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Dyslexia and DLD: comorbid disorders with distinct effects on reading comprehension

Snowling, M. J., Hayiou-Thomas, M. E., Nash, H. M., & Hulme, C. (2020). Dyslexia and Developmental Language Disorder: comorbid disorders with distinct effects on reading comprehension. Journal of Child Psychology and Psychiatry, 61(6), 672–680.

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Key terms in this paper:

  • Decoding: The ability to recognise familiar written words and to work out words you have not seen before (poor decoding is strongly associated with dyslexia).

  • Linguistic comprehension: The ability to understand spoken or written language.

  • Reading comprehension: The ability to read and understand written language. It combines both decoding and linguistic comprehension.

  • Comorbidity: When a person has two or more medical diagnoses at the same time.

What was the aim of the study?

Oral language creates the basis for learning how to read. A link between DLD and poorer reading comprehension has been suggested. Dyslexia also relates to weaker reading comprehension. This is because children with dyslexia have difficulties decoding written text. Although DLD and dyslexia are separate disorders, comorbidity between them is common.

The study followed the literacy development of three groups of children:

  1. Children with a family history of dyslexia

  2. Children with preschool language impairment

  3. Children with typical language development

Children began the study when they were three and a half years old. When they were 8 and 9, their reading comprehension was assessed. The children were then classified into 3 groups: (a) DLD, (b) dyslexia, and (c) comorbid dyslexia + DLD.

What was found?

  • All 3 groups had reading comprehension difficulties, however...

(a) In the DLD group, children’s decoding skills were in the typical range.

(b) The group with dyslexia showed better reading comprehension abilities than the DLD group and the DLD + dyslexia group.

(c) The DLD + dyslexia group had the most severe reading comprehension difficulties.

  • Preschool language difficulties predicted later reading comprehension difficulties.

  • The rate of comorbidity between DLD and dyslexia was very high: almost half of the children diagnosed with DLD also met the criteria for a diagnosis of dyslexia.

  • Children whose language disorder had resolved by age 5½ years were less likely to experience later reading comprehension difficulties, compared to children whose problems persisted or emerged later.

What do these findings mean?

Children with a family history of reading or preschool language problems, have an increased risk of experiencing reading and language difficulties later in childhood. Early language profiles could inform educational interventions. Deficits in either decoding or language comprehension abilities (or both) can impact reading comprehension abilities. Whilst children with DLD can decode text, their language comprehension abilities limit their reading comprehension. The opposite occurs in dyslexia. This confirms that dyslexia and DLD develop differentially. The findings have important implications for educational practice.

Where can I read this paper?

This paper is open access, which means everyone can read it. Please click here to read the full paper:


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