Symptoms of PDD may include behavioral and communication problems such as:

- Difficulty using and understanding language.

- Difficulty relating to people, objects, and events; for example, lack of eye contact, pointing behavior, and lack of facial responses.

- Unusual play with toys and other objects.

- Difficulty with changes in routine or familiar surroundings.

- Repetitive body movements or behavior patterns, such as hand flapping, hair twirling, foot tapping, or more complex movements.

- Inability to cuddle or be comforted.

- Difficulty regulating behaviors and emotions, which may result in temper tantrums, anxiety, and aggression.

- Emotional breakdowns.

Children with PDD vary widely in abilities, intelligence, and behaviors. Some children do not speak at all, others speak in limited phrases or conversations, and some have relatively normal language development. Repetitive play skills and limited social skills are generally evident as well. Unusual responses to sensory information – loud noises, lights – they also are common.

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.

In the third edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-III), SDD was opposed to the pervasive developmental disorders (PDD). There were two factors that were considered:

- The specificity of the impairment: in SDD there is one single domain that is affected, whereas in PDD multiple areas of functioning are affected.

- The nature of the impairment: development in SDD is delayed but not otherwise abnormal, whereas in PDD there are behavioral deviations that are not typical for any developmental stage.

In the fourth edition of the DSM specific developmental disorders are no longer grouped together. Instead they are reclassified as communication disorders, learning disorders, and motor skills disorders.

The tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) has four categories of specific developmental disorder: specific developmental disorders of speech and language, specific developmental disorders of scholastic skills, specific developmental disorder of motor function, and mixed specific developmental disorder.

Mental retardation is coded on Axis II of the DSM-IV-TR. The diagnostic criteria necessary in order to diagnose intellectual disability consists of:

There are varying degrees of intellectual disability, which are identified by an IQ test.

Mental retardation, Severity Unspecified: This unspecified diagnosis is given when there is a strong assumption that the child is mentally retarded, but cannot be tested because the individual is too impaired, not willing to take the IQ test or is an infant.

Separation anxiety disorder

- excessive stress when separated from home or family

- fear of being alone

- refusal to sleep alone

- clinginess

- excessive worry about safety

- excessive worry about getting lost

- frequent medical complaints with no cause

- refusal to go to school

Selective mutism

- unable to speak in certain social situations, even though they are comfortable speaking at home or with friends

- difficulty maintaining eye contact

- may have blank facial expressions

- stiff body movements

- may have a worrisome personality

- may be incredibly sensitive to sound

- difficulty with verbal and non-verbal expression

- may appear shy, when in reality, they have a fear of people.

Reactive attachment disorder of infancy or early childhood

- withdrawing from others

- aggressive attitude towards peers

- awkwardness or discomfort

- watching others but not engaging in social interaction

Stereotypic movement disorder

- head banging

- nail biting

- hitting or biting oneself

- hand waving or shaking

- rocking back and forth

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.

Whole body movement and motor coordination issues mean that major developmental targets including walking, running, climbing and jumping can be affected. The difficulties vary from person to person and can include the following:

- Poor timing

- Poor balance (sometimes even falling over in mid-step). Tripping over one's own feet is also common.

- Difficulty combining movements into a controlled sequence.

- Difficulty remembering the next movement in a sequence.

- Problems with spatial awareness, or proprioception.

- Trouble picking up and holding onto simple objects such as pencils, owing to poor muscle tone or proprioception.

- Clumsiness to the point of knocking things over and bumping into people accidentally.

- Difficulty in determining left from right.

- Cross-laterality, ambidexterity, and a shift in the preferred hand are also common in people with developmental coordination disorder.

- Problems with chewing foods.

Various areas of development can be affected by developmental coordination disorder and these will persist into adulthood, as DCD has no cure. Often various coping strategies are developed, and these can be enhanced through occupational therapy, psychomotor therapy, physiotherapy, speech therapy, or psychological training.

In addition to the physical impairments, developmental coordination disorder is associated with problems with memory, especially working memory. This typically results in difficulty remembering instructions, difficulty organizing one's time and remembering deadlines, increased propensity to lose things or problems carrying out tasks which require remembering several steps in sequence (such as cooking). Whilst most of the general population experience these problems to some extent, they have a much more significant impact on the lives of dyspraxic people. However, many dyspraxics have excellent long-term memories, despite poor short-term memory. Many dyspraxics benefit from working in a structured environment, as repeating the same routine minimises difficulty with time-management and allows them to commit procedures to long-term memory.

People with developmental coordination disorder sometimes have difficulty moderating the amount of sensory information that their body is constantly sending them, so as a result dyspraxics are prone to sensory overload and panic attacks.

Many dyspraxics struggle to distinguish left from right, even as adults, and have extremely poor sense of direction generally.

