Capturing the dynamic nature of dyslexia: The Delphi Definition

Why was a new definition needed?

Dyslexia is a term that has been the subject of much controversy over decades, with different diagnostic criteria being used in research and practice across different countries and within and between different groups of professionals. Doubt has also been cast on whether the term should be used at all (Elliot, 2020). This has created confusion for educators, parents, assessors and individuals with literacy difficulties themselves. There has been an urgent need for clarity. Consistency matters too, not least because in the UK assessors can find themselves acting as a ‘gatekeeper’ for resources and provision that depend on a diagnosis.

Methodology

The Delphi method is a structured process used for gathering opinions from a large group of professionals in a particular field and reaching a consensus on a complex topic. Its successful use in establishing terminology and diagnostic criteria for developmental language disorder (DLD) in 2016 (Bishop et al, 2016, Bishop et al, 2017), was one of the reasons it was chosen for this study, which was to address many similar issues. The research was a collaboration between the University of Birmingham, the Specific Learning Difficulties Assessment Standards Committee (SASC), King’s College London and the University of Oxford. A panel of 58 specialists in dyslexia were asked to rate a collection of 55 statements about dyslexia, according to their level of agreement with them. A range of perspectives was considered essential, and the panel included academics, practitioners and individuals with dyslexia representing a variety of countries and professions. Ratings from each participant were collected, some statements re-written or abandoned, and the process was repeated, giving everyone a chance to see anonymous feedback, comments from others, and to revise their views if they wished to. A moderating group met several times to select statements and formulate the final definition.

Other definitions

A definition of dyslexia that is widely used by assessors in the UK is the Rose definition, which came from a government-commissioned report (Rose, 2009) on identifying and teaching children with dyslexia and literacy difficulties. The International Dyslexia Association (IDA) in the USA also has an influential definition (IDA, 2002). Two other classification systems are also widely referred to when identifying difficulties with reading, particularly in an international context, although neither use the term dyslexia. There is the eleventh version of the International Classification of Diseases, the ICD-11 (Word Health Organisation, 2018) and the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, the DSM-5 (American Psychiatric Association, 2013).

Use of the term ‘dyslexia’

As in the Rose definition and that of the IDA, dyslexia is still seen as a useful term for referring to persistent difficulties with literacy. It is not used within the ICD-11 or DSM-5 definitions.

Emphasis on phonological processing

Phonological processing, recognised since the 1980s as having a central role in dyslexia, is still identified as the key area of cognitive processing where inefficiency is likely to be seen. 

A continuum not a category

Within the Delphi definition, dyslexic difficulties are stated to be on a continuum. This is in line with the explicit statement to this effect in the Rose definition, though is not highlighted in the IDA, ICD-11 or DSM-5 definitions.

Unexpectedness is expected

The idea of literacy attainment being below expectations in some way is central to the Delphi definition, as it is to others, though there is variation between definitions in the factors that determine what is expected. The ICD-11 for example requires reading to be below intellectual ability, while the Delphi, Rose, IDA and DSM-5 definitions are more flexible in their view about other strengths that might be used as a meaningful basis for comparison. Unexpectedness in relation to opportunities for learning is universally agreed to be essential. The role of intellectual ability in dyslexia was debated at length by the Delphi panel but the conclusion was that its measurement should not be used to create a rigid cut-off point in diagnosis.

Dynamic nature with multiple causes

The Delphi definition explicitly acknowledges the changing manifestation, experience and impact of dyslexia throughout the lifespan. This is of great importance for those seeking to identify it, as it means that diagnosis cannot depend on a rigid set of criteria. Multiple factors create variability in its manifestation and impact. These include both genetic and environmental factors, for example educational experiences and intervention, various cognitive processing skills and also changing literacy needs and demands at different stages in life. As well as identifying factors likely to have an adverse impact on literacy development, the Delphi papers also emphasise the role of positive factors with a protective effect, such as strong language skills.

Emphasis on processing skills

Whereas other definitions place more emphasis on different behavioural manifestations of dyslexia (reading and spelling weakness), a strong focus on processing difficulties is an important element of the Delphi definition. Although phonological difficulties are still viewed as being the most common area of cognitive inefficiency in dyslexia (namely phonological awareness, phonological memory and phonological processing speed), other areas of processing are also recognised as having an impact. Working memory, general processing speed and orthographic skills are all specifically identified. Orthographic processing in particular is noted as being 'commonly impaired' in dyslexia and is an area that should be assessed. Evidence for it as a causative factor rather than a secondary consequence however, was considered inconclusive.

Emphasis on fluency over attainment

Alongside the acknowledgement of a wide range of factors that influence the developmental trajectory of dyslexia, the definition identifies reading fluency and spelling as core and persistent areas of difficulty. Unlike other definitions, it does not mention reading accuracy, which is more likely to improve over time. This emphasis on fluency makes the definition more applicable to different age groups and also aligns better with how dyslexia is viewed in languages with more transparent orthographies, where accuracy is likely to be less of a challenge than speed.

Move away from specificity

The ICD-11 and DSM-5 inclusion of reading difficulties are based on an idea of specificity in those difficulties. In contrast, one of the Delphi statements that received a high level of consensus refers to the ‘highly overlapping nature’ of dyslexia. The recognition that it frequently co-occurs with other developmental difficulties is an integral part of the Delphi definition, though it is noted in the papers that more research is needed to understand the shared and specific risk factors that might account for these relationships. This view of dyslexia as just one of a set of overlapping conditions aligns with current interest and perspectives on neurodiversity.

Bridging theory and practice

The second Delphi paper discusses the Delphi statements related to why, when and what to assess and presents multi-stage ‘hypothesis-testing’ models for approaching the assessment of children and adults. The key areas of consensus that emerged from the Delphi study inform this model, including recognition of the need for gathering information from multiple sources. For younger children, assessment of need is prioritised over a rush to a diagnostic decision. More precise guidelines are available for assessors on the SASC website, with more detailed versions of the models presented in the research papers.

Support for all

Most importantly, Delphi panel members were in agreement that ‘support should be given to all individuals with literacy difficulties, regardless of definition or accompanying factors’ (Carroll et al, 2025). The new definition, with the comprehensive assessment framework that has been created from it are likely to prove valuable tools in the identification of need and the provision of such support.

References

Bishop, D.V., Snowling, M.J., Thompson, P.A., Greenhalgh, T., & Catalise Consortium. (2016). CATALISE: A multinational and multidisciplinary Delphi consensus study. Identifying language impairments in children. PLoS One, 11, e0158753.

Bishop, D.V., Snowling, M.J., Thompson, P.A., Greenhalgh, T., Catalise-2 Consortium, Adams, C., & House, A. (2017). Phase 2 of CATALISE: A multinational and multidisciplinary Delphi consensus study of problems with language development: Terminology. Journal of Child Psychology and Psychiatry, 58, 1068–1080.

Carroll, J.M., Holden, C., Kirby, P., Thompson, P.A., Snowling, M.J. and (2025), Toward a consensus on dyslexia: findings from a Delphi study. J Child Psychol Psychiatr. https://doi.org/10.1111/jcpp.14123

Elliott, J.G. (2020). It's time to be scientific about dyslexia. Reading Research Quarterly, 55, S61–S75.

Holden, C., Kirby, P., Snowling, M.J., Thompson, P.A. and Carroll, J.M. (2025), Towards a Consensus for Dyslexia Practice: Findings of a Delphi Study on Assessment and Identification. Dyslexia, 31: e1800. https://doi.org/10.1002/dys.1800

IDA. (2002). https://dyslexiaida.org/definition-of-dyslexia/