Exactly what causes autism is unknown although it is believed to be a neurological condition. Medical conditions that could be considered to be causal are found in only about 5-10% of cases.
It seems likely that a predisposition to autism is inherited. The evidence of the heritability of autism comes from twin and family studies. In twin studies, unusually high rates of autism are found in identical twins, and relatively low rates in fraternal or non-identical twins. Studies of families have indicated that 2-6% of the siblings of children with autism are also autistic, and that 8% of the extended families will include another member who has autism. Family studies also reveal an increased prevalence of mental retardation and cognitive difficulties in the siblings of children with autism especially those who have mentally retardation themselves. This suggests that what may be inherited is not an "autism gene" but rather a nonspecific factor which increases the likelihood of various cognitive problems including autism.
It is believed that autism may be related to damage to the prefrontal cortex and limbic region of the brain and to the connections between the two regions. The most consistent findings are of brain stem and cerebellum abnormalities. P.E.T. scans of children with autism show the location of problems to be in these areas of the brain. Adults with this kind of brain damage, as a result of accident or stroke, display similar difficulties to those experienced by individuals with autism.
There is some indication that the brains of children with autism may have increased cell density suggesting that the cells do not get pruned back as they do in normal development. Other researchers have found that this is particularly true within the dopamine system suggesting that there may be an excess of dopamine which could contribute to an overactive system. Studies of glucose metabolism and blood flow have failed to reveal consistent global or regional abnormalities, although co-relational studies do show some promise.
The developmental history of individuals with autism seldom reveals medical conditions that can be linked to the disorder. However, certain other illnesses place children at risk for developing autism. These are neurofibromatosis, tuberous sclerosis, and fragile X syndrome. Many individuals with autism (approximately one fifth to one third) develop seizures. Most of these occur in less able individuals and usually develop in later childhood or adolescence.
Many studies have shown that the number of perinatal problems experienced by children with autism is exceptionally high including: difficult delivery, infantile seizures, delayed breathing and neonatal convulsions. Some children appear to have normal development early on and only show the symptoms of autism in the second year of life. There have been two explanations given for this: (1) the child did show problems earlier but they only became obvious when speech failed to develop and the pressure for socialization was greater, or (2) the child was born with a vulnerability to acquiring the syndrome and it was triggered by a virus or other insult.