Autism Spectrum: a developmental disability that affects social interaction, communication, and imaginative play. (Blakeslee, 211) One of a group of serious developmental problems. (RamachandranTTB, 295) “Autism” is the most debilitating form while other disorders ... produce milder symptoms. (NIMH, 2011) There are many genetic causes and contributing factors to autism spectrum disorders. Known environmental triggers include prenatal exposures to 'rubella' and the drug 'valproate.' "Heritability" of autism is high - about 90 percent. Many genes contribute to autism risk. Two in particular, which encode the "receptor proteins" 'neuroligin' and 'neurexin,' may explain how the condition arises. (Lewis, 149-150) Adjective - 'autistic.' Also referred to as ‘autism spectrum disorders,’ and ‘pervasive developmental disorders.’

At one end of the spectrum, you might find the severely disabled. At the other end, you might encounter an Einstein or a Steve Jobs.
— Temple Grandin, The Autistic Brain

Asperger’s Syndrome: a rare and relatively mild 'autistic' disorder which develops in early childhood and persists in adult life. (Oxford) Hans Asperger identified a class of children who shared several distinct behaviors: “a lack of empathy, little ability to form friendships, one-sided conversations, intense absorption in a special interest, and clumsy movements.” He also noted that these children could talk endlessly about their favorite subjects; he dubbed them ‘little professors.’ (Grandin, 15) A type of autism in which people have normal language skills and cognitive development but have significant problems with social interaction. (RamachandranTTB, 295) The exact cause is unknown. More than likely, an abnormality in the brain is the cause of Asperger syndrome. Genetic factors may play a role, since the disorder tends to run in families. A specific gene has not been identified. The main difference between Asperger syndrome and autistic disorder is that children with Asperger syndrome do not have speech or cognitive delays. The condition appears to be more common in boys than in girls. Although people with Asperger syndrome often have difficulty socially, many have above-average intelligence. They may excel in fields such as computer programming and science. There is no delay in their cognitive development, ability to take care of themselves, or curiosity about their environment. (PubMedHealth2) Autistic kids can seem rude when they’re actually just oblivious to social cues. (Grandin, 7) Editor’s note - Asperger’s Syndrome is not included in "DSM-5."

Autism: brain disorder characterized by impaired social interactions, communication problems, and repetitive behaviors. Symptoms usually appear before the age of three. The exact cause of autism is not known. (NHGRI) Marked by severe difficulties in communication and forming relationships with other people, in developing language, and in using abstract concepts. (Includes) repetitive and limited patterns of behavior and obsessive resistance to tiny changes in familiar surroundings. Often progresses into adulthood. (OxfordMed) An autistic person has a human mind that cannot conceive of other minds. It is called a ‘pervasive developmental disorder,’ because so many aspects of development are disturbed: “intelligence,"  "perception," socializing skills, "language," and "emotion." (Doidge, 74-77) Autism and other ASDs develop in childhood and generally are diagnosed by age three. Autism is about four times more common in boys than girls. Girls with the disorder, however, tend to have more severe symptoms and greater cognitive impairment. (NIMH, 2011) May be related to defects in the “mirror neuron” system or the "circuits" it projects to. (RamachandranTTB, 295) Autism is a physical condition linked to abnormal biology and chemistry in the brain. The exact causes of these abnormalities remain unknown, but this is a very active area of research. Genetic factors seem to be important. For example, "identical twins" are much more likely than "fraternal twins" or siblings to both have autism. (PubMedHealth2)

Rett Syndrome: disorder of the nervous system that leads to developmental reversals, especially in the areas of expressive language and hand use. Occurs almost always in girls. It may be misdiagnosed as autism or cerebral palsy. Studies have linked many Rett syndrome cases to a defect in a gene on the "X chromosome." Females have two X chromosomes. Even when one chromosome has this defect, the other X chromosome is normal enough for the child to survive. Males born with this defective gene do not have a second X chromosome to make up for the problem. Therefore, the defect usually results in miscarriage, stillbirth, or very early death. The condition affects about 1 out of 10,000 children. (PubMed Health) Inherited neurological developmental disorder that is associated with X-Linked inheritance. The affected female is normal until the age of 6-25 months when progressive loss of voluntary control of hand movements and communication skills; seizures; and (autistic-like-behavior) appear. (MeSH) The gene behind the disorder (‘MECP2’) is on the X-chromosome. Affects several organ systems because the gene adds "methyl groups" to other genes, silencing them. Difficult to study because nerve cells do not divide. (Lewis, 125) No Rett syndrome risk factors have been identified. There is no known method for preventing Rett syndrome. (WebMD) Editor’s note - Rett Syndrome is no longer considered one of the Autism Spectrum Disorders in "DSM-5."