Incidence of written language disorders refers to the number of new cases identified in a specified time period. Prevalence of written language disorders refers to the number of people
who are living with the condition in a given time period.
Incidence and prevalence of written language disorders vary across research studies due to differences in participant characteristics, study designs, and
diagnostic classification criteria within and across subtypes of written language disorder. As an example of how classification criteria may differ,
cut-point scores for distinguishing typical learners from those with developmental dyslexia may be 1 standard deviation (SD) below the mean in one
study and 1.5 SDs below the mean in another study.
Further, with regard to study design, although two studies may evaluate the same aspect of reading (e.g., reading decoding accuracy), one study may
evaluate decoding words in isolation, whereas another focuses on decoding words in connected text, yielding different results. As such, the statistics that
follow should be interpreted with these types of variations in mind.
- 7.6% of second and third grade children in the Connecticut Longitudinal Study were identified as having a reading disability, regardless of whether
researcher- or school system–identification criteria were applied (Shaywitz, Shaywitz, Fletcher, & Escobar, 1990).
- A longitudinal study of a birth cohort born between January 1, 1976, and December 31, 1982, in Rochester, Minnesota, revealed that 6.9%–14.7%
of participants had a writing disorder at age 19 years (Katusic, Colligan, Weaver, & Barbaresi, 2009).
- Studies of elementary-age children from Iowa (Catts, Fey, Tomblin, & Zhang, 2002) and New Jersey (Flax et al., 2003) revealed a reading
disability and specific language impairment comorbidity of 50% and 68%, respectively.
- A longitudinal study of a birth cohort born between January 1, 1976, and December 31, 1982, in Rochester, Minnesota, revealed that among children
with speech and language impairment, the cumulative incidence of a writing disorder was 61.4% for boys and 55.1% for girls at age 19 years (Katusic et al.,
- Prevalence of reading disability and ADHD co-morbidity from general population samples primarily from the United States ranged from 0.4% to 3.7%
(Sexton, Gelhorn, Bell, & Classi, 2012).
- A longitudinal study of a birth cohort born between January 1, 1976, and December 31, 1982, in Rochester, Minnesota, revealed that 10%–18.9%
of boys and 3.6%–10.1% of girls had a writing disorder at age 19 years (Katusic et al., 2009).
- Several studies (Badian, 1999; Flannery, Liederman, Daly, & Schultz, 2000; Katusic et al., 2009) have reported that boys are 2–3 times
more likely than girls to have a literacy disorder.