Of the problems that can occur with autism, mental retardation is the most widespread.

Seventy-five to 80 percent of people with autism are mentally retarded to some extent. Fifteen to 20 percent are considered severely retarded, with IQs below 35.

But autism does not necessarily correspond with mental impairment. More than 10 percent of people with autism have an average or above average IQ. A few show exceptional intelligence.

Interpreting IQ scores is difficult, however, because most intelligence tests are not designed for people with autism. People with autism do not perceive or relate to their environment in typical ways.

When tested, some areas of ability are normal or even above average, and some areas may be especially weak. For example, a child with autism may do extremely well on the parts of the test that measure visual skills but earn low scores on the language subtests.