Epidemiology of Developmental Dyslexia: A Comparison of DSM-5 and ICD-11 Criteria

The two major medical classifications (ICD-11 and DSM-5), define diagnostic criteria for developmental dyslexia that partly differ and that are open to multiple interpretations, inducing different prevalence estimates and discordant cases. The present study evaluates the prevalence of developmental dyslexia for the first time in France in an extensive population representative of French sixth-graders (N=25,000), investigating the consequences of using one classification or the other, and of the different ways of implementing each criterion. Moreover, students diagnosed with dyslexia were compared with the reference population in the other available characteristics. Overall, prevalence estimates ranged from 1.3% to 17.2% depending on the criteria and thresholds used. A reasonable set of criteria and thresholds (-1.5 SD below mean for reading score, -0.5 SD for achievement) yielded a prevalence of dyslexia in France of 6.6% according to DSM-5 and 3.5% according to ICD-11. Factors that had the greatest influence on prevalence estimates were the criteria relative to 1) IQ, and 2) impact on academic achievement. DSM-5, being more liberal than ICD-11 on the IQ criterion, included more cases with relatively low IQ and thus yielded higher prevalence estimates. Compared with the reference population, children with dyslexia were more likely to be boys, to be schooled in a disadvantaged area, and to have lower SES, IQ, and math results. Our results emphasize that the choice of classification and the operationalization of specific criteria have a large impact on who is diagnosed with dyslexia.