The first English-language IQ test, the Stanford–Binet Intelligence Scales, was adapted from a test battery designed for school placement by Alfred Binet in France. Lewis Terman adapted Binet's test and promoted it as a test measuring "general intelligence." Terman's test was the first widely used mental test to report scores in "intelligence quotient" form ("mental age" divided by chronological age, multiplied by 100). Current tests are scored in "deviation IQ" form, with a performance level by a test-taker two standard deviations below the median score for the test-taker's age group defined as IQ 70. Until the most recent revision of diagnostic standards, an IQ of 70 or below was a primary factor for intellectual disability diagnosis, and IQ scores were used to categorize degrees of intellectual disability.

Since current diagnosis of intellectual disability is not based on IQ scores alone, but must also take into consideration a person's adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and such like. IQ assessment must be based on a current test. This enables diagnosis to avoid the pitfall of the Flynn effect, which is a consequence of changes in population IQ test performance changing IQ test norms over time.

Adaptive behavior, or adaptive functioning, refers to the skills needed to live independently (or at the minimally acceptable level for age). To assess adaptive behavior, professionals compare the functional abilities of a child to those of other children of similar age. To measure adaptive behavior, professionals use structured interviews, with which they systematically elicit information about persons' functioning in the community from people who know them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgment as well. Certain skills are important to adaptive behavior, such as:

- Daily living skills, such as getting dressed, using the bathroom, and feeding oneself

- Communication skills, such as understanding what is said and being able to answer

- Social skills with peers, family members, spouses, adults, and others

About half of parents of children with ASD notice their child's unusual behaviors by age 18 months, and about four-fifths notice by age 24 months. According to an article, failure to meet any of the following milestones "is an absolute indication to proceed with further evaluations. Delay in referral for such testing may delay early diagnosis and treatment and affect the long-term outcome".

- No babbling by 12 months.

- No gesturing (pointing, waving, etc.) by 12 months.

- No single words by 16 months.

- No two-word (spontaneous, not just echolalic) phrases by 24 months.

- Any loss of any language or social skills, at any age.

The United States Preventive Services Task Force in 2016 found it was unclear if screening was beneficial or harmful among children in whom there is no concerns. The Japanese practice is to screen all children for ASD at 18 and 24 months, using autism-specific formal screening tests. In contrast, in the UK, children whose families or doctors recognize possible signs of autism are screened. It is not known which approach is more effective. Screening tools include the Modified Checklist for Autism in Toddlers (M-CHAT), the Early Screening of Autistic Traits Questionnaire, and the First Year Inventory; initial data on M-CHAT and its predecessor, the Checklist for Autism in Toddlers (CHAT), on children aged 18–30 months suggests that it is best used in a clinical setting and that it has low sensitivity (many false-negatives) but good specificity (few false-positives). It may be more accurate to precede these tests with a broadband screener that does not distinguish ASD from other developmental disorders. Screening tools designed for one culture's norms for behaviors like eye contact may be inappropriate for a different culture. Although genetic screening for autism is generally still impractical, it can be considered in some cases, such as children with neurological symptoms and dysmorphic features.

Many normed assessments can be used in evaluating skills in the primary academic domains: reading, including word recognition, fluency, and comprehension; mathematics, including computation and problem solving; and written expression, including handwriting, spelling and composition.

The most commonly used comprehensive achievement tests include the Woodcock-Johnson IV (WJ IV), Wechsler Individual Achievement Test II (WIAT II), the Wide Range Achievement Test III (WRAT III), and the Stanford Achievement Test–10th edition. These tests include measures of many academic domains that are reliable in identifying areas of difficulty.

In the reading domain, there are also specialized tests that can be used to obtain details about specific reading deficits. Assessments that measure multiple domains of reading include Gray's Diagnostic Reading Tests–2nd edition (GDRT II) and the Stanford Diagnostic Reading Assessment. Assessments that measure reading subskills include the Gray Oral Reading Test IV – Fourth Edition (GORT IV), Gray Silent Reading Test, Comprehensive Test of Phonological Processing (CTOPP), Tests of Oral Reading and Comprehension Skills (TORCS), Test of Reading Comprehension 3 (TORC-3), Test of Word Reading Efficiency (TOWRE), and the Test of Reading Fluency. A more comprehensive list of reading assessments may be obtained from the Southwest Educational Development Laboratory.

The purpose of assessment is to determine what is needed for intervention, which also requires consideration of contextual variables and whether there are comorbid disorders that must also be identified and treated, such as behavioral issues or language delays. These contextual variables are often assessed using parent and teacher questionnaire forms that rate the students' behaviors and compares them to standardized norms.

