Intellectual disability (ID) begins during childhood and involves deficits in mental abilities, social skills, and core activities of daily living (ADLs) when compared to same-aged peers. There often are no physical signs of mild forms of ID, although there may be characteristic physical traits when it is associated with a genetic disorder (e.g., Down syndrome).

The level of impairment ranges in severity for each person. Some of the early signs can include:

- Delays in reaching or failure to achieve milestones in motor skills development (sitting, crawling, walking)

- Slowness learning to talk or continued difficulties with speech and language skills after starting to talk

- Difficulty with self-help and self-care skills (e.g., getting dressed, washing, and feeding themselves)

- Poor planning or problem solving abilities

- Behavioral and social problems

- Failure to grow intellectually or continued infant-like behavior

- Problems keeping up in school

- Failure to adapt or adjust to new situations

- Difficulty understanding and following social rules

In early childhood, mild ID (IQ 50–69) may not be obvious or identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild intellectual disability from specific learning disability or emotional/behavioral disorders. People with mild ID are capable of learning reading and mathematics skills to approximately the level of a typical child aged nine to twelve. They can learn self-care and practical skills, such as cooking or using the local mass transit system. As individuals with intellectual disability reach adulthood, many learn to live independently and maintain gainful employment.

Moderate ID (IQ 35–49) is nearly always apparent within the first years of life. Speech delays are particularly common signs of moderate ID. People with moderate intellectual disability need considerable supports in school, at home, and in the community in order to fully participate. While their academic potential is limited, they can learn simple health and safety skills and to participate in simple activities. As adults, they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, manage their finances. As adults, they may work in a sheltered workshop.

People with severe or profound ID need more intensive support and supervision their entire lives. They may learn some ADLs, but an intellectual disability is considered severe or profound when individuals are unable to independently care for themselves without ongoing significant assistance from a caregiver throughout adulthood. Individuals with profound ID are completely dependent on others for all ADLs and to maintain their physical health and safety, although they may be able to learn to participate in some of these activities to limited degree.

Intellectual disability (ID), also known as general learning disability, and mental retardation (MR), is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ score under 70 in addition to deficits in two or more adaptive behaviors that affect everyday, general living.

Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environments. As a result of this focus on the person's abilities in practice, a person with an unusually low IQ may not be considered to have intellectually disability.

Intellectual disability is subdivided into syndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.

Intellectual disability affects about 2–3% of the general population. Seventy-five to ninety percent of the affected people have mild intellectual disability. Non-syndromic or idiopathic cases account for 30–50% of cases. About a quarter of cases are caused by a genetic disorder, and about 5% of cases are inherited from a person's parents. Cases of unknown cause affect about 95 million people as of 2013.

Disorders considered in origin include:

1. Intellectual disability (ID) or intellectual and developmental disability (IDD), previously called mental retardation

2. Autism spectrum disorders, such as Asperger's syndrome or Kanner syndrome

3. Motor disorders including developmental coordination disorder and stereotypic movement disorder Tic disorders including Tourette's syndrome

4. Traumatic brain injury (including congenital injuries such as those that cause cerebral palsy)

5. Communication, speech and language disorders

6. genetic disorders, such as fragile-X syndrome, Down syndrome, attention deficit hyperactivity disorder, schizophrenia, schizotypal disorder, hypogonadotropic hypogonadal syndromes

7. disorders due to neurotoxicants like fetal alcohol spectrum disorder, Minamata disease caused by mercury, behavioral disorders including conduct disorder etc caused by other heavy metals, such as lead, chromium, platinum etc, hydrocarbons like dioxin, PBDEs and PCBs, medications and illegal drugs, like cocaine and others.

Neurodevelopmental disorders are impairments of the growth and development of the brain or central nervous system. A narrower use of the term refers to a disorder of brain function that affects emotion, learning ability, self-control and memory and that unfolds as an individual develops and grows.

The term is sometimes erroneously used as an exclusive synonym for autism spectrum disorders.

Borderline intellectual functioning, also called borderline mental disability, is a categorization of intelligence wherein a person has below average cognitive ability (generally an IQ of 70–85), but the deficit is not as severe as intellectual disability (below 70). It is sometimes called below average IQ (BAIQ). This is technically a cognitive impairment; however, this group may not be sufficiently mentally disabled to be eligible for specialized services. The DSM-IV-TR codes borderline intellectual functioning as V62.89.

