How developmental disorders cluster
Imagine learning to talk and read, thread beads, and pay attention. These three skills – examples of language skills, motor skills, and executive function – seem like three distinct developmental areas. So why do problems in one area often forecast problems in another? Researchers are working to understand their shared roots.
Dyslexia, language impairment, ADHD, and coordination disorders often come together. About 40% of school-aged children who have one of these developmental disorders also meet the criteria for at least one of the other conditions.
But we know less about the clustering of these problems in preschoolers. How do the early signs of developmental disorders relate to each other? A recent research project by a team of psychologists in London and Oxford sought to untangle this question. They investigated the co-morbidity of language, motor, and executive function impairments in children at family risk of dyslexia between the ages of three and four.
The researchers recruited 242 three-year-olds in Yorkshire (UK) and screened the children to form four participant groups: typically developing children, children with language impairment, children at family risk of dyslexia, and children with both language impairment and a family risk of dyslexia.
The children in the “family risk of dyslexia” group were those whose parents or siblings had dyslexia, or whose parents had low scores on literacy tests. The aim of this screening was to identify a group of children with a higher genetic chance of developing dyslexia.
Children were tested on their motor skills and on their attention and self-regulation (both aspects of executive function). Parents also completed questionnaires about their children’s attention, behavior and motor coordination.
At one year’s follow up, aged four, the children were tested again on motor coordination and executive function and on their early literacy abilities.
Developmental problems do cluster in preschool children
The researchers found that developmental problems do cluster in preschool children. Children with language impairment were more likely to have problems with motor skills and executive function than their peers without language impairment.
On average, children with a family risk of dyslexia (but no language impairment) did worse on almost every test than typically developing children. However, the differences between the two groups were small.
Across the board, children with language impairments did much worse on the tests than those without. Among those with language impairments, the presence or absence of a family risk of dyslexia made little average difference.
This finding supports the theory that language disorders often occur with symptoms from motor or attention disorders.
Over time, motor ability predicts literacy
This second finding of the study was that among children with the same level of language ability at age three, those who had worse motor ability at age three were more likely to have lower literacy at age four. In other words, language and motor ability at age three independently predicted differences in literacy at age four.
This provides provisional support for the idea that difficulties beyond those in literacy, and indeed language, increase the risk of developing reading difficulties and dyslexia.
Despite the strength in behavioral assessment in the study, the connections drawn between impairments in language and motor skills and literacy attainment are tentative. The outcomes on the early years literacy test suggest there is relationship, but the children were still too young to determine whether they will go on to have reading difficulties.
What’s the link?
The clustering of developmental disabilities suggests that “factors that adversely influence brain development can have diverse effects,” the researchers note.
One such factor could be family background. “On average, the typically developing children were from higher SES backgrounds than children with language impairment,” the researchers note. On average, families with typically developing children came from UK postcodes that were better off than about 70% of all UK postcodes. Families whose children had language impairments came from postcodes at the 50th percentile of socioeconomic status, on average. A lack of family resources could contribute to multiple developmental problems.
This study was the first stage of a longer longitudinal study. The researchers can continue to test the way that developmental disorders cluster, especially in the development of dyslexia, and provide more insight into the relationships between early cognitive markers and later literacy outcomes.
Gooch, D., Hulme, C., Nash, H.M., & Snowling, M.J. (2013). Comorbidities in preschool children at family risk of dyslexia. Journal of Child Psychology and Psychiatry. doi:10.1111/jcpp.12139
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