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As the name implies, ASDs are spectrum disorders, ranging from mild to severe. A child on the severe end of the spectrum may be unable to speak and also have mental retardation. A child on the mild end of the spectrum may be able to function in a regular classroom and even reach the point where he or she no longer meets the criteria for autism. No two children with ASDs are alike, even if they have the same diagnosis. One child with an ASD may be nonverbal and have a low IQ. Another child with the exact same diagnosis may have an above-average IQ. A third child may be verbally and intellectually precocious. The terms high-functioning and low-functioning are sometimes used to describe where a child is on the autism spectrum.
You can't tell that a child has an ASD simply by looking at a picture of him or her. A two-year-old with an ASD can be the same height and weight and be just as adorable as a "typical" two-year-old. ("Normal" is a relative term, and one that is not widely accepted in the ASD world. The Autism Network International introduced a new term, neurologically typical or NT, to describe people without ASDs, which has been shortened to typical as the acceptable term in many publications). What distinguishes a child with an ASD from a typical peer is what you can't see: the brain. This is why ASDs are known as invisible disabilities. Because there is no medical test for an ASD, a child is diagnosed based on either the absence or presence of certain behaviors and skills. For example, if a child is still not speaking by the age of three, that is considered the absence of an age-appropriate behavior. If a three-year-old child engages in odd or idiosyncratic behavior, such as excessive hand flapping, grimacing, or aimlessly running back and forth across a room that may be an indication of a developmental disorder.
Excerpted from The Autism Sourcebook