The question of whether dyslexia can be restricted to a single language frequently arises for multilingual individuals. Dyslexia is the most common learning disability, affecting reading and spelling despite adequate intelligence and instruction. While reading difficulties may appear in one language and not another, the core issue is not language-specific. The way a language is constructed, particularly its writing system, alters how the underlying difficulties of dyslexia become apparent, leading to the perception that the condition is selective.
Dyslexia: A Neurobiological Origin
Dyslexia is fundamentally a neurobiological disorder rooted in a core difficulty with the phonological component of language. This deficit involves trouble recognizing and manipulating phonemes, the smallest sound units of speech. Because this processing issue resides in the brain’s circuitry, it is considered a universal cognitive deficit independent of any specific language.
The underlying problem is characterized by a weakness in phonological awareness—the ability to consciously manipulate the sound structure of spoken words. This difficulty in breaking words down makes mapping letters to sounds extremely challenging. Another element is a deficit in rapid automatized naming (RAN), which reflects difficulty with the speed of lexical retrieval. Individuals with dyslexia often struggle to quickly name familiar items, such as letters, numbers, or colors, measuring processing speed related to reading fluency. These neurocognitive markers, phonological awareness and RAN, are present regardless of the language a person is learning to read.
How Linguistic Depth Affects Manifestation
Symptoms appear to vary across languages due to orthographic depth, which describes the consistency of a language’s letter-to-sound correspondence. Languages with “shallow” orthographies, such as Italian, Spanish, or Finnish, have a highly consistent and predictable mapping system. In these languages, one letter almost always corresponds to a single sound, making the initial process of decoding words straightforward.
In contrast, “deep” orthographies, like English or French, have a complex and inconsistent letter-to-sound relationship. For example, the letters ‘ough’ in English can be pronounced in multiple ways, such as in though, through, or bough. This lack of transparency forces readers to rely on memorizing whole word spellings and complex rules.
For a person with the phonological processing deficit of dyslexia, deep orthography, like English, is a greater challenge than shallow orthography. In a shallow language, the consistency of the spelling system allows for faster and more accurate decoding, often making reading accuracy symptoms negligible. The primary outward sign of dyslexia in shallow languages often becomes slow reading speed or lack of fluency, reflecting the underlying RAN deficit. However, in a deep language, the same individual experiences severe difficulty with both reading accuracy and speed, making the disability more pronounced and meeting diagnostic criteria.
The Reality of Cross-Linguistic Transfer
The fundamental cause of dyslexia is present across all languages an individual speaks, but the resulting reading disability is not always equally visible. The underlying phonological weakness transfers to every language learned. However, the linguistic structure of a shallow language can mitigate the outward severity of reading difficulties, sometimes to the point where the person no longer meets the formal diagnostic criteria for dyslexia in that language.
An individual may be diagnosed with dyslexia in English, a deep orthography, due to marked reading and spelling errors, but appear to be a typically developing reader in Spanish, a shallow orthography, aside from slower reading speed. This difference in manifestation creates the illusion of being dyslexic in only one language. Cognitive skills gained in one language, such as phonological awareness, can positively transfer to the other, helping a bilingual person with dyslexia navigate a second language.
For a proper diagnosis in multilingual individuals, clinicians must look beyond simple reading performance in a single language. The assessment should focus on language-independent skills, such as phonological awareness and rapid automatized naming, to identify the universal neurobiological deficit. Relying solely on reading tests, which measure the symptom, can lead to misdiagnosis where one language’s structure has masked the disability. The core deficit remains, but its behavioral expression is filtered through the specific rules of the language being used.