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Examining the Etiology of Reading Disability as Conceptualized by the Hybrid Model


When we feel ill, we go see a doctor, and she diagnoses us with an ailment based on our symptoms. We trust the diagnosis because not only are the symptoms of an illness precisely listed and universally agreed upon, but they are also universally distributed to doctors (thanks to medical handbooks) and the general public (thanks to the Internet). Diagnosing learning disabilities is a different story. Despite over 30 years of research efforts, learning disability diagnosis has not come close to the level of standardization and ease of diagnosis provided by the symptom checklist model followed by the medical field. For example, in the case of reading disability, the most common learning disorder, we still disagree on how to define it. As a result, reading disability diagnosis tends to be unreliable and uninformative. Without a universal definition, a child who is considered reading disabled by one reading specialist may be considered typically developing by another. In addition, not knowing the precise roots of a child’s struggles with reading makes it really hard to figure out how to help that child improve in whichever area needs improving. In response to these problems, and in an attempt to mirror the symptom checklist format utilized by medical doctors, many reading researchers have adopted a new way of measuring reading disability, known as the “hybrid model”.

By bringing together many kinds of reading deficits, the hybrid model can account for the many different forms that reading disability can take, while also accounting for children who have more than one kind of reading-related difficulty. Consequently, the hybrid model is meant to be more informative and powerful than a single definition of reading disability.

Based on the consensus reached at the Learning Disabilities Summit convened by the US Office of Special Education Programs in 2002, a hybrid model of reading disability should include two essential components. The first basic component is called “unexpected underachievement,” which describes a phenomenon in which a child struggles with reading, even though there appears to be no obvious underlying reason. In this scenario, the child has received adequate reading instruction and does not suffer from any known causes of poor achievement, like emotional or intellectual deficits (e.g., Attention Deficit Hyperactivity Disorder), but he continues to perform poorly in reading. This form of contrasting reading achievement and other achievement areas helps us pinpoint that a deficit is reading-specific. The second basic component is called “inadequate response-to-intervention (RTI).” Inadequate RTI refers to the phenomenon in which a child receives both adequate instruction and special assistance in reading but continues to fall behind in reading achievement. In this scenario, a child is failing to respond to both instruction and intervention, indicating that the child’s deficit is reading-specific and difficult to improve.

Using a large sample of over 30,000 kids, work by Spencer and colleagues (2014) found empirical support for a hybrid model definition of reading disability that was composed of four reading disability deficits that met both of the requirements set by the Learning Disabilities Summit. One deficit captured inadequate RTI and the remaining three deficits captured the unexpected underachievement facet of reading disability. They were unexpected underachievement in sight word reading, phonological decoding, and reading comprehension relative to listening comprehension. Spencer and colleagues’ (2014) results showed that all four symptoms were distinct while still belonging to the same overarching category of reading disability, supporting the use of the hybrid model to define reading disability.

In an effort to find further scientific support for the use of a hybrid model in defining and identifying reading disability, Erbeli and colleagues (2018) recently conducted a follow-up study to confirm the structure of the hybrid model. Using a sample of 2,737 seven to ten-year old twins from the Florida Twin Project on Reading, they examined the nature of the hybrid model in order to determine if the grouping of related reading disability deficits is an accurate way to identify a reading disability. Twin studies allow researchers to understand the “nature” (i.e., genetic effects) and “nurture” (i.e., environmental effects) of something, like the hybrid model of reading disability. Overall, Erbeli and colleagues (2018) found that instead of being attributable to any one specific deficit, the nature and nurture of the hybrid model deficits were common to the overall hybrid model of reading disability. These results justify the characterization of reading disability as a hybrid model that includes several related reading disability deficits in a single model.

With empirical support for a multi-deficit model, like the hybrid model, which mirrors the multi-symptom models used in the medical field, we hope to get closer to a universal method for diagnosing reading disability that will make it more reliable and precise.

Erbeli, F., Hart, S.A., Wagner, R.W., & Taylor, J. (2018). Examining the etiology of reading disability as conceptualized by the hybrid model. Scientific Studies of Reading, 22(2), 167-180.