Children whose experiences with early reading (e.g. preschool) are limited or of poor quality are at risk for reading failure. Dyslexia refers to reading that is impaired due to developmental neural and genetic anomalies that affect this skill, It focuses on reading impairments that are neurobiological and genetic rather than environmental.

The proximal causes are impairments in cognitive, perceptual or motor functions that affects components of reading. Dysfluencies in reading words can arise from several causes (e.g. slow recognition of letter combinations). Genetics can contribute to understanding reading disorders by specifying mechanisms that influence brain development in ways that underlie the proximal causes. Dyslexia is also associated with several types of anomalies in the structure and function of the neural systems for reading.

There are several common behaviours in children with dyslexia:

1. Phonology
   The impaired performance on phonemic tasks includes deletion (1), matching (2) and blending (3).
2. Reading aloud
   The performance is often slow, dysfluent and error-prone. There is particular difficulty with irregularly pronounced words.
3. Processing speed
   The performance is often slow when it comes to naming familiar digits, colours and objects.
4. Orthography
   The limited knowledge of orthographic structure expresses itself in difficulty distinguishing valid from invalid letter strings (1), weak knowledge of word spellings (2), misspellings (3), misidentification (4) and dysfluency in generating spellings (5).
5. Working memory
   There is a deficit on working memory tasks.
6. Language
   There is a limit on vocabulary size and lexical quality (1), familiarity with a narrower range of sentence structures and expressions (2) and difficulty reading texts aloud with appropriate intonation (3).

It was important to distinguish reading difficulties that result from a general intellectual impairment from readings-specific ones. There is no consensus regarding the incidence of dyslexia due to arbitrary cut-off points for when someone can be considered to be dyslexic.

The genetic component of dyslexia is greater for higher-IQ children than for lower-IQ children. Higher IQ acts as a protective factor against falling into the dyslexic range. The behavioural characteristics of poor readers are very similar across a wide IQ range. The skills that pose difficulty for children with reading difficulty are not closely related to the skills that IQ tests measure.

All children face the same challenges in learning to read but dyslexics have more difficulty with the essential components. The accumulation of these deficits and their multiplicative effect on each other affect performance to the point where a child can be described as dyslexic.

It is not clear whether dyslexia is a valid developmental disorder since it entails the children who fall on the low end of a reading continuum. This does not necessarily mean that it is a disorder. How far an individual with dyslexia progresses with reading depends on the severity of impairment (1), timing and quality of intervention (2), protective factors (3) and the environment (4). Dyslexia may be at the far-end of a normal behaviour (e.g. continuum of reading) but require treatment and extra help because the behaviour (i.e. reading) is crucial and easier to address in children than in adults.

The boundary between having the condition or not having the condition is neither fixed nor categorical but determined by conventions that periodically change. Extreme scepticism about dyslexia encourages treating symptoms without regard to underlying causes. The effectiveness of a treatment depends on whether it addresses the relevant causes. Furthermore, neglecting the existence of dyslexia puts unrealistic demands on teachers. Lastly, it creates additional barriers to treatment as the child may not get the proper help the child needs.

The best treatment to dyslexia is prevention. This requires identifying at-risk children and this is difficult. Children who have a slow start with learning to read may seem like they are falling behind but this may be typical development. This demonstrates how difficult it is to recognize children at-risk for dyslexia. Definitive identification is not possible until a later age when the intervention is far less effective.

The response to intervention (RTI) is a multistep procedure. It first screens for at-risk children. Their classroom progress is then assessed using frequent short tests of skills and teachers with extra training are used. This is a tier 1 intervention. The tier 2 intervention includes the children who did not benefit from the tier 1 intervention and this consists of additional instruction in small groups. The tier 3 interventions include children who did not benefit from the tier 2 intervention and takes on several forms (e.g. special education classroom). The disadvantage is that the effectiveness of RTI depends on how it is implemented and it is often improperly implemented.

The problems of a dyslexic child multiply because they cannot obtain the amount and variety of experience on which skilled reading depends. Other behaviours can be affected because reading impairments result from anomalies in capacities that are not just for reading. Reading affects other areas of life as well because a lot of things depend on reading (e.g. learning). The underlying cause of dyslexia will also influence other areas of life (e.g. visual problems in recognizing letters will also result in other visual problems). The same problem (i.e. reading difficulty) can have different causes.

Dyslexia is comorbid with other developmental disorders (e.g. ADHD). It is possible that they co-occur because of deficits in the shared capacities of these disorders. The factors that affect reading influence each other, making it difficult to track their impact.

Dyslexia does not have a single cause and underlying deficits vary in severity, the behaviours they affect and their persistence. Impaired reading can result from the co-occurrence of several relatively mild deficits that would not be debilitating in isolation. The effects of these deficits can be modulated by protective factors. The manifestations of the deficits change over time as children develop.

Children who developed reading difficulties by age five appeared to have spoken-language deficits at thirty months. At 2.5 years, the dyslexic child produced sentences with simpler syntax and pronounced words less accurately than non-dyslexics. By age three to four, deficits in vocabulary and phonology emerged. At five years, children were impaired on pre-reading skills and vocabulary. Speech sounds appear to elicit atypical evoked potentials in infants at familial risk for dyslexia. Furthermore, variation in spoken-language skills at age 3 to 4 was predictive of reading comprehension at age 8.

The developmental anomalies that underlie dyslexia are initially manifested in children’s spoken language.

A minor phonological aberration may have a heavy impact on learning to read because impairment interferes with discovering the phonological components of words. This leads to words that overlap to be treated as less alike. It is possible that the dyslexic brain develops in ways that interfere with discovering commonalities among words involving orthography and phonology.

Non-dyslexic readers show an adaptation effect when listening to people’s speech. Their brain activity in speech-related areas was reduced when the words were spoken by the same person rather than by multiple. This is because the listener picks up on characteristics of an individual’s speech. This adaptation effect was weaker in dyslexic readers. Dyslexics may have difficulty with generalizing patterns of words (e.g. learning the plural of a noun). This leads to dyslexics needing to learn the same things multiple times. Dyslexics fail to benefit from shared structure including exact repetitions and there may be oversensitivity to irrelevant information (e.g. font).

In shallow orthographies, dyslexia is manifested in slower but more accurate responses than in deeper orthographies. The same underlying phonological deficit can affect either speed or accuracy of reading aloud. Performance is probably impaired because the reader treats stimuli as different rather than picking up on their commonalities. This results in less efficient learning, comprehension and generalization. Phonology is a big umbrella covering all of the ways in which knowledge derived from pronunciation and sound is used in reading, speaking and other tasks. Impaired phonology influences performance on all these tasks.

Dyslexia may have beneficial effects as it is accompanied by a greater sensitivity to differences. The conditions that cause dyslexia in some individuals may promote the development of other skills.