However, caution should be made when suspecting the person with a learning disability may also have dementia, especially as people with Down's syndrome may have the neuroanatomical profile but not the associated clinical signs and symptoms. Examination can be carried out of executive functioning as well as social and cognitive abilities but may need adaptation of standardized tests to take account of special needs.

While infection with rubella during pregnancy causes fewer than 1% of cases of autism, vaccination against rubella can prevent many of those cases.

ASD can be detected as early as 18 months or even younger in some cases. A reliable diagnosis can usually be made by the age of two years. The diverse expressions of ASD symptoms pose diagnostic challenges to clinicians. Individuals with an ASD may present at various times of development (e.g., toddler, child, or adolescent), and symptom expression may vary over the course of development. Furthermore, clinicians must differentiate among pervasive developmental disorders, and may also consider similar conditions, including intellectual disability not associated with a pervasive developmental disorder, specific language disorders, ADHD, anxiety, and psychotic disorders.

Considering the unique challenges in diagnosing ASD, specific practice parameters for its assessment have been published by the American Academy of Neurology, the American Academy of Child and Adolescent Psychiatry, and a consensus panel with representation from various professional societies. The practice parameters outlined by these societies include an initial screening of children by general practitioners (i.e., "Level 1 screening") and for children who fail the initial screening, a comprehensive diagnostic assessment by experienced clinicians (i.e. "Level 2 evaluation"). Furthermore, it has been suggested that assessments of children with suspected ASD be evaluated within a developmental framework, include multiple informants (e.g., parents and teachers) from diverse contexts (e.g., home and school), and employ a multidisciplinary team of professionals (e.g., clinical psychologists, neuropsychologists, and psychiatrists).

After a child shows initial evidence of ASD tendencies, psychologists administer various psychological assessment tools to assess for ASD. Among these measurements, the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (ADOS) are considered the "gold standards" for assessing autistic children. The ADI-R is a semi-structured parent interview that probes for symptoms of autism by evaluating a child's current behavior and developmental history. The ADOS is a semistructured interactive evaluation of ASD symptoms that is used to measure social and communication abilities by eliciting several opportunities (or "presses") for spontaneous behaviors (e.g., eye contact) in standardized context. Various other questionnaires (e.g., The Childhood Autism Rating Scale, Autism Treatment Evaluation Checklist) and tests of cognitive functioning (e.g., The Peabody Picture Vocabulary Test) are typically included in an ASD assessment battery.

In the UK, there is some diagnostic use of the Diagnostic Interview for Social and Communication Disorders (DISCO) was which was developed for use at The Centre for Social and Communication Disorders, by Lorna Wing and Judith Gould, as both a clinical and a research instrument for use with children and adults of any age. The DISCO is designed to elicit a picture of the whole person through the story of their development and behaviour. In clinical work, the primary purpose is to facilitate understanding of the pattern over time of the specific skills and impairments that underlie the overt behaviour. If no information is available, the clinician has to obtain as much information as possible concerning the details of current skills and pattern of behaviour of the person. This type of dimensional approach to clinical description is useful for prescribing treatment.

At its most basic level, dyscalculia is a learning disability affecting the normal development of arithmetic skills.

A consensus has not yet been reached on appropriate diagnostic criteria for dyscalculia. Mathematics is a specific domain that is complex (i.e. includes many different processes, such as arithmetic, algebra, word problems, geometry, etc.) and cumulative (i.e. the processes build on each other such that mastery of an advanced skill requires mastery of many basic skills). Thus dyscalculia can be diagnosed using different criteria, and frequently is; this variety in diagnostic criteria leads to variability in identified samples, and thus variability in research findings regarding dyscalculia.

Other than using achievement tests as diagnostic criteria, researchers often rely on domain-specific tests (i.e. tests of working memory, executive function, inhibition, intelligence, etc.) and teacher evaluations to create a more comprehensive diagnosis. Alternatively, fMRI research has shown that the brains of the neurotypical children can be reliably distinguished from the brains of the dyscalculic children based on the activation in the prefrontal cortex. However, due to the cost and time limitations associated with brain and neural research, these methods will likely not be incorporated into diagnostic criteria despite their effectiveness.

Learning disabilities can be categorized by either the type of information processing affected by the disability or by the specific difficulties caused by a processing deficit.

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.

To date, very few interventions have been developed specifically for individuals with dyscalculia. Concrete manipulation activities have been used for decades to train basic number concepts for remediation purposes. This method facilitates the intrinsic relationship between a goal, the learner’s action, and the informational feedback on the action. A one-to-one tutoring paradigm designed by Lynn Fuchs and colleagues which teaches concepts in arithmetic, number concepts, counting, and number families using games, flash cards, and manipulables has proven successful in children with generalized math learning difficulties, but intervention has yet to be tested specifically on children with dyscalculia. These methods require specially trained teachers working directly with small groups or individual students. As such, instruction time in the classroom is necessarily limited. For this reason, several research groups have developed computer adaptive training programs designed to target deficits unique to dyscalculic individuals.

