Autism rates reported to authorities is increasing, without apparent cause.
The apparent withdrawal of the autistic child has traditionally been seen as resulting from an unresolved conflict between fear and curiosity in social encounters. Recent evidence, however, leads researchers to believe there are genetic preconditions, such as a "fragile X" chromosome, that increase the likelihood of the condition. Rather than a single flawed gene causing a disease autism appears to be brought on by unknown environmental influences coupled with a dimly understood combination of genes that makes people vulnerable. According to another hypothesis, autism stems from immature development of some parts of the brain in combination with hyperdevelopment of other parts, and possibly abnormal brain chemistry.
Evidence for a genetic root for autism is that the rate of autism in the general population is about two-tenths of a percent or less, but for siblings of an autism patient it jumps to around 3 percent. And for an identical twin of an autism patient, someone who shares all the patient's genes, the rate is 60 percent or more. In 1997, an American study linked subliminal autism in parents to full-blown autism in their 380 children.
Autism rates reported to authorities in California nearly quadrupled between 1980 and 1994, to 208 cases per 100,000 children.
A 1997 study estimated that 80% of the 400,000 autistic people in the USA were also mentally retarded, while a few had IQ's in the genius range.