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.

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.

A lack of demonstrated empathy affects aspects of communal living for persons with Asperger syndrome. Individuals with AS experience difficulties in basic elements of social interaction, which may include a failure to develop friendships or to seek shared enjoyments or achievements with others (for example, showing others objects of interest), a lack of social or emotional reciprocity (social "games" give-and-take mechanic), and impaired nonverbal behaviors in areas such as eye contact, facial expression, posture, and gesture.

People with AS may not be as withdrawn around others, compared with those with other, more debilitating forms of autism; they approach others, even if awkwardly. For example, a person with AS may engage in a one-sided, long-winded speech about a favorite topic, while misunderstanding or not recognizing the listener's feelings or reactions, such as a wish to change the topic of talk or end the interaction. This social awkwardness has been called "active but odd". This failure to react appropriately to social interaction may appear as disregard for other people's feelings, and may come across as insensitive. However, not all individuals with AS will approach others. Some of them may even display selective mutism, speaking not at all to most people and excessively to specific people. Some may choose only to talk to people they like.

The cognitive ability of children with AS often allows them to articulate social norms in a laboratory context, where they may be able to show a theoretical understanding of other people's emotions; however, they typically have difficulty acting on this knowledge in fluid, real-life situations. People with AS may analyze and distill their observations of social interaction into rigid behavioral guidelines, and apply these rules in awkward ways, such as forced eye contact, resulting in a demeanor that appears rigid or socially naive. Childhood desire for companionship can become numbed through a history of failed social encounters.

Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. People with autism may be severely impaired in some respects but normal, or even superior, in others. Overt symptoms gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. It is distinguished not by a single symptom, but by a characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis. Autism's individual symptoms occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.

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.

In the United States, a revision to autism spectrum disorder (ASD) was presented in the "Diagnostic and Statistical Manual of Mental Disorders" version 5 (DSM-5), released May 2013. The new diagnosis encompasses previous diagnoses of autistic disorder, Asperger's disorder, childhood disintegrative disorder, and PDD-NOS. Compared with the DSM-IV diagnosis of autistic disorder, the DSM-5 diagnosis of ASD no longer includes communication as a separate criterion, and has merged social interaction and communication into one category. Slightly different diagnostic definitions are used in other countries. For example, the ICD-10 is the most commonly-used diagnostic manual in the UK and European Union.

Rather than categorizing these diagnoses, the DSM-5 has adopted a dimensional approach to diagnosing disorders that fall underneath the autism spectrum umbrella. Some have proposed that individuals on the autism spectrum may be better represented as a single diagnostic category. Within this category, the DSM-5 has proposed a framework of differentiating each individual by dimensions of severity, as well as associated features (i.e., known genetic disorders, and intellectual disability).

Another change to the DSM includes collapsing social and communication deficits into one domain. Thus, an individual with an ASD diagnosis will be described in terms of severity of social communication symptoms, severity of fixated or restricted behaviors or interests, and associated features. The restricting of onset age has also been loosened from 3 years of age to "early developmental period", with a note that symptoms may manifest later when social demands exceed capabilities.

Autism forms the core of the autism spectrum disorders. Asperger syndrome is closest to autism in signs and likely causes; unlike autism, people with Asperger syndrome usually have no significant delay in language development, according to the older DSM-4 criteria. PDD-NOS is diagnosed when the criteria are not met for a more specific disorder. Some sources also include Rett syndrome and childhood disintegrative disorder, which share several signs with autism but may have unrelated causes; other sources differentiate them from ASD, but group all of the above conditions into the pervasive developmental disorders.

Autism, Asperger syndrome, and PDD-NOS are sometimes called the "autistic disorders" instead of ASD, whereas autism itself is often called "autistic disorder", "childhood autism", or "infantile autism". Although the older term "pervasive developmental disorder" and the newer term "autism spectrum disorder" largely or entirely overlap, the earlier was intended to describe a specific set of diagnostic labels, whereas the latter refers to a postulated spectrum disorder linking various conditions. ASD is a subset of the broader autism phenotype (BAP), which describes individuals who may not have ASD but do have autistic-like traits, such as avoiding eye contact.

