Developmental dyslexia is characterized by unexpected difficulty in reading in children and adults who otherwise possess the intelligence and motivation considered necessary for accurate and fluent reading. Dyslexia is one of the most common problems affecting children and adults; in the United States, the prevalence of dyslexia is estimated to range from 5 to 17% of school-age children, with as many as 40% reading below grade level. It is the most common and most carefully studied of the learning disabilities, affecting at least 80% of all individuals identified as learning-disabled. Recent epidemiologic data indicate that, like hypertension and obesity, dyslexia fits a dimensional model—within the population, reading ability occurs along a continuum, with reading disability representing the tail of a normal distribution.

Good evidence based on sample surveys of randomly selected populations of children indicates that dyslexia affects boys and girls comparably. (The long-held belief that only boys suffer from dyslexia reflected bias in school-identified samples.) Dyslexia is a persistent, chronic condition that stays with the individual for a lifetime. It is both familial and heritable; about half of children who have a parent with dyslexia, as well as half of the siblings of dyslexics and half of the parents of dyslexics, may have the disorder.