Software intended to remediate dyscalculia has been developed. While computer adaptive training programs are modeled after one-to-one type interventions, they provide several advantages. Most notably, individuals are able to practice more with a digital intervention than is typically possible with a class or teacher. As with one-to-one interventions, several digital interventions have also proven successful in children with generalized math learning difficulties. Räsänen and colleagues have found that games such as The Number Race and Graphogame-math can improve performance on number comparison tasks in children with generalized math learning difficulties.

Several digital interventions have been developed for dyscalculics specifically. Each attempts to target basic processes that are associated with maths difficulties. Rescue Calcularis was one early computerized intervention that sought to improve the integrity of and access to the mental number line. Other digital interventions for dyscalculia adapt games, flash cards, and manipulables to function through technology.

While each intervention claims to improve basic numerosity skills, the authors of these interventions do admit that repetition and practice effects may be a factor involved in reported performance gains. An additional criticism is that these digital interventions lack the option to manipulate numerical quantities. While the previous two games provide the correct answer, the individual using the intervention cannot actively determine, through manipulation, what the correct answer should be. Butterworth and colleagues argued that games like The Number Bonds, which allows an individual to compare different sized rods, should be the direction that digital interventions move towards. Such games use manipulation activities to provide intrinsic motivation towards content guided by dyscalculia research. One of these serious games is Meister Cody – Talasia, an online training that includes the CODY Assessment – a diagnostic test for detecting dyscalculia. Based on these findings, Rescue Calcluaris was extended by adaptation algorithms and game forms allowing manipulation by the learners. It was found to improve addition, subtraction and number line tasks, and was made available as Dybuster Calcularis.

A study used transcranial direct current stimulation (TDCS) to the parietal lobe during numerical learning and demonstrated selective improvement of numerical abilities that was still present six months later in typically developing individuals. Improvement were achieved by applying anodal current to the right parietal lobe and cathodal current to the left parietal lobe and contrasting it with the reverse setup. When the same research group used tDCS in a training study with two dyscalculic individuals, the reverse setup (left anodal, right cathodal) demonstrated improvement of numerical abilities.

Remediation includes both appropriate remedial instruction and classroom accommodations.

Autism spectrum disorders tend to be highly comorbid with other disorders. Comorbidity may increase with age and may worsen the course of youth with ASDs and make intervention/treatment more difficult. Distinguishing between ASDs and other diagnoses can be challenging, because the traits of ASDs often overlap with symptoms of other disorders, and the characteristics of ASDs make traditional diagnostic procedures difficult.

The most common medical condition occurring in individuals with autism spectrum disorders is seizure disorder or epilepsy, which occurs in 11-39% of individuals with ASD. Tuberous sclerosis, a medical condition in which non-malignant tumors grow in the brain and on other vital organs, occurs in 1-4% of individuals with ASDs.

Intellectual disabilities are some of the most common comorbid disorders with ASDs. Recent estimates suggest that 40-69% of individuals with ASD have some degree of an intellectual disability, more likely to be severe for females. A number of genetic syndromes causing intellectual disability may also be comorbid with ASD, including fragile X syndrome, Down syndrome, Prader-Willi and Angelman syndromes, and Williams syndrome.

Learning disabilities are also highly comorbid in individuals with an ASD. Approximately 25-75% of individuals with an ASD also have some degree of a learning disability.

Various anxiety disorders tend to co-occur with autism spectrum disorders, with overall comorbidity rates of 7-84%. Rates of comorbid depression in individuals with an ASD range from 4–58%. The relationship between ASD and schizophrenia remains a controversial subject under continued investigation, and recent meta-analyses have examined genetic, environmental, infectious, and immune risk factors that may be shared between the two conditions.

Deficits in ASD are often linked to behavior problems, such as difficulties following directions, being cooperative, and doing things on other people's terms. Symptoms similar to those of attention deficit hyperactivity disorder (ADHD) can be part of an ASD diagnosis.

Sensory processing disorder is also comorbid with ASD, with comorbidity rates of 42–88%.

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.

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.

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.

Pragmatic language impairment (PLI), or social (pragmatic) communication disorder (SCD), is an impairment in understanding pragmatic aspects of language. This type of impairment was previously called semantic-pragmatic disorder (SPD). People with these impairments have special challenges with the semantic aspect of language (the meaning of what is being said) and the pragmatics of language (using language appropriately in social situations). It is assumed that those with autism have difficulty with "the meaning of what is being said" due to different ways of responding to social situations.