During school years, individuals with borderline intellectual functioning are often "slow learners." Although a large percentage of this group fails to complete high school and can often achieve only a low socioeconomic status, most adults in this group blend in with the rest of the population.

There are a variety of medical conditions affecting cognitive ability. This is a broad concept encompassing various intellectual or cognitive deficits, including intellectual disability, deficits too mild to properly qualify as intellectual disability, various specific conditions (such as specific learning disability), and problems acquired later in life through acquired brain injuries or neurodegenerative diseases like dementia. These disabilities may appear at any age.

Under the DSM-5, autism is characterized by persistent deficits in social communication and interaction across multiple contexts, as well as restricted, repetitive patterns of behavior, interests, or activities. These deficits are present in early childhood, and lead to clinically significant functional impairment. There is also a unique form of autism called autistic savantism, where a child can display outstanding skills in music, art, and numbers with no practice. Because of its relevance to different populations, self-injurious behaviors (SIB) are not considered a core characteristic of the ASD population however approximately 50% of those with ASD take part in some type of SIB (head-banging, self-biting) and are more at risk than other groups with developmental disabilities.

Other characteristics of ASD include Restricted and Repetitive Behaviors (RRBs) which include a large range of specific gestures and acts, it can even include certain behavioral traits as defined in the Diagnostic and Statistic Manual for Mental Disorders.

Asperger syndrome was distinguished from autism in the DSM-IV by the lack of delay or deviance in early language development. Additionally, individuals diagnosed with Asperger syndrome did not have significant cognitive delays. PDD-NOS was considered "subthreshold autism" and "atypical autism" because it was often characterized by milder symptoms of autism or symptoms in only one domain (such as social difficulties).

The DSM-5 eliminated the four separate diagnoses: Asperger Syndrome, Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS), Childhood Degenerative Disorder, and Autistic Disorder and combined them under the diagnosis of Autism Spectrum Disorder.

Learning disability is a classification that includes several areas of functioning in which a person has difficulty learning in a typical manner, usually caused by an unknown factor or factors. Given the "difficulty learning in a typical manner", this does not exclude the ability to learn in a different manner. Therefore, some people can be more accurately described as having a "Learning Difference", thus avoiding any misconception of being disabled with a lack of ability to learn and possible negative stereotyping.

In the UK, the term "learning disability" generally refers to an intellectual disability, while difficulties such as dyslexia and dyspraxia are usually referred to as "learning difficulties".

While "learning disability, learning disorder" and "learning difficulty" are often used interchangeably, they differ in many ways. Disorder refers to significant learning problems in an academic area. These problems, however, are not enough to warrant an official diagnosis. Learning disability, on the other hand, is an official clinical diagnosis, whereby the individual meets certain criteria, as determined by a professional (psychologist, pediatrician, etc.). The difference is in degree, frequency, and intensity of reported symptoms and problems, and thus the two should not be confused. When the term "learning disorder" is used, it describes a group of disorders characterized by inadequate development of specific academic, language, and speech skills. Types of learning disorders include reading (dyslexia), mathematics (dyscalculia) and writing (dysgraphia).

The unknown factor is the disorder that affects the brain's ability to receive and process information. This disorder can make it problematic for a person to learn as quickly or in the same way as someone who is not affected by a learning disability. People with a learning disability have trouble performing specific types of skills or completing tasks if left to figure things out by themselves or if taught in conventional ways.

Individuals with learning disabilities can face unique challenges that are often pervasive throughout the lifespan. Depending on the type and severity of the disability, interventions, and current technologies may be used to help the individual learn strategies that will foster future success. Some interventions can be quite simplistic, while others are intricate and complex. Current technologies may require student training to be effective classroom supports. Teachers, parents, and schools can create plans together that tailor intervention and accommodations to aid the individuals in successfully becoming independent learners. School psychologists and other qualified professionals quite often help design the intervention and coordinate the execution of the intervention with teachers and parents.

Autism spectrum disorders are thought to follow two possible developmental courses, although most parents report that symptom onset occurred within the first year of life. One course of development is more gradual in nature, in which parents report concerns in development over the first two years of life and diagnosis is made around 3–4 years of age. Some of the early signs of ASDs in this course include decreased looking at faces, failure to turn when name is called, failure to show interests by showing or pointing, and delayed pretend play.

A second course of development is characterized by normal or near-normal development followed by loss of skills or regression in the first 2–3 years. Regression may occur in a variety of domains, including communication, social, cognitive, and self-help skills; however, the most common regression is loss of language.

