A pervasive developmental disorder not otherwise specified (Including atypical autism) (PDD-NOS) is one of the four autistic spectrum disorders in the DSM-5 and also was one of the five disorders classified as a pervasive developmental disorder (PDD) in the DSM-IV. According to the DSM-4, PDD-NOS is a diagnosis that is used for "severe or pervasive impairment in the development of reciprocal social interaction and/or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and/or activities are present, but the criteria are not met for a specific PDD" or for several other disorders. PDD-NOS includes atypical autism, because the criteria for autistic disorder are not met, for instance because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these. Even though PDD-NOS is considered milder than typical autism, this is not always true. While some characteristics may be milder, others may be more severe. Signs and symptoms
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
Paranoia, a characteristic form of social anxiety, derealization, transient psychosis, and unconventional beliefs if the environment changed without further notice
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 fantasies that interfere and that are not normal to have at a certain age depending on social, cultural and religious norms.
While its true that children diagnosed with PDD-NOS, as a whole, show fewer intellectual deficits and are higher-functioning than autistic children, many others who fit the criteria for PDD-NOS have some autistic features but also have intellectual deficits that are so severe that its difficult or impossible to tell whether some of the deficits come from the autism or from the severe to profound degree of intellectual disability itself. Furthermore, some others who fit the criteria for PDD-NOS come to professional attention at a later age, compared to those diagnosed with autism. Subgroups
Studies suggest that persons with PDD-NOS belong to one of three very different subgroups:
A high-functioning group (around 25 percent) whose symptoms more or less overlap with that of Asperger syndrome, while also not meeting the criteria for autistic disorder, but who completely differ from those with Asperger syndrome 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 delay in early life.) Another group (around 25 percent) whose symptoms more closely resemble those of autism, but do not fully meet all its diagnostic signs and symptoms. This is because either the symptoms were recognized at a later age or because they were too young or have cognitive deficits that are too severe to properly identify all the symptoms of autism that they may have.