Because these are frequently found in cases of autistic disorders, criteria could be met for multiple neurological disorders, or cause severe symptoms.

Some examples include:

1. Learning difficulties symptoms such as dyslexia, dysgraphia, dyscalcula, NVLD, slow learning, poor memory, etc.

2. AD/HD symptoms such as poor concentration, poor decision making, poor judgement, impulsiveness, difficulty sitting still, etc.

3. Synesthesia.

4. Neurological sleep disorders such as narcolepsy, insomnia, circadian rhythm disorder, etc.

5. Conditions affecting perceptions and/or cognition, such as agnosia, aphasia, etc.

6. Tourette syndrome or Tic disorder.

7. Epilepsy or Seizure disorder.

8. Parkinsonian syndrome features such as tremors, stiff movements, etc.

The current diagnostic criteria for MCDD are a matter of debate due to it not being in the DSM-IV or ICD-10. Various websites contain various diagnostic criteria. At least three of the following categories should be present. Co-occurring clusters of symptoms must also not be better explained by being symptoms of another disorder such as experiencing mood swings due to autism, cognitive difficulties due to schizophrenia, and so on. The exact diagnostic criteria for MCDD remain unclear but may be a useful diagnosis for people who do not fall into any specific category. It could also be argued that MCDD is a vague and unhelpful term for these patients.

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

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.

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 .

One of the symptoms of conduct disorder is a lower level of fear. Research performed on the impact of toddlers exposed to fear and distress shows that negative emotionality (fear) predicts toddlers' empathy-related response to distress. The findings support that if a caregiver is able to respond to infant cues, the toddler has a better ability to respond to fear and distress. If a child does not learn how to handle fear or distress the child will be more likely to lash out at other children. If the caregiver is able to provide therapeutic intervention teaching children at risk better empathy skills, the child will have a lower incident level of conduct disorder.

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.

In psychology and neuroscience, executive dysfunction, or executive function deficit, is a disruption to the efficacy of the executive functions, which is a group of cognitive processes that regulate, control, and manage other cognitive processes. Executive dysfunction can refer to both neurocognitive deficits and behavioural symptoms. It is implicated in numerous psychopathologies and mental disorders, as well as short-term and long-term changes in non-clinical executive control.

Executive dysfunction is not the same as dysexecutive syndrome, a term coined by Alan Baddeley to describe a common pattern of dysfunction in executive functions, such as deficiencies in planning, abstract thinking, flexibility and behavioural control. This group of symptoms, usually resulting from brain damage, tend to occur together. However, the existence of dysexecutive syndrome is controversial.

Specific developmental disorders are disorders in which development is delayed in one specific area or areas, and in which basically all other areas of development are not affected. Specific developmental disorders are as opposed to pervasive developmental disorders that are characterized by delays in the development of multiple basic functions including socialization and communication.

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.

Conduct disorder (CD) is a mental disorder diagnosed in childhood or adolescence that presents itself through a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate norms are violated. These behaviors are often referred to as "antisocial behaviors." It is often seen as the precursor to antisocial personality disorder, which is per definition not diagnosed until the individual is 18 years old.

Conduct disorder is estimated to affect 51.1 million people globally as of 2013.

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.

Pathological Demand Avoidance is not recognised by the DSM-5 or ICD-10, the two main classification systems for mental disorder.

To be recognized a sufficient amount of consensus and clinical history needs to be present, and as a newly proposed condition PDA had not met the standard of evidence required at the time of recent revisions. In April 2014 the UK Minister of State for Care and Support Norman Lamb stated that the Department of Health, "In the course of the development of the National Institute for Health and Care Excellence (NICE) clinical guideline on the treatment of autism in children and young people (CG128), the developers looked at differential diagnoses for autism. In this, they did consider PDA, identifying it as a particular subgroup of autism that could also be described as oppositional defiant disorder (ODD). The guidance recommends that consideration should be given to differential diagnoses for autism (including ODD) and whether specific assessments are needed to help interpret the autism history and observations. However, due to the lack of evidence and the fact that the syndrome is not recognised within the DSM or ICD classifications, NICE was unable to develop specific recommendations on the assessment and treatment of PDA."

