The Dyslexia Handbook 2013 69 Aryan van der Leij, Elsje van Bergen and Peter de Jong Longitudinal family-risk studies of dyslexia: why some children develop dyslexia and others don t. Longitudinal family-risk studies of dyslexia: why some children develop dyslexia and others don t. Aryan van der Leij 1, Elsje van Bergen 2 and Peter de Jong 3 Prospective studies which involve children who have a dyslexic parent and, hence, have a family risk of becoming dyslexic, provide the opportunity to examine the differential development of at-risk children who do and do not become dyslexic and controls (i.e., not at-risk non-dyslexics). The question is in what respect at-risk children who do not develop dyslexia differ from those who do. First, it is interesting to know whether atrisk children without dyslexia perform at the level of controls in reading, spelling and related skills, or, alternatively, also show impairments but to a lesser degree than at-risk children with dyslexia. In addition, the question where these group differences stem from can be addressed. As learning to read is often believed to build on (oral) language abilities, one possibility is that at-risk non-dyslexic children show differences in preceding verbal development in comparison to at-risk children with dyslexia. Another possibility is that this group is at lower genetic risk, i.e. has a less dyslexic parent. Finally, it could be that this group experiences more advantageous environmental factors. Literacy and its cognitive underpinnings Elbro, Borstrøm, and Petersen (1998) followed the progress of Danish children with and without family risk from kindergarten through the beginning of second grade, when dyslexia was determined. In kindergarten, the at-risk dyslexics showed deficits as compared to the control group in phoneme awareness, verbal short-term memory, and distinctness of phonological 1 Research Institute Child Development and Education, University of Amsterdam, The Netherlands 2 Department of Experimental Psychology, University of Oxford, UK 3 See 1
70 The Dyslexia Handbook 2013 representations, whereas the at-risk non-dyslexics did not. However, the at-risk non-dyslexics did perform as poorly as the at-risk dyslexics on morpheme awareness and articulation, whereas they performed better than the at-risk dyslexics but less well than the controls on letter knowledge. Pennington and Lefly (2001) also conducted an at-risk study. Dyslexia was assessed in second grade. At the start of first grade, the at-risk non-dyslexics performed poorer than controls but better than atrisk dyslexics on phoneme awareness and speech perception. Relating letter knowledge, verbal short-term memory, phonological awareness and rapid naming in kindergarten to reading in Grade 2, Boets et al. (2010; 2007) found that the performance of the at-risk non-dyslexic group was at an intermediate position between that of the other two groups, but did not differ significantly from either of them. In one of the samples of the Dutch Dyslexia Programme (DDP), at-risk children were followed from kindergarten through Grade 5 (van Bergen, de Jong, Regtvoort, Oort, van Otterloo, & van der Leij (2011). The at-risk children who went on to have dyslexia had poor letter knowledge in kindergarten. In addition, they were impaired on rapid naming. The at-risk children without dyslexia performed better on letter knowledge and rapid naming, but still below the level of controls. The groups did not differ on phonological awareness. At-risk dyslexics read less fluently from first grade onwards than the other groups. At-risk non-dyslexics reading fluency was at an intermediate position between the other groups at the start of reading. By fifth grade they had reached a similar level as the controls on word reading, but still lagged significantly behind on pseudoword reading. Replicating this study in a larger sample of the DDP, it was confirmed that, at the end of Grade 2, the at-risk children with dyslexia performed poorer than the other two groups on all measures, whereas the at-risk no-dyslexia children performed better than the at-risk dyslexia children, but still below the level of the controls on all tasks (van Bergen, de Jong, Plakas, Maassen & van der Leij, 2012). Interestingly, on rapid naming the at-risk no-dyslexia children were as fast as the controls.
The Dyslexia Handbook 2013 71 In sum, although not all differences reach significance, the evidence suggests that at-risk children who do not develop dyslexia show signs of milder, i.e. not clinical, impairments. The possible exception may be rapid naming which is comparable to controls when reading development is well under way (Grade 2). Environmental factors With regard to the possible origins of the differences between at-risk groups, the findings of several studies do not support an explanation in terms of environmental differences. As part of the Jyväskylä Longitudinal Study of Dyslexia (JLD), Torppa, Poikkeus, Laakso, Tolvanen, Leskinen, Leppänen, and Lyytinen (2007) assessed the relation between home literacy, reading interest, phonological awareness, vocabulary, and emergent literacy variables. They found a direct effect of home-literacy environment (shared reading) on vocabulary growth, but not on emergent literacy. Similarly, Elbro, Borstrøm and Petersen (1998) did not find differences between dyslexic and non-dyslexic parents, nor between parents of dyslexic and non-dyslexic children in the amount of shared reading in kindergarten. Snowling, Miter and Carroll (2007) did not find differences between the two at-risk groups in parent reading behavior, family literacy behavior, and socioeconomic background. Finally, van Bergen et al. (2011) found that, although the parents of the groups of at-risk children differed in educational level and reading skills, there were no differences in literacy environment. In sum, there is hardly any evidence that children who develop dyslexia grow up in a relatively disadvantageous literacy environment. Verbal development In her seminal study, Scarborough (1990) compared at-risk children who later became dyslexic with control children with no family background of reading problems. She found that the at-risk dyslexic children were deficient in several language skills from age 2½ onwards. Slow language development was also shown by a prospective study of Snowling, Gallagher, and Frith (2003). When the children were 3 years and 9 months,