Moderate to extreme difficulty doing physical tasks is experienced by some dyspraxics, and fatigue is common because so much extra energy is expended while trying to execute physical movements correctly. Some (but not all) dyspraxics suffer from hypotonia, low muscle tone, which like DCD can detrimentally affect balance.

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.

Individuals with Asperger syndrome may have signs or symptoms that are independent of the diagnosis, but can affect the individual or the family. These include differences in perception and problems with motor skills, sleep, and emotions.

Individuals with AS often have excellent auditory and visual perception. Children with ASD often demonstrate enhanced perception of small changes in patterns such as arrangements of objects or well-known images; typically this is domain-specific and involves processing of fine-grained features. Conversely, compared with individuals with high-functioning autism, individuals with AS have deficits in some tasks involving visual-spatial perception, auditory perception, or visual memory. Many accounts of individuals with AS and ASD report other unusual sensory and perceptual skills and experiences. They may be unusually sensitive or insensitive to sound, light, and other stimuli; these sensory responses are found in other developmental disorders and are not specific to AS or to ASD. There is little support for increased fight-or-flight response or failure of habituation in autism; there is more evidence of decreased responsiveness to sensory stimuli, although several studies show no differences.

Hans Asperger's initial accounts and other diagnostic schemes include descriptions of physical clumsiness. Children with AS may be delayed in acquiring skills requiring motor dexterity, such as riding a bicycle or opening a jar, and may seem to move awkwardly or feel "uncomfortable in their own skin". They may be poorly coordinated, or have an odd or bouncy gait or posture, poor handwriting, or problems with visual-motor integration. They may show problems with proprioception (sensation of body position) on measures of developmental coordination disorder (motor planning disorder), balance, tandem gait, and finger-thumb apposition. There is no evidence that these motor skills problems differentiate AS from other high-functioning ASDs.

Children with AS are more likely to have sleep problems, including difficulty in falling asleep, frequent nocturnal awakenings, and early morning awakenings. AS is also associated with high levels of alexithymia, which is difficulty in identifying and describing one's emotions. Although AS, lower sleep quality, and alexithymia are associated, their causal relationship is unclear.

As a pervasive developmental disorder, Asperger syndrome is distinguished by a pattern of symptoms rather than a single symptom. It is characterized by qualitative impairment in social interaction, by stereotyped and restricted patterns of behavior, activities and interests, and by no clinically significant delay in cognitive development or general delay in language. Intense preoccupation with a narrow subject, one-sided verbosity, restricted prosody, and physical clumsiness are typical of the condition, but are not required for diagnosis. Suicidal behavior appears to occur at rates similar to those without ASD.

Nonverbal learning disorder (also known as nonverbal learning disability, NLD, or NVLD) is a learning disorder characterized by verbal strengths as well as visual-spatial, motor, and social skills difficulties. It is sometimes confused with Asperger Syndrome or high IQ. Nonverbal learning disorder has never been included in the American Psychiatric Association's "Diagnostic and Statistical Manual of Mental Disorders" or the World Health Organization's "International Classification of Diseases".

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.

Considered to be neurologically based, nonverbal learning disorder is characterized by verbal strengths as well as visual-spatial, motor, and social skills difficulties. People with this disorder may not at times comprehend nonverbal cues such as facial expression or tone of voice. Challenges with mathematics and handwriting are common.

While various nonverbal impairments were recognized since early studies in child neurology, there is ongoing debate as to whether/or the extent to which existing conceptions of NLD provide a valid diagnostic framework. As originally presented "nonverbal disabilities" (p. 44) or "disorders of nonverbal learning" (p. 272) was a category encompassing non-linguistic learning problems (Johnson and Myklebust, 1967). "Nonverbal learning disabilities" were further discussed by Myklebust in 1975 as representing a subtype of learning disability with a range of presentations involving "mainly visual cognitive processing," social imperception, a gap between higher verbal ability and lower performance IQ, as well as difficulty with handwriting. Later neuropsychologist Byron Rourke sought to develop consistent criteria with a theory and model of brain functioning that would establish NLD as a distinct syndrome (1989).

Questions remain about how best to frame the perceptual, cognitive and motor issues associated with NLD.

The DSM-5 (Diagnostic and Statistical Manual) and ICD-10 (International Classification of Diseases) do not include NLD as a diagnosis.

Assorted diagnoses have been discussed as sharing symptoms with NLD—these conditions include Right hemisphere brain damage and Developmental Right Hemisphere Syndrome, Developmental Coordination Disorder, Social-Emotional Processing Disorder, Asperger syndrome, Gerstmann syndrome and others.

Labels for specific associated issues include visual-spatial deficit, dyscalculia, dysgraphia, as well as dyspraxia.

