Understanding Delay in Developmental Disorders

Researchers in developmental disorders frequently refer to abilities that are in line with mental age as simply delayed. The qualifier simply might imply an existing theory of developmental delay that is well understood and uninteresting (perhaps because it is an exaggerated form of individual differences, the responsibility of other researchers). In this article, I argue that the notion of delay can be separated into descriptive and explanatory versions. The descriptive version is often used too coarsely to be helpful. Instead, we need an approach based on developmental trajectories to separate types of descriptive delay, which may then have different underlying causes. The explanatory version is poorly articulated in developmental theory. One useful way to deepen our understanding of delay is by building computational models that simulate development in large populations of individuals and explicitly implementing factors that cause variations in development. Finally, I suggest that dividing research among separate investigators of typical development, individual differences, and developmental disorders may be counterproductive if the underlying mechanisms recognize no such distinction. KEYWORDS—developmental disorders; delay; trajectories; computational modeling; intervention When researchers who investigate the causes of developmental disorders examine an impaired cognitive ability in a given disorder, they frequently ask whether the ability is atypical or simply delayed. Why attach the qualifier simply to the notion of delay? The implication is twofold: Researchers already have a theory of developmental delay, and then find it uninteresting. In this article, I express reservations with these implications and argue that the concept of delay is both poorly characterized and lacking explanation at a mechanistic level. The assumption of delayed development—that abilities will catch up eventually—is usually wrong. In the first part of the article, I argue that characterizing development in terms of trajectories provides a richer vocabulary to describe delay, and I offer examples of these methods. In the second part, I consider explanations of delay and argue that computational modeling allows us to focus on the mechanisms that influence rates of development. I also provide examples from language development and show how these models may provide a basis to inform intervention. I finish with a look ahead at areas for research. DELAY AS A DESCRIPTIVE TERM The notion of delay, which originated in a clinical context, identifies when a child fails to show age-appropriate developmental milestones or falls quantitatively below certain thresholds on standardized tests of core domains, such as motor skills, speech and language, personal–social cognition, or daily living (1). Implicit in the quantitative definition is the recognition that in typical children, early trajectories of development vary. Variation below a certain range is deemed problematic. The label delay may also serve as a temporary placeholder in the dialog between clinicians and parents, indicating that development is not on track; a specific diagnosis may became apparent when the child is older (1). The clinical notion of delay is frequently criticized for failing to recognize that in many cases, a slower rate of development is also associated with long-term impairment rather than resolution into the normal range (2, 3). The concern here is when the clinical label of delay is imported into theoretical explanations of developmental deficits. Michael S. C. Thomas, Birkbeck, University of London. The research reported in this article was supported by UK ESRC grant RES-062-23-2721, a Leverhulme Study Abroad Fellowship held at the University of Chicago, and a joint grant from the Wellcome Trust and the Education Endowment Foundation awarded to the University of London Centre for Educational Neuroscience. Correspondence concerning this article should be addressed to Michael S. C. Thomas, Department of Psychological Sciences, Birkbeck, University of London, Malet Street, Bloomsbury, London WC1E 7HX, United Kingdom; e-mail: [email protected].