72 The Dyslexia Handbook 2013 the at-risk children who became dyslexic had deficiencies in vocabulary, narrative skills, and verbal short-term memory. At 6 years of age verbal ability of the dyslexic children was significantly lower than that of their peers without familial risk. Torppa, Lyytinen, Erskine, Eklund, and Lyytinen (2010) examined very early language markers of dyslexia. From age 2 onwards, the at-risk children who later became dyslexic showed impairments relative to the controls. At-risk children who became normal readers also tended to show weak expressive syntax and vocabulary at age 5, but this difference was not significant. In a related study (van Bergen, de Jong, Maassen, Krikhaar, Plakas, & van der Leij, in revision) it was detected that the at-risk group which developed dyslexia was characterized by relatively poor sentence comprehension, poor expressive syntax, weak vocabulary, and poor verbal short-term memory at 4 years of age. The study also revealed that, relative to the controls, the at-risk group without dyslexia had subtle, but significant, impairments in the verbal domain, while the at-risk group with dyslexia lagged more behind on verbal IQ. Because the described studies were executed in USA, UK, Finland and The Netherlands, it may be concluded that poor language development across a wide range of verbal abilities is an early marker of dyslexia, independent of orthography and language. Genetic risk Behavioral studies (e.g., Snowling et al., 2003) have indicated that the family risk of dyslexia is continuous. As a consequence, it may be hypothesized that at-risk dyslexic children have a higher genetic liability than at-risk non-dyslexics: their parents have more severe reading and spelling problems. However, Snowling et al. (2007) reported that the parents of at-risk dyslexics did not differ from the parents of at-risk non-dyslexics on spelling and word-reading accuracy. In contrast, van Bergen et al. (2011) found relatively better reading skills of parents of at-risk non-dyslexics, which suggests that these children might have a lower genetic liability. The findings of the study of Torppa, Eklund, van Bergen and Lyytinen (2011) supported the differences in liability of at-risk children with and without
The Dyslexia Handbook 2013 73 dyslexia: the parents of the at-risk children with dyslexia had more severe difficulties in pseudoword reading, spelling accuracy, rapid word recognition and in text reading fluency, than the parents of the at-risk children without dyslexia. Van Bergen et al. (2012) also investigated rapid naming, phonology, spelling, and word and pseudoword reading of parents and their children. With regard to parental reading, the conclusion of the earlier study (van Bergen et al., 2011) that the at-risk children who develop dyslexia are likely to have a higher liability was supported, although less outspoken and restricted to word reading (and not pseudoword reading). Interestingly, the non-dyslexic parents of the at-risk children who developed dyslexia, had poorer reading performance than its counterpart of the at-risk non-dyslexics, indicating that genetic liability may also be a result of genetic influences from both parents. In addition, an important finding was that the parents of the at-risk children without dyslexia, relative to those of dyslexic children, were better in rapid naming (as were their children). Both the group comparisons and the parent-child relations highlight the importance of good rapid naming skills for reading acquisition. Conclusion The described prospective studies, in particular the comparison of at-risk children who do and do not become dyslexic and controls, have added much to our understanding of developmental dyslexia. First, genetic risk affects reading and spelling performance on a scale of more to less. Although the performance in reading and spelling of at-risk children without dyslexia is within the normal range, mild impairments have been detected with regard to (nearly) all the relevant determinants of reading and spelling (phonological awareness, letter knowledge, rapid naming). Possibly, these impairments give them a slow start which they overcome in due time. Apart from the fact that their impairments differ quantitatively from the more severe deficits shown by the at-risk children who develop dyslexia, the question whether there are compensating mechanisms has still to be answered. The mechanism that enables rapid serial
74 The Dyslexia Handbook 2013 naming seems to be an interesting candidate that deserves further study. Second, the prospective studies indicate that poor language development across a wide range of verbal abilities, including verbal IQ, is an early marker of dyslexia, independent of orthography and language. As was the case with the determinants of reading and spelling, the at-risk group which does not develop dyslexia seems to have an intermediate position between the at-risk, dyslexic, group and the non atrisk controls. It should be noted, however, that the differences in early language markers are small and that even the poorer performances of the children who develop dyslexia do not qualify for the clinical range. Third, supporting the hypothesis of a transgenerational continuum of genetic risk, the Finnish JLD and Dutch DDP studies have provided evidence that the chance of becoming dyslexic partly depends on the severity of the reading problems of the dyslexic parent. On addition, two intriguing findings warrant further investigation. As was the case with the children, the performance in rapid naming of the dyslexic parent seems to have an impact on the chance of becoming dyslexic as a child. In addition, the reading performance of the other, i.e. non dyslexic, parent might be of importance. That genetic liability may depend on these genetic factors is also indicated by the fact that it is unlikely that children who develop dyslexia grow up in a relatively disadvantageous literacy environment. At-risk families seem to cope with the problem of stimulating emergent literacy at the same level as control families. References Bishop, D. V. M. (2009). Genes, Cognition, and Communication. Annals of the New York Academy of Sciences, 1156 (The Year in Cognitive Neuroscience 2009), 1-18. Boets, B., De Smedt, B., Cleuren, L., Vandewalle, E., Wouters, J., & Ghesquière, P. (2010). Towards a further characterization of phonological and literacy problems in
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76 The Dyslexia Handbook 2013 connections to reading in Finnish children with and without familial risk for dyslexia. Journal of Learning Disabilities, 43, 308-321. van Bergen, E., de Jong, P.F., Regtvoort, A., Oort, F., van Otterloo, S., & van der Leij, A. (2011). Dutch children at family risk of dyslexia: Precursors, reading development, and parental effects. Dyslexia, 17 (1), 2-18. van Bergen, E., de Jong, P.F., Plakas, A., Maassen, B., & van der Leij, A. (2012). Child and parental literacy levels within families with a history of dyslexia. Journal of Child Psychology and Psychiatry, 53 (1), 28-36. van Bergen, E., de Jong, P.F., Maassen, B., Krikhaar, E., Plakas, A., & van der Leij, A. (submitted). IQ of four-year-olds who go on to develop dyslexia.