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.

High-functioning autism is characterized by features very similar to those of Asperger syndrome. The defining characteristic most widely recognized by psychologists is a significant delay in the development of early speech and language skills, before the age of three years. The diagnostic criteria of Asperger syndrome exclude a general language delay.

Further differences in features between people with high-functioning autism and those with Asperger syndrome, include the following:

- People with HFA have a lower verbal reasoning ability

- Better visual/spatial skills (higher performance IQ) than people with Asperger syndrome

- Less deviating locomotion than people with Asperger syndrome

- People with HFA more often have problems functioning independently

- Curiosity and interest for many different things, in contrast to people with Asperger syndrome

- People with Asperger syndrome are better at empathizing with another

- The male to female ratio of 4:1 for HFA is much smaller than that of Asperger syndrome

Individuals with autism spectrum disorders, including high-functioning autism, risk developing symptoms of anxiety. While anxiety is one of the most commonly occurring mental health symptoms, children and adolescents with high functioning autism are at an even greater risk of developing symptoms.

There are other comorbidities, the presence of one or more disorders in addition to the primary disorder, associated with high-functioning autism. Some of these include depression, bipolar disorder, and obsessive compulsive disorder (OCD). In particular the link between HFA and OCD, has been studied; both have abnormalities associated with serotonin.

Observable comorbidities associated with HFA include ADHD, Tourette syndrome, and possibly criminal behavior. While the association between HFA and criminal behavior is not completely characterized, several studies have shown that the features associated with HFA may increase the probability of engaging in criminal behavior. While there is still a great deal of research that needs to be done in this area, recent studies on the correlation between HFA and criminal actions suggest that there is a need to understand the attributes of HFA that may lead to violent behavior. There have been several case studies that link the lack of empathy and social naïveté associated with HFA to criminal actions.

HFA does not cause nor include intellectual disabilities. This characteristic distinguishes HFA from the rest of the autism spectrum; between 40 and 55% of individuals with autism also have an intellectual disability.

It is common for individuals with PDD-NOS to have more intact social skills and a lower level of intellectual deficit than individuals with other PDDs. Characteristics of many individuals with PDD-NOS are:

- Communication difficulties (e.g., using and understanding language)

- Difficulty with social behavior

- Difficulty with changes in routines or environments

- Uneven skill development (strengths in some areas and delays in others)

- Unusual play with toys and other objects

- Repetitive body movements or behavior patterns

- Preoccupation with fantasy, such as imaginary friends in childhood

High-functioning autism (HFA) is a term applied to people with autism who are deemed to be cognitively "higher functioning" (with an IQ of 70 or greater) than other people with autism. Individuals with HFA or Asperger syndrome may exhibit deficits in areas of communication, emotion recognition and expression, and social interaction. HFA is not a recognized diagnosis in the DSM-5 or the ICD-10.

The amount of overlap between HFA and Asperger syndrome is disputed.

CDD is a rare condition, with only 1.7 cases per 100,000.

A child affected with childhood disintegrative disorder shows normal development and he/she acquires "normal development of age-appropriate verbal and nonverbal communication, social relationships, motor, play and self-care skills" comparable to other children of the same age. However, between the ages of 2 and 10, skills acquired are lost almost completely in at least two of the following six functional areas:

- Expressive language skills (being able to produce speech and communicate a message)

- Receptive language skills (comprehension of language - listening and understanding what is communicated)

- Social skills and self care skills

- Control over bowel and bladder

- Play skills

- Motor skills

Lack of normal function or impairment also occurs in at least two of the following three areas:

- Social interaction

- Communication

- Repetitive behavior and interest patterns

In her book, "Thinking in Pictures", Temple Grandin argues that compared to "Kanner's classic autism" and to Asperger syndrome, CDD is characterized with more severe sensory processing disorder but less severe cognitive problems. She also argues that compared to most autistic people, persons with CDD have more severe speech pathology and they usually do not respond well to stimulants.

