*2.1. Participants*

A Norwegian national age cohort of six-year-old children with Down syndrome (including every registered child across the country) was invited to participate; all habilitation services in Norway forwarded an informational letter and a consent form to the registered parents of each child with Down syndrome. The letter and consent form were approved in advance by the Regional Committees for Medical and Health Research Ethics. The families of forty-three children with Down syndrome accepted the invitation on the children's behalf (22 boys and 21 girls; chronological age: mean (*M*) = 75.78 months, *SD* = 3.48 months; nonverbal mental ability raw score (Block Design): *M* = 12.23, *SD* = 5.40). The families who accepted the invitation returned the consent form to the principal investigator. In addition to being 6 years old at the start of the study, the inclusion criteria were that the child did not have a comorbid diagnosis of autism spectrum disorder (ASD) and that Norwegian was the first language.

Among the readers, all children had trisomy 21 except one who had translocation. The non-readers showed almost identical percentages. All except two children had trisomy 21. One of the two had translocation, and the other had mosaic. All participants in both groups except for one of the non-readers went to ordinary primary school. At T1, there were quite similar occurrences between the two groups in regard to permanent hearing

disability (25% vs. 30% for non-readers and readers, respectively, odds ratio (*OR*) = 0.77, 95% confidence interval (*CI*) 0.13–4.48, *p* = 0.77) and mean parental education (*M* = 2.51, *SD* = 1.07 vs. *M* = 2.56, *SD* = 1.02 for non-readers and readers, respectively, *OR* = 1.05, 95% *CI* 0.50–2.20, *p* = 0.91). However, at T1, the nonverbal mental ability of readers (*M* = 3.13, *SD* = 2.10) was substantially better than that of non-readers (*M* = 1.49, *SD* = 1.07, *OR* = 1.95, 95% *CI* 1.13–3.41, *p* = 0.01) (scaled scores based on the Block Design; for a description of the measure, see Section 2.3.3).
