Autism is usually categorised as primary or secondary. Primary cases (known as idiopathic ASD) are not associated with any other health conditions, while secondary cases are related to an underlying medical condition which causes symptoms of autism. 90 percent of cases of autism are classed as primary and the remaining ten percent are classed as secondary.

There is no single cause of autism and there is some uncertainty still surrounding the cause or possible causes of autism. Scientists and experts generally accept that autism is caused by abnormalities in either the function or structure of the brain, but there are other factors which may be associated with the condition. Research has focused on four main areas, including:

• Genetic factors
• Environmental factors
• Neurological factors
• Psychological factors

Genetic factors

Many researchers believe that certain genes can contribute to a higher risk of developing autism. This means that certain genes which a baby inherits may make them more likely to suffer from autism. There is evidence to suggest that genetics play a part in determining whether a child has autism. Autism runs in families and a baby is around 5-6 percent more likely to develop the condition if an older sibling has autism. If a twin has autism, the other twin has a 60 percent chance of developing autism if the twins are identical.

Although research suggests that certain genes can contribute to a higher risk of autism, the specific genes have not yet been identified. As a result, there is currently no screening test for autism.

Environmental factors

Some researchers believe environmental factors can cause autism. This is based on the belief that a person is born with a pre-disposition to autistic spectrum disorders but the condition only develops when they are exposed to certain triggers or environments. Scientists have identified the following risk factors:

• Smoking during pregnancy: women who smoke on a daily basis throughout their pregnancy are around 40 percent more likely to have a child with autism.
• Viral infections during pregnancy: if a woman has a viral infection, such as rubella, they are 7 percent more likely to have a child with autism.
• Age of the father: if the father is over 40 they have a greater chance of having a child with autism. It is estimated that new fathers over the age of 40 are 6 times more likely to have a child with autism that fathers under the age of 40.
• Pollution and pesticides: the influence of pollution and pesticides is currently being studied as part of the CHARGE research project. The results of the study have not yet been released.

Neurological factors

Research into neurological factors is centred on a region of the brain known as the amygdala, which acts as a kind of switch between the cerebral cortex and the limbic system; the cerebral cortex processes the senses, while the limbic system controls emotions. The amygdala is responsible for matching the emotional response to an individual situation – for example, in a sad situation, an individual will feel upset.

Psychological factors

Research surrounding psychological factors is focused on the Theory of Mind concept. This concept revolves around the idea of being able to see the world through another person’s eyes. People with autism usually have a limited understanding of the Theory of Mind concept.

Causes of secondary autism include Rