Autism and several other related disorders are grouped under the heading of pervasive developmental disorders (PDDs). These disorders all have similar symptoms.

In addition, there are several medical conditions that can be confused with autism because of similar symptoms. Some of these medical conditions can occur along with autism but are not classified as PDDs. They include:

• Mental retardation. People with severe mental retardation may have many behaviors similar to those of autism but are not necessarily autistic. People who are autistic can also be mentally retarded.
• Specific developmental disorders. Some developmental disorders, particularly language disorders, can mimic autism.
• Schizophrenia. When schizophrenia develops in childhood, which is rare, it can be confused with autism.
• Selective mutism. In selective mutism, a child is able to speak in some situations but unable to speak (is mute) in others.
• Obsessive-compulsive disorder (OCD). Some children with obsessive-compulsive disorder, like those with autism, have unusual interests and repetitive behaviors. Unlike with autism, children with OCD are able to develop social and communication skills.
• Reactive attachment disorder. In this disorder, there is usually a history of severe neglect. Language and social skills usually begin to develop after a child is given love and attention.
• Avoidant personality disorder. This disorder is characterized by anxiety in dealing with social situations.

Specific diagnostic guidelines are used to distinguish autism from other conditions.¹ In general, autism is unique because symptoms always develop within the first year of life (although they are not always recognized at that time), language is typically delayed or absent, and certain social skills are compromised. Although other PDDs and certain medical conditions share some of these symptoms, they do not meet all of the diagnostic criteria for autism.