So the National Institute for Health and Care Excellence (which provides guidelines on best practice for UK clinicians) makes no mention of PDA in its guidelines for diagnosis of autism either in children or adults.

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.

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.

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.

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.

Sensory-based motor disorder shows motor output that is disorganized as a result of incorrect processing of sensory information affecting postural control challenges, resulting in postural disorder, or developmental coordination disorder.

The SBMD subtypes are:

1. Dyspraxia

2. Postural disorder

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.

Autistic individuals can display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R) categorizes as follows.

- Stereotyped behaviors: Repetitive movements, such as hand flapping, head rolling, or body rocking.

- Compulsive behaviors: Time-consuming behaviors intended to reduce anxiety that an individual feels compelled to perform repeatedly or according to rigid rules, such as placing objects in a specific order, checking things, or hand washing.

- Sameness: Resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.

- Ritualistic behavior: Unvarying pattern of daily activities, such as an unchanging menu or a dressing ritual. This is closely associated with sameness and an independent validation has suggested combining the two factors.

- Restricted interests: Interests or fixations that are abnormal in theme or intensity of focus, such as preoccupation with a single television program, toy, or game.

- Self-injury: Behaviors such as eye-poking, skin-picking, hand-biting and head-banging.

No single repetitive or self-injurious behavior seems to be specific to autism, but autism appears to have an elevated pattern of occurrence and severity of these behaviors.

Individuals with PLI have particular trouble understanding the meaning of what others are saying, and they are challenged in using language appropriately to get their needs met and interact with others. Children with the disorder often exhibit:

- delayed language development

- aphasic speech (such as word search pauses, jargoning, word order errors, word category errors, verb tense errors)

- Stuttering or cluttering speech

- Repeating words or phrases

- difficulty with pronouns or pronoun reversal

- difficulty understanding questions

- difficulty understanding choices and making decisions.

- difficulty following conversations or stories. Conversations are "off-topic" or "one-sided".

- difficulty extracting the key points from a conversation or story; they tend to get lost in the details

- difficulty with verb tenses

- difficulty explaining or describing an event

- tendency to be concrete or prefer facts to stories

- difficulty understanding satire or jokes

- difficulty understanding contextual cues

- difficulty in reading comprehension

- difficulty with reading body language

- difficulty in making and maintaining friendships and relationships because of delayed language development.

- difficulty in distinguishing offensive remarks

- difficulty with organizational skills

Symptoms may vary according to the disorder's type and subtype present. SPD can affect one sense or multiple senses. While many people can present one or two symptoms, sensory processing disorder has to have a clear functional impact on the person's life.

Autistic individuals may have symptoms that are independent of the diagnosis, but that can affect the individual or the family.

An estimated 0.5% to 10% of individuals with ASD show unusual abilities, ranging from splinter skills such as the memorization of trivia to the extraordinarily rare talents of prodigious autistic savants. Many individuals with ASD show superior skills in perception and attention, relative to the general population. Sensory abnormalities are found in over 90% of those with autism, and are considered core features by some, although there is no good evidence that sensory symptoms differentiate autism from other developmental disorders. Differences are greater for under-responsivity (for example, walking into things) than for over-responsivity (for example, distress from loud noises) or for sensation seeking (for example, rhythmic movements). An estimated 60%–80% of autistic people have motor signs that include poor muscle tone, poor motor planning, and toe walking; deficits in motor coordination are pervasive across ASD and are greater in autism proper.

Unusual eating behavior occurs in about three-quarters of children with ASD, to the extent that it was formerly a diagnostic indicator. Selectivity is the most common problem, although eating rituals and food refusal also occur; this does not appear to result in malnutrition. Although some children with autism also have gastrointestinal symptoms, there is a lack of published rigorous data to support the theory that children with autism have more or different gastrointestinal symptoms than usual; studies report conflicting results, and the relationship between gastrointestinal problems and ASD is unclear.

Parents of children with ASD have higher levels of stress. Siblings of children with ASD report greater admiration of and less conflict with the affected sibling than siblings of unaffected children and were similar to siblings of children with Down syndrome in these aspects of the sibling relationship. However, they reported lower levels of closeness and intimacy than siblings of children with Down syndrome; siblings of individuals with ASD have greater risk of negative well-being and poorer sibling relationships as adults.

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.

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.