Nobody yet knows the causes of autism. What is clear is that autism is not caused by parental attitudes or behaviour, nor is there any evidence to suggest a link between autism and the MMR vaccine.
In about 6% of cases it is thought that autism may be linked certain physical factors which can affect brain development. These include maternal rubella, tuberous sclerosis, a lack of oxygen at birth and encephalitis.
It is thought that pollution may also play a part but we don't understand how this may impact on autism.
Studies of identical twins have shown that if one twin has autism, there is a 60% chance of the other being affected as well. This, coupled with a much higher than average rate between siblings, points to a strong genetic link. Research is currently being undertaken to identify the genes involved, this is a complex and arduous process as more than one gene is likely to be involved and there may be some interaction with environmental factors that we don't yet understand.
The BMJ states that "studies have indentified possible candidate genes on chromosomes 2q, 7q, 16p and 19p. No specific candidate genes have yet emerged".
It is likely that the genes associated with autism are complex and a child's genetic inclination towards autism may be initiated or "triggered" by an environmental event. Some researchers are currently investigating whether a viral infection or air pollutants can play a part in triggering autism.
It has been shown that the brains of people with autism have certain abnormalities, certain parts of the brain may be enlarged, the autistic brain contains a greater number of minicolumns (groups of cells in the outer part of the brain) and that the neurons inside the minicolumns are smaller than average. Neuron size has a direct relationship with the efficiency of the connections between the neurons. It is thought that this may be why people with autism have difficulty with activities that require longer complex connections like language.
The National Autistic Society points out that there may be a number of different causes of autism for different subtypes which further complicates things.
