic and neurological factors as well as spoken language deficits (see relationship between spoken and
written language in the overview section of this page).
Genetic factors—Several genes have been associated with reading disabilities or with increased susceptibility to reading disabilities (see Meaburn, Harlaar,
Craig, Schalkwyk, & Plomin, 2008; Paracchini, Scerri, & Monaco, 2007).
Studies comparing identical and fraternal twins report a higher co-occurrence of reading disabilities in identical twins than in fraternal twins (DeFries
& Alarcon, 1996; Light & DeFries, 1995). Genetic influences are reflected in early reading performance, but environmental factors (e.g., family and
school) can influence subsequent growth in early reading skills (Petrill et al., 2010).
- Neurological basis—Differences in brain structure and function have been found in individuals with reading disabilities as compared with typical
readers, although the relationship is not clear. It may be that some differences are the result—rather than the cause of—reading problems
(Catts, Kamhi, & Adlof, 2012).
- Structural differences include the following:
- Atypical patterns of symmetry in the temporal lobe (e.g., Galaburda, 1988)
- Presence of focal dysplasias in the cortex (Galaburda, 1991)
- Differences in the corpus callosum (e.g., Duara et al., 1991), inferior parietal lobe (e.g., W. E. Brown et al., 2001), and cerebellum (e.g., Eckert et
- Functional differences include the following:
- Less left-hemisphere dominance than typical readers (see Bryden, 1982; Gerber, 1993)
- Differences in activation (over- or underactivation) in various brain regions (e.g., Meyler, Keller, Cherkassky, Gabrieli, & Just, 2008; Paulesu et
al., 1995; Salmelin, Service, Kiesilä, Uutela, & Salonen, 1996; Shaywitz et al., 1998)
Visually based deficits, auditory processing deficits, and attention-based deficits have often been proposed as causes for reading disabilities, but most
are not supported by scientific evidence. (See Catts et al., 2012)
- Visually-based deficits include reversal errors (e.g., reading/writing b for d), erratic eye movements (e.g., more or longer fixations than typical readers),
and transient processing deficits (i.e., problems processing global visual features). In spite of the popularity of this explanation in the past, there
is no general support that visually based deficits cause reading disabilities. In fact, these observed visual phenomena may actually reflect typical
development (reversal error) or cognitive processing difficulties (erratic eye movements) during reading—or may be linked to phonological
processing deficits (transient processing deficits).
- Auditory processing deficits include deficits in auditory perception (e.g., problems perceiving rapid sound changes) and lack of sensitivity to syllable-level prosodic information.
Research findings are inconsistent with regard to the presence of auditory processing deficits in poor readers and whether or not these deficits, if
present, are sufficient to cause reading problems.
- Attention-based deficits (particularly inattention) are thought to be associated with reading difficulties because reading demands significant attention. However, research does
not support a causal relationship. Although attention deficits and reading difficulties can occur together, they appear to be distinct developmental
disorders with different causes. When they co-occur, inattention may contribute to reading comprehension difficulties.