There is a significant overlap between language disorders and literacy difficulties. 50% of young people with Developmental Language Disorder (DLD) are unable to read simple texts, and more than 80% struggle to understand what they read (RCSLT et al, 2023). When a survey of children attending a special school for those with Specific Language Impairment (SLI) was taken, 75 out of 82 had reading difficulties (Haynes and Naidoo, 1991, cited in Bishop and Snowling, 2004).
There is also evidence of oral language difficulties in those with dyslexia. In one study of 110 children diagnosed with specific reading difficulties, just over half scored more than one standard deviation below the mean on the CELF-R (Clinical Evaluation of Language Fundamentals – Revised: a language battery designed to identify language disorders) (McArthur et al, 2000, cited in Bishop and Snowling, 2004).
In another landmark study, Scarborough and Dobrich (1990) monitored a small group of children diagnosed with “early language difficulties” over the years. Although their deficits grew milder over time and appeared to naturally resolve by the age of 5 or 6, when they were followed up again 3 years later, 3 out of 4 had severe reading difficulties.
Bishop and colleagues observed a similar pattern with older students. 60 children with language disorders were identified at age 4, and reassessed at different times over the years. Strikingly, at ages 15-16, 52% of the resolved group, and 93% of the continuing poor outcome group scored below a twelve year old level on a reading test (Bishop & Adams, 1990; Bishop & Edmundson, 1987; Stothard, Snowling, Bishop, Chipchase, & Kaplan, 1998 cited in Dale et al, 2014).
Bishop (2014) points out that since oral and written language difficulties have traditionally been dealt with by different professionals (Speech and Language Therapists (SLTs) and Educational Psychologists/ Specialist Dyslexia Tutors respectively), this overlap has not always been appreciated. However, as our understanding of these conditions has evolved, the sharp dividing line between them has been questioned.
Some have gone so far as to argue that DLD and dyslexia are not two separate conditions, but one, presenting at different points on a continuum, or at different points of development. This has even led to some researchers adopting new terms, such as “language learning impairment”, or “language learning disorder” to encompass oral and/ or written language difficulties (Bishop and Snowling, 2004, Paul and Norbury, 2012).
The reconceptualization of dyslexia has contributed to this converging. Historically known as “word blindness”, dyslexia was seen as a visual processing disorder where words would appear to “jump” out of the page, and students would mix letters up or reverse them. However, most cases of dyslexia are now considered to be linked to poor phonological awareness, which is the ability to recognize, segment, blend and manipulate sounds in words.
Those who see DLD and dyslexia as part of a continuum point to the presence of similar phonological processing difficulties in some students with language disorders as well as other shared underlying difficulties. For example, some studies have highlighted difficulties with auditory perception (the brain’s ability to make sense of sounds), as well as poor performance on nonword repetition tasks (repeating made up words) in both groups (Bishop and Snowling, 2004).
Given the degree of overlap between DLD and dyslexia, it seems likely that there are some shared etiologies. However, Bishop and Snowling (2004) argue that talking of the two as one is an oversimplification, and that it is useful to keep the categories separate. The signs of classic dyslexia look quite different to the main behaviours that characterize DLD.
For one thing, a significant proportion of those identified with poor phonological awareness and decoding (word reading) ability do not have any major problems understanding text (Bishop and Snowling, 2004). In fact, there is some evidence that these “poor decoders” develop more advanced reading comprehension strategies and exploit their background knowledge to compensate for their difficulties (Catts et al, 2006, Snyder and Caccamise, 2010).
This is in direct contrast to many students with DLD, and those known as “poor comprehenders” who often struggle to understand text despite being able to read it (although some students with DLD may be in the unlucky position of struggling with both decoding and understanding). (See my post: https://secondaryschoolslt.wordpress.com/2026/03/26/why-do-some-students-struggle-to-understand-what-they-read/ for more information.)
For students like these, poor vocabulary and knowledge of advanced syntax may lead to difficulties with understanding, especially at secondary level, when the complexity of written language increases significantly. (See my post: https://secondaryschoolslt.wordpress.com/2026/04/15/what-is-expository-discourse-and-how-is-it-relevant-at-secondary-school/ for more information about this type of academic language.) As Paul and Norbury (2012, p.410) put it, “reading is a language-based skill, and understanding meaning through reading makes use of all the same processes used to extract meaning from oral language”.
There may also be other reasons for the apparent overlap between the two disorders. Students with dyslexia who lack the ability or motivation to read age-appropriate texts will not be exposed to the same range of vocabulary and sentence structures as their peers, potentially causing them to fall behind with their language skills.
Conversely, certain areas of language may be more connected to decoding and spelling than they appear at first glance. Students with language disorders with a weak grasp of grammatical morphology (the awareness of how words are made up of smaller, meaningful units, called “morphemes”, including prefixes, roots and suffixes), for example, may struggle to spell.
To sum up, DLD frequently occurs alongside other neurodevelopmental disorders, including literacy difficulties. As SLTs, we’re likely to have many students on our caseloads who are struggling with their reading and writing for a variety of reasons. As specialists in language, we’re being increasingly expected to play a role in supporting our students with these difficulties, and are well placed to do so (Paul and Norbury, 2012). But first we need to understand the exact nature of our students’ difficulties if we are to help them.
Notes
Bishop, Dorothy V. M., and Margaret J. Snowling. 2004. “Developmental Dyslexia and Specific Language Impairment: Same or Different?” Psychological Bulletin 130 (6): 858–886.
Bishop, D.V.M. (2014). Ten questions about terminology for children with unexplained language problems. International Journal of Language & Communication Disorders, 49(4), pp. 381-415.
Catts, Hugh W., Suzanne M. Adlof, and Susan Ellis Weismer. “Language Deficits in Poor Comprehenders: A Case for the Simple View of Reading.” Journal of Speech, Language, and Hearing Research 49, no. 2 (2006): 278–293.
Dale, Philip S., Angela J. McMillan, M. E. Hayiou-Thomas, and Robert Plomin. “‘Illusory Recovery’: Are Recovered Children With Early Language Delay at Continuing Elevated Risk?” American Journal of Speech-Language Pathology 23, no. 3 (August 2014): 437–447.
Paul, R. and Norbury, C. (2012) Language Disorders from Infancy Through Adolescence: Listening, Speaking, Reading, Writing, and Communicating. 4th edn. St. Louis, MO: Elsevier.
Royal College of Speech and Language Therapists (RCSLT). “A Vision for Developmental Language Disorder (DLD) for the UK” London: RCSLT, 2023. https://www.rcslt.org/wp-content/uploads/2023/10/A-Vision-for-Developmental-Language-Disorder.pdf.
Scarborough, Hollis S., and Wanda Dobrich. “Development of Children with Early Language Delay.” Journal of Speech and Hearing Research 33, no. 1 (1990): 70–83.
Snyder, Lynn, and Donna Caccamise. “Comprehension Processes for Expository Text: Building Meaning and Making Sense.” In Expository Discourse in Children, Adolescents, and Adults: Development and Disorders, edited by Marilyn A. Nippold and Cheryl M. Scott, 13–39. New York: Psychology Press, 2010.
Thoughts of a secondary school SLT 