There continues to be a debate over the differential outcomes based on these two developmental courses. Some studies suggest that regression is associated with poorer outcomes and others report no differences between those with early gradual onset and those who experience a regression period. While there is conflicting evidence surrounding language outcomes in ASD, some studies have shown that cognitive and language abilities at age may help predict language proficiency and production after age 5. Overall, the literature stresses the importance of early intervention in achieving positive longitudinal outcomes.

Global developmental delay is an umbrella term used when children are significantly delayed in their cognitive and physical development. There is usually a more specific condition which causes this delay, such as Fragile X syndrome or other chromosonal abnormalities. However, it is sometimes difficult to identify this underlying condition.

Other terms associated with this condition are failure to thrive (which focuses on lack of weight gain and physical development), intellectual disability (which focuses on intellectual deficits and the changes they cause to development) and developmental disability (which can refer to both intellectual and physical disability altering development).

Hyperlexic children are characterized by word-reading ability well above what would be expected given their ages and IQs. Hyperlexia can be viewed as a superability in which word recognition ability goes far above expected levels of skill. However, in spite of few problems with decoding, comprehension is poor. Some hyperlexics also have trouble understanding speech. Most or perhaps all children with hyperlexia lie on the autism spectrum. Between 5–10% of autistic children have been estimated to be hyperlexic.

Dyslexia is a learning disability that manifests itself as a difficulty with word decoding and/or reading fluency. Comprehension may be affected as a result of difficulties with decoding, but is not a primary feature of dyslexia. It is separate and distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction. It is estimated that dyslexia affects between 5–17% of the population. Dyslexia has been proposed to have three cognitive subtypes (auditory, visual and attentional), although individual cases of dyslexia are better explained by the underlying neuropsychological deficits and co-occurring learning disabilities (e.g. attention-deficit/hyperactivity disorder, math disability, etc.). Although not an intellectual disability, it is considered both a learning disability and a reading disability.

Dyslexia and IQ are not interrelated, since reading and cognition develop independently in individuals who have dyslexia."Nerve problems can cause damage to the control of eye muscles which can also cause diplopia" (WEBMD, 2005)

Intellectual disability, also known as "general learning disability", and previously known as "mental retardation" (a term now widely considered to be offensive), is a generalized disorder characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors that appears before adulthood. It has historically been defined as an Intelligence Quotient (IQ) score under 70, but the definition now includes both one component relating to mental functioning and one relating to individuals' functional skills in their environment, so IQ is not the only factor.

Intellectual disability must have appeared in the developmental period, not only as an adult. By contrast, people with cognitive impairment have, or previously had, normal IQ, but now show confusion, forgetfulness and difficulty concentrating; cognitive impairment is typical of brain injuries, side effects from medications, and dementia. Many of these disabilities have an effect on memory, which refers to the ability to recall what has been learned over time. Typically memory is moved from sensory memory to working memory then finally into long term memory. People with cognitive disabilities typically will have have trouble with one of these types of memory.

Overactive disorder associated with mental retardation and stereotyped movements is a pervasive developmental disorder (PDD) listed in Chapter V(F) of the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10); its diagnostic code is F84.4.

Affected newborns generally have striking neurological defects and seizures. Severely impaired development is common, but disturbances in motor functions may not appear until later in life.

Infants with microcephaly are born with either a normal or reduced head size. Subsequently, the head fails to grow, while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed. Development of motor functions and speech may be delayed. Hyperactivity and intellectual disability are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from in some to spastic quadriplegia in others.

Deficits in any area of information processing can manifest in a variety of specific learning disabilities. It is possible for an individual to have more than one of these difficulties. This is referred to as comorbidity or co-occurrence of learning disabilities. In the UK, the term "dual diagnosis" is often used to refer to co-occurrence of learning difficulties.

Microcephaly is a type of cephalic disorder. It has been classified in two types based on the onset:

As an infant, the individual with intellectual disability might sit up, crawl, or walk later than what is developmentally appropriate. He or she may have trouble talking or learn to talk late. The infant with intellectual disability will probably have trouble learning to potty train, feeding himself or herself, remembering things, with problem-solving, and may have recurrent explosive tantrums.

Some symptoms that a child with intellectual disability might show are continued infant-like behavior, a lack of curiosity, the inability to meet educational demands, learning ability that is below average, and the failure to meet developmentally appropriate intellectual goals. Some children with severe intellectual disability may have seizures, mobility problems, vision problem, or hearing problems.