Studies suggest that persons with PDD-NOS belong to one of three very different subgroups:

- A high-functioning group (around 25 percent) whose symptoms largely overlap with that of Asperger syndrome, but who differ in terms of having a lag in language development and/or mild cognitive impairment. (The criteria for Asperger syndrome excludes a speech delay or a cognitive impairment.)

- A group (around 25 percent) whose symptoms more closely resemble those of autism spectrum disorder, but do not fully meet all its diagnostic signs and symptoms.

- The biggest group (around 50 percent) consists of those who meet all the diagnostic criteria for autism spectrum disorder, but whose stereotypical and repetitive behaviors are noticeably mild.

The childhood disintegrative disorder (CDD), also known as Heller's syndrome and disintegrative psychosis, is a rare condition characterized by late onset of developmental delays—or stunning reversals—in language, social function, and motor skills. Researchers have not been successful in finding a cause for the disorder. CDD has some similarity to autism, and is sometimes considered a low-functioning form of it. In May 2013, the term CDD, along with other types of autism, was fused into a single diagnostic term called "autism spectrum disorder" under the new DSM-5 manual. Therefore, CDD is now also called "regressive autism", being that this term can now refer to any type of autism spectrum disorder that involves regression, including CDD.

CDD was originally described by Austrian educator Theodor Heller (1869–1938) in 1908, 35 years before Leo Kanner and Hans Asperger described autism. Heller had previously used the name "dementia infantilis" for the syndrome.

An apparent period of fairly normal development is often noted before a regression in skills or a series of regressions in skills. The age at which this regression can occur varies, but typically after 3 years of normal development. The regression can be so dramatic that the child may be aware of it, and may in its beginning even ask, vocally, what is happening to them. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost.

Many children are already somewhat delayed when the disorder becomes apparent, but these delays are not always obvious in young children. This has been described by many writers as a devastating condition, affecting both the family and the individual's future. As is the case with all pervasive developmental disorder categories, there is considerable controversy about the right treatment for CDD.

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".

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)

Late talker is a term used for exceptionally bright people who experience a delay in the development of speech. Commonalities include usually being boys, delayed speech development, highly educated parents, musically gifted families, puzzle-solving abilities, and lagging social development. Many high-achieving late talkers were notoriously strong willed and noncompliant as children. Late talkers can often be misdiagnosed early on as having severe ("low-functioning") autism spectrum disorder (a category known simply as "autism", prior to the DSM-5), and careful professional evaluation is necessary for differential diagnosis, according to Darold Treffert and other experts. One major difference between late talkers and low-functioning autistic children is that for late talkers, communication skills automatically reach a normal level and the child requires no further special treatment with regards to speech. Outlook for late talkers with or without intervention is generally favorable. However, late language emergence can also be an early or secondary sign of high-functioning autism spectrum disorder / Asperger syndrome, or other developmental disorders, such as attention deficit hyperactivity disorder, intellectual disability, learning disability, social communication disorder, or specific language impairment.

Einstein syndrome, a term coined by the economist Thomas Sowell, is also sometimes used to describe late talkers. The term is named after Albert Einstein (often said to have been a late talker, though with questionable evidence), whom Sowell used as the primary example of a late talker in his work. Sowell also included Edward Teller, Srinivasa Ramanujan, the mathematician Julia Robinson, Richard Feynman, and the pianists Clara Schumann and Arthur Rubinstein to be in the late talkers group. As a toddler, the scientist John Clive Ward showed similar behavioral traits to those described by Sowell, according to a brief sketch of his biography.

Sowell claimed late talkers are often inaccurately categorized as having an autism spectrum disorder (ASD), and that a small subset of late talkers are highly intelligent children with common characteristics concentrated in music, memory, math or the sciences. However, as reported by Simon Baron-Cohen, such characteristics are often found in high-functioning autism / Asperger syndrome.

