Saturday 9 June 2018

Developmental language disorder: the need for a clinically relevant definition

There's been debate over the new terminology for Developmental Language Disorder (DLD) at a meeting (SRCLD) in the USA. I've not got any of the nuance here, but I feel I should make a quick comment on one issue I was specifically asked about, viz:

As background: the field of children's language disorders has been a terminological minefield. The term Specific Language Impairment (SLI) began to be used widely in the 1980s as a diagnosis for children who had problems acquiring language for no apparent reason. One criterion for the diagnosis was that the child's language problems should be out of line with other aspects of development, and hence 'specific', and this was interpreted as requiring normal range nonverbal IQ (nviq).

The term SLI was never adopted by the two main diagnostic systems -WHO's International Classification of Diseases (ICD) or the American Psychiatric Association's Diagnostic and Statistical Manual (DSM), but the notion that IQ should play a part in the diagnosis became prevalent.

In 2016-7 I headed up the CATALISE project with the specific goal of achieving some consensus about the diagnostic criteria and terminology for children's language disorders: the published papers about this are openly available for all to read (see below). The consensus of a group of experts from a range of professions and countries was to reject SLI in favour of the term DLD.

Any child who meets criteria for SLI will meet criteria for DLD: the main difference is that the use of an IQ cutoff is no longer part of the definition. This does not mean that all children with language difficulties are regarded as having DLD: those who meet criteria for intellectual disability, known syndromes or biomedical conditions are treated separately (see these slides for summary).

The tweet seems to suggest we should retain the term SLI, with its IQ cutoff, because it allows us to do neatly controlled research studies. I realise a brief, second-hand tweet about Rice's views may not be a fair portrayal of what she said, but it does emphasise a bone of contention that was thoroughly gnawed in the discussions of the CATALISE panel, namely, what is the purpose of diagnostic terminology? I would argue its primary purpose is clinical, and clinical considerations are not well-served by research criteria.

The traditional approach to selecting groups for research is to find 'pure' cases - quite simply, if you include children who have other problems beyond language (including other neurodevelopmental difficulties) then it is much harder to know how far you are assessing correlates or causes of language problems: things get messy and associations get hard to interpret. The importance of controlling for nonverbal IQ has been particularly emphasised over many years: quite simply, if you compare language-impaired vs comparison (typically-developing, or td) children on a language or cognitive measure, and the language-impaired group has lower nonverbal ability, then it may be that you are looking at a correlate of nonverbal ability rather than language. Restricting consideration to those who meet stringent IQ criteria to equalise the groups is one way of addressing the issue.

However, there are three big problems with this approach:

1. A child's nonverbal IQ can vary from time to time and it will depend on the test that is used. However, although this is problematic, it's not the main reason for dropping IQ cutoffs; the strongest arguments concern validity rather than reliability of an IQ-based approach.

2. The use of IQ-cutoffs ignores the fact that pure cases of language impairment are the exception rather than the rule. In CATALISE we looked at the evidence and concluded that if we were going to insist that you could only get a diagnosis of DLD if you had no developmental problems beyond language, then we'd exclude many children with language problems (see also this old blogpost). If our main purpose is to get a diagnostic system that is clinically workable, it should be applicable to the children who turn up in our clinics - not just a rarefied few who meet research criteria. An analogy can be drawn with medicine: imagine if your doctor identified you with high blood pressure but refused to treat you unless you were in every other regard fit and healthy. That would seem both unfair and ill-judged. Presence of co-occurring conditions might be important for tracking down underlying causes and determining a treatment path, but it's not a reason for excluding someone from receiving services.

3. Even for research purposes, it is not clear that a focus on highly specific disorders makes sense. An underlying assumption, which I remember starting out with, was the idea that the specific cases were in some important sense different from those who had additional problems. Yet, as noted in the CATALISE papers, the evidence for this assumption is missing: nonverbal IQ has very little bearing on a child's clinical profile, response to intervention, or aetiology. For me, what really knocked my belief in the reality of SLI as a category was doing twin studies: typically, I'd find that identical twins were very similar in their language abilities, but they sometimes differed in nonverbal ability, to the extent that one met criteria for SLI and the other did not. Researchers who treat SLI as a distinct category are at risk of doing research that has no application to the real world.

There is nothing to stop researchers focusing on 'pure' cases of language disorder to answer research questions of theoretical interest, such as questions about the modularity of language. This kind of research uses children with a language disorder as a kind of 'natural experiment' that may inform our understanding of broader issues. It is, however, important not to confuse such research with work whose goal is to discover clinically relevant information.

If practitioners let the theoretical interests of researchers dictate their diagnostic criteria, then they are doing a huge disservice to the many children who end up in a no-man's-land, without either diagnosis or access to intervention. 


Bishop, D. V. M. (2017). Why is it so hard to reach agreement on terminology? The case of developmental language disorder (DLD). International Journal of Language & Communication Disorders, 52(6), 671-680. doi:10.1111/1460-6984.12335

Bishop, D. V. M., 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(7), e0158753. doi:10.1371/journal.pone.0158753

Bishop, D. V. M., Snowling, M. J., Thompson, P. A., Greenhalgh, T., & CATALISE Consortium. (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(10), 1068-1080. doi:10.1111/jcpp.12721


  1. This discussion and its conclusions are very similar to the arguments about dyslexia and IQ. The empirical evidence (from a number of studies in different countries) is that individual with a reading problem with a reading IQ discrepancy and those with a similar reading problem and no discrepancy do not differ in basic cognitive processes related to reading or in the ability to benefit from remediation. For a detailed review see my book, Not Stupid, Not Lazy: Understanding Dyslexia and Other Learning Disabilities.

  2. Problem 1 strikes me as a fictitious problem. You could say just the same of performance in any language test, yet you wouldn't want to drop language cut-offs from the diagnosis of DLD. And IQ is probably more reliable than most language tests.

  3. Problem 3: For research purposes, focusing on specific or pure cases should not be an end in itself. But if your research hypothesis depends on performance in a given test, and performance in that test can be affected by other cognitive functions than language, then you had better control those other cognitive functions if you want to be able to draw a clear interpretation.
    So research exclusion criteria can be legitimate, but must be justified by specific hypotheses and the necessity to control confounding factors.

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