Dyslexia can be understood as reading abilities which are either below the child’s overall cognitive abilities (IQ) or age. Children who meet only IQ-discrepant criteria have above average overall cognitive abilities with reading abilities that are lower than expected but still appropriate for their age. Conversely, children who only meet age-discrepant criteria have lower overall cognitive abilities and low reading abilities. Some children meet both IQ-discrepant and age-discrepant dyslexia (e.g., average or above IQ and below average reading), but many fall into one category or the other.
Genetic differences play a greater difference in those with a high IQ who struggle to read. Further, genetics play a greater role in dyslexia in those from a high socioeconomic status. Taken together, these findings suggest that bright children whose families have greater resources are more likely to be strong readers unless they are genetically predisposed to be poor decoders. Conversely, less advantaged children are more prone to environmental factors which contribute to poor reading. This pattern is known as a biological gene x environment (G x E) interaction.
Children with both forms of dyslexia demonstrate the same underlying brain differences, neuropsychological deficits, and benefit from the same treatments. Specifically, children who struggle to read all show deficits in left-hemisphere reading and language networks. They both demonstrate disproportionately lower phonological processing and respond best to phonics-based reading instruction.
Reference:
Pennington, B.F., McGrath, L.M., & Peterson, R.L. (2019). Diagnosing learning disorders: From science to practice (3rd ed.). New York, NY: The Guilford Press