Autism Spectrum Disorder is not an Intellectual Disability or a Lack of Intelligence
Intelligence is communicated through skillful use of language. Because of the social and communication issues of those with autism spectrum disorder, it can make them appear unintelligent. People on the spectrum vary in intelligence just as other people do. But sometimes, their language and social problems can make that intelligence harder to discover. It should be kept in mind that there are multiple kinds of intelligence and not all of them involve language.
Historically, autism was confused with intellectual disabilities (formerly known as mental retardation). Today, intellectual disabilities and autism are known to be distinct and separate conditions.
Autism Spectrum Disorder is not Obsessive-Compulsive Disorder
People with autism spectrum disorder have ritual preoccupations and behaviors that are similar to the obsessions and compulsions seen in Obsessive Compulsive Disorder (OCD). They also show a need for order and perfection like that seen in Obsessive Compulsive Personality Disorder (OCPD). Obsessions are intrusive thoughts that happen constantly, and which cannot be stopped or escaped. They cause affected people to become anxious and to behave in ways that will reduce that anxiety. For example, someone might develop an obsession about whether a door handle is dirty. Anxiety around that contamination fear would motivate action; once the door handle was cleaned, some anxiety relief might be experienced. This creates compulsions, which are anxiety motivated behaviors that once performed, help to reduce distress (however short lived that relief might be). OCPD is a distinct condition from OCD. People with OCPD do not experience true obsessions or compulsions. Instead, they are rule-governed, inflexible people who are rigidly concerned with their position within their social hierarchy, and with the orderliness and perfection of their personal environments and projects.
Thought there are similarities between autism spectrum disorder and the obsessive-compulsive disorders, these disorders are separate from one another. Autism spectrum disorder begins in early childhood while the other disorders do not tend to develop until early adulthood. Autism spectrum disorder is a brain disease reducing affected people's capability for communicating and socializing with people around them. People with moderate or severe autism spectrum disorder have difficulty understanding that other people have different interests or thoughts than they do. This contrasts with people with OCPD who are hyper-aware of their social position. Both those with OCD and autism spectrum disorder display repetitive obsessive behaviors. However, OCD compulsiveness appears to be motivated more by anxious attempts to self-sooth. Compulsiveness in autism spectrum disorder may be motivated by a need to self-stimulate. The disease process that creates obsessions and compulsions in OCD appears to have a different method than the one responsible for issues on the autism spectrum. This means that people with autism spectrum disorder could develop OCD as well and receive both diagnoses. However, if this were to be the case, both conditions would be happening at the same time. They are not the same disorder.