Infantile autism is a serious disorder clearly recognizable by age three, but it is defined (or diagnosed) in terms of a set of behaviors, not yet by any known impairment of brain function. Failure to learn language is the most serious aspect of the disorder, at least for parents who must cope day by day with a non-communicative child.

The prevailing opinion is that autism is caused by an obscure and mysterious combination of bad genes. Currently there is more research on chromosome loci associated with autism than on which systems of the brain might be affected.

Many genetic disorders have been found among children with autism, including phenylketonuria, tuberous sclerosis, and fragile X syndrome. But children with autism are also found among those with prenatal rubella infection, and autistic behaviors have been observed in children with prenatal exposure to alcohol, thalidomide, the anti-seizure medication valproic acid, and drugs of abuse.