Hyperkinetic disorder is a psychiatric neurodevelopmental condition emerging in early childhood that features an enduring pattern of severe, developmentally inappropriate symptoms, namely inattention, hyperactivity, and impulsivity across different settings (e.g., home and school) that significantly impair academic, social and work performance. The disorder, which appears in the World Health Organization's ICD-10, is roughly similiar to the "combined presentation" of attention deficit hyperactivity disorder in the American Psychiatric Association's DSM-V.

Hyperkinetic people display disorganized, poorly controlled and excessive activity; they lack perseverance in tasks involving thought and attention, and tend to move from one activity to the next without completing any. They are frequently accident-prone, reckless and impulsive, and may thoughtlessly (rather than defiantly) break rules. Cognitive impairment and delayed language and motor development are more common in this group than in the general population; and they may experience low self-esteem and engage in dissocial behavior as a consequence of the disorder.

While hyperkinetic children are commonly incautious and unreserved with adults, they might be isolated and unpopular with other children.

Though the American Psychiatric Association's criteria for attention deficit hyperactivity disorder (ADHD), and the World Health Organization's criteria for hyperkinetic disorder each list a very similar set of 18 symptoms, the differing rules governing diagnosis mean that hyperkinetic disorder features greater impairment and more impulse-control difficulties than typical ADHD, and it most resembles a severe case of ADHD combined type.

Unlike ADHD, a diagnosis of hyperkinetic disorder requires that the clinician directly observes the symptoms (rather than relying only on parent and teacher reports); that onset must be by age 6 not 7; and that at least six inattention, three hyperactivity and one impulsivity symptom be present in two or more settings. While ADHD may exist comorbid with (in the presence of) mania or a depressive or anxiety disorder, the presence of one of these rules out a diagnosis of hyperkinetic disorder. Most cases of hyperkinetic disorder appear to meet the broader criteria of ADHD.

Hyperkinetic disorder may exist comorbid with conduct disorder, in which case the diagnosis is hyperkinetic conduct disorder.

Other conditions which feature repetitive behaviors in the differential diagnosis include autism spectrum disorders, obsessive–compulsive disorder, tic disorders (e.g., Tourette syndrome), and other conditions including dyskinesias.

Stereotypic movement disorder is often misdiagnosed as tics or Tourette syndrome (TS). Unlike the tics of TS, which tend to appear around age six or seven, repetitive movements typically start before age three, are more bilateral than tics, and consist of intense patterns of movement for longer runs than tics. Tics are less likely to be stimulated by excitement. Children with stereotypic movement disorder do not always report being bothered by the movements as a child with tics might.

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.

Stereotyped movements are common in infants and young children; if the child is not distressed by movements and daily activities are not impaired, diagnosis is not warranted. When stereotyped behaviors cause significant impairment in functioning, an evaluation for stereotypic movement disorder is warranted. There are no specific tests for diagnosing this disorder, although some tests may be ordered to rule out other conditions. SMD may occur with Lesch-Nyhan syndrome, intellectual disability, and fetal alcohol exposure or as a result of amphetamine intoxication.

When diagnosing stereotypic movement disorder, DSM-5 calls for specification of:

- with or without self-injurious behavior;

- association with another known medical condition or environmental factor;

- severity (mild, moderate or severe).

Children with conduct disorder have a high risk of developing other adjustment problems. Specifically, risk factors associated with conduct disorder and the effects of conduct disorder symptomatology on a child's psychosocial context have been linked to overlap with other psychological disorders. In this way, there seems to be reciprocal effects of comorbidity with certain disorders, leading to increased overall risk for these youth.

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.

Fine-motor problems can cause difficulty with a wide variety of other tasks such as using a knife and fork, fastening buttons and shoelaces, cooking, brushing one's teeth, styling one's hair, shaving, applying cosmetics, opening jars and packets, locking and unlocking doors, and doing housework.

Difficulties with fine motor co-ordination lead to problems with handwriting, which may be due to either ideational or ideo-motor difficulties.

Problems associated with this area may include:

- Learning basic movement patterns.

- Developing a desired writing speed.

- Establishing the correct pencil grip

- The acquisition of graphemes – e.g. the letters of the Latin alphabet, as well as numbers.