PLI is now a diagnosis in DSM-5, and is called social (pragmatic) communication disorder. Communication problems are also part of the autism spectrum disorders (ASD); however, the latter also show a restricted pattern of behavior, according to behavioral psychology. The diagnosis SCD can only be given if ASD has been ruled out.

Studies have failed to find clear evidence that language delay can be prevented by training or educating health care professionals in the subject. Overall, some of the reviews show positive results regarding interventions in language delay, but are not curative. (Commentary - Early Identification of Language Delays, 2005)

There are no objectively definitive statistics about how many people have savant skills. The estimates range from "exceedingly rare" to one in ten people with autism having savant skills in varying degrees. A 2009 British study of 137 parents of autistic children found that 28% believe their children met the criteria for a savant skill, defined as a skill or power "at a level that would be unusual even for 'normal' people". As many as 50 cases of sudden or acquired savant syndrome have been reported.

Males with savant syndrome outnumber females by roughly 6:1, slightly higher than the sex ratio disparity for autism spectrum disorders of 4.3:1.

Treatment for dysgraphia varies and may include treatment for motor disorders to help control writing movements. The use of occupational therapy can be effective in the school setting, and teachers should be well informed about dysgraphia to aid in carry-over of the occupational therapist's interventions. Treatments may address impaired memory or other neurological problems. Some physicians recommend that individuals with dysgraphia use computers to avoid the problems of handwriting. Dysgraphia can sometimes be partially overcome with appropriate and conscious effort and training. The International Dyslexia Association suggests the use of kinesthetic memory through early training by having the child overlearn how to write letters and to later practice writing with their eyes closed or averted to reinforce the feel of the letters being written. They also suggest teaching the students cursive writing as it has fewer reversible letters and can help lessen spacing problems, at least within words, because cursive letters are generally attached within a word.

Diagnosing dysgraphia can be challenging but can be done at facilities specializing in learning disabilities. It is suggested that those who believe they may have dysgraphia seek a qualified clinician to be tested. Clinicians will have the client self-generate written sentences and paragraphs, and copy age-appropriate text. They will assess the output of writing, as well as observe the client's posture while writing, their grip on the writing instrument, and will ask the client to either tap their finger or turn their wrists repeatedly to assess fine motor skills.

Intellectual disability in children can be caused by genetic or environmental factors. The individual could have a natural brain malformation or pre or postnatal damage done to the brain caused by drowning or a traumatic brain injury, for example. Nearly 30 to 50% of individuals with intellectual disability will never know the cause of their diagnosis even after thorough investigation.

Prenatal causes of intellectual disability include:

- Congenital infections such as cytomegalovirus, toxoplasmosis, herpes, syphilis, rubella and human immunodeficiency virus

- Prolonged maternal fever in the first trimester

- Exposure to anticonvulsants or alcohol

- Untreated maternal phenylketonuria (PKU)

- Complications of prematurity, especially in extremely low-birth-weight infants

- Postnatal exposure to lead

Single-gene disorders that result in intellectual disability include:

- Fragile X syndrome

- Neurofibromatosis

- Tuberous sclerosis

- Noonan's syndrome

- Cornelia de Lange's syndrome

These single-gene disorders are usually associated with atypical physical characteristics.

About 1/4 of individuals with intellectual disability have a detectable chromosomal abnormality. Others may have small amounts of deletion or duplication of chromosomes, which may go unnoticed and therefore, undetermined.

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.

In 1983, Rapin and Allen suggested the term "semantic pragmatic disorder" to describe the communicative behavior of children who presented traits such as pathological talkativeness, deficient access to vocabulary and discourse comprehension, atypical choice of terms and inappropriate conversational skills. They referred to a group of children who presented with mild autistic features and specific semantic pragmatic language problems. More recently, the term "pragmatic language impairment" (PLI) has been proposed.

Rapin and Allen's definition has been expanded and refined by therapists who include communication disorders that involve difficulty in understanding the meaning of words, grammar, syntax, prosody, eye gaze, body language, gestures, or social context. While autistic children exhibit pragmatic language impairment, this type of communication disorder can also be found in individuals with other types of disorders including auditory processing disorders, neuropathies, encephalopathies and certain genetic disorders.

There is no treatment for intellectual disability but there are plenty of services offered for those diagnosed to help them function in their everyday lives. Professionals will sometimes work out an Individualized Family Service Plan (IFSP), which documents the child's needs, as well as the services that would best help them specifically. Speech, physical, and occupational therapy may be offered. Intellectually disabled children can be placed in special education classes through the public school system, where the school and parents will map out an Individualized Education Program (IEP). This program lays out all of the services and classes the child will become involved in during their time in school.

Older people with cognitive impairment appear to improve somewhat with light therapy.

Although not necessary for the diagnosis, individuals with intellectual disability are at higher risk for SMD. It is more common in boys, and can occur at any age.