Symptoms of M2DS include infantile hypotonia and failure to thrive, delayed psychomotor development, impaired speech, abnormal or absent gait, epilepsy, spasticity, gastrointestinal motility problems, recurrent infections, and genitourinary abnormalities. Many of those affected by M2DS also fit diagnostic criteria for autism. M2DS can be associated with syndromic facies, namely an abnormally flat back of the head, underdevelopment of the midface, ear anomalies, deep-set eyes, prominent chin, pointed nose, and a flat nasal bridge.

This is an ill-defined disorder of uncertain nosological validity. The category is included here because of the evidence that children with moderate to severe intellectual disability (IQ below 35) who exhibit major problems in hyperactivity and inattention frequently show stereotyped behaviours; such children tend not to benefit from stimulant drugs (unlike those with an IQ in the normal range) and may exhibit a severe dysphoric reaction (sometimes with psychomotor retardation) when given stimulants; in adolescence the overactivity tends to be replaced by underactivity (a pattern that is not usual in hyperkinetic children with normal intelligence). It is also common for the syndrome to be associated with a variety of developmental delays, either specific or global. The extent to which the behavioural pattern is a function of low IQ or of organic brain damage is not known, neither is it clear whether the disorders in children with mild intellectual disability who show the hyperkinetic syndrome would be better classified here or under F90.- (Hyperkinetic disorders); at present they are included in F90-.

Diagnostic guidelines

Diagnosis depends on the combination of developmentally inappropriate severe overactivity, motor stereotypies, and moderate to severe intellectual disability; all three must be present for the diagnosis. If the diagnostic criteria for F84.0 (childhood autism), F84.1 (atypical autism) or F84.2 (Rett's syndrome) are met, that condition should be diagnosed instead.

Children with Attention Deficit and Disruptive Behavior Disorders may show the following symptoms:

- impulsivity or distractibility

- difficulty socializing

- aggressive behavior

- difficulty following rules or directions

- difficulty completing a task

- problems at school

- frustration

- alcohol or drug use

Ophthalmologic problems include strabismus. This requires early identification to avoid amblyopia. Surgery or patching are usually necessary to treat strabismus if diagnosed early. Refractive errors in patients with FXS are also common.

Children with fragile X have very short attention spans, are hyperactive, and show hypersensitivity to visual, auditory, tactile, and olfactory stimuli. These children have difficulty in large crowds due to the loud noises and this can lead to tantrums due to hyperarousal. Children with FXS pull away from light touch and can find textures of materials to be irritating. Transitions from one location to another can be difficult for children with FXS. Behavioral therapy can be used to decrease the child’s sensitivity in some cases.

Perseveration is a common communicative and behavioral characteristic in FXS. Children with FXS may repeat a certain ordinary activity over and over. In speech, the trend is not only in repeating the same phrase but also talking about the same subject continually. Cluttered speech and self-talk are commonly seen. Self-talk includes talking with oneself using different tones and pitches.

Signs and symptoms of CBPS typically appear in infancy or at birth, but can appear later in childhood. These include facial diplegia (paralysis on both sides), facial muscle spasms, pseudobulbar palsy, dysarthria (difficulty speaking), difficulty chewing, dysphagia (difficulty swallowing), epilepsy, and intellectual disability. Epileptic seizures in individuals with CBPS are different between individuals and can vary between episodes.

Psychopathology and related behavioral abnormalities are typically seen in LFS, and they may be considered in the diagnosis of the disorder. The most common of these in LFS is an autism-like spectrum disorder, and LFS is considered as one of a number of genetic disorders associated with autism. Additional alterations of psychopathology with behavioral manifestations that have been observed in LFS include: psychotic behavior, schizophrenia, hyperactivity and attention-deficit hyperactivity disorder, aggression, oppositional defiant disorder, obsessive compulsive disorder, extreme shyness, learning disability, cognitive impairment, short-term memory deficit, low frustration tolerance, social dysfunction, lack of impulse control, eating disorder and associated malnutrition, attributed to psychogenic loss of appetite; and pyromania.

While psychiatric conditions like these are to be expected with LFS, there have also been cases of the disorder with some preservation of mental and behavioral abilities, such as problem solving, reasoning and normal intelligence.

The psychopathology of LFS usually exhibits schizophrenia. When schizophrenia is diagnosed in an individual known to be affected by intellectual disability, LFS may be considered in the differential diagnosis of schizophrenia, with confirmation of cause through appropriate psychiatric and genetic evaluation methods.