In their 1967 book "Learning Disabilities; Educational Principles and Practices", Doris J. Johnson and Helmer R. Myklebust characterize how someone with these kinds of disabilities appears in a classroom: "An example is the child who fails to learn the meaning of the actions of others...We categorize this child as having a deficiency in social perception, meaning that he has an inability which precludes acquiring the significance of basic nonverbal aspects of daily living, though his verbal level of intelligence falls within or above the average." (p. 272). In their chapter "Nonverbal Disorders Of Learning" (p. 272-306) are sections titled "Learning Though Pictures," (274) "Gesture," (281) "Nonverbal Motor Learning," (282) "Body Image," (285) "Spatial Orientation," (290) "Right-Left Orientation," (292) "Social Imperception," (295) "Distractibility, Perseveration, and Disinhibition." (298)

Developmental disability is a diverse group of chronic conditions that are due to mental or physical impairments. Developmental disabilities cause individuals living with them many difficulties in certain areas of life, especially in "language, mobility, learning, self-help, and independent living". Developmental disabilities can be detected early on, and do persist throughout an individual's lifespan. Developmental disability that affects all areas of a child's development is sometimes referred to as global developmental delay.

Most common developmental disabilities:

- Down syndrome is a condition in which people are born with an extra copy of chromosome 21. Normally, a person is born with two copies of chromosome 21. However, if they are born with Down syndrome, they have an extra copy of this chromosome. This extra copy affects the development of the body and brain, causing physical and mental challenges for the individual.

- Fragile X syndrome (FXS) is thought to cause autism and intellectual disability, usually among boys.

- Pervasive developmental disorders (PDD) are a group of developmental disabilities that can cause significant social, communication and behavioral challenges.

- Fetal alcohol spectrum disorders (FASD) are a group of conditions that can occur in a person whose mother drank alcohol during pregnancy.

- Cerebral palsy (CP) is a group of disorders that affect a person’s ability to move and maintain balance and posture. CP is the most common motor disability in childhood.

- Intellectual disability, also (sometimes proscriptively) known as mental retardation, is defined as an IQ below 70 along with limitations in adaptive functioning and onset before the age of 18 years.

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.

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.

The term "multisystem developmental disorder" has also been used to describe various developmental disorders. These include:

- Alagille syndrome, an autosomal dominant disorder with a wide range of features and manifestations. Its five most significant features are chronic cholestasis, a condition where bile cannot flow from the liver to the duodenum, occurring in 95% of cases; heart abnormalities (over 90%); butterfly vertebrae; posterior embryotoxon and a distinctive face (prominent forehead, deep-set eyes, and a pointed chin).

- Rubinstein-Taybi syndrome, a mental retardation syndrome characterized by broad thumbs, facial abnormalities, and big toes alongside mental retardation.

- Williams syndrome, a neurodevelopmental disorder characterized by a unique profile of strengths and deficits; most with the condition have mild mental retardation but have grammatical and lexical abilities above what would be expected from their IQs. They are hypersocial and empathetic, but social isolation is commonly experienced.

- Proteus syndrome, a congenital disorder causing disproportionate growth of skin, bone, and other tissues.

- Asphyxiating thoracic dysplasia, a autosomal recessive skeletal disorder with an estimated prevalence of between 1 in 100,000 and 1 in 130,000 live births.

Pathological demand avoidance (PDA) is a proposed subtype of autism characterized by an avoidance of demand-framed requests by an individual. It was proposed in 1980 by the UK child psychologist Elizabeth Newson. The Elizabeth Newson Centre in Nottingham, England carries out assessments for the NHS, local authorities and private patients around autism spectrum disorder, which include, but are not exclusively PDA.

PDA behaviours are consistent with autism, but have differences from other autism subtypes diagnoses. It is not recognised by either the DSM-5 or the .

The causes of developmental disabilities are varied and remain unknown in a large proportion of cases. Even in cases of known etiology the line between "cause" and "effect" is not always clear, leading to difficulty in categorizing causes.

Genetic factors have long been implicated in the causation of developmental disabilities. There is also a large environmental component to these conditions, and the relative contributions of nature versus nurture have been debated for decades.

Current theories on causation focus on genetic factors, and over 1,000 known genetic conditions include developmental disabilities as a symptom.

Developmental disabilities affect between 1 and 2% of the population in most western countries, although many government sources acknowledge that statistics are flawed in this area. The worldwide proportion of people with developmental disabilities is believed to be approximately 1.4%. It is twice as common in males as in females, and some researchers have found that the prevalence of mild developmental disabilities is likely to be higher in areas of poverty and deprivation, and among people of certain ethnicities.

Multisystem developmental disorder (MSDD) is a term used by Stanley Greenspan to describe children under age 3 who exhibit signs of impaired communication as in autism, but with strong emotional attachments atypical of autism. It is described in the DC:0-3R manual as an optional diagnosis for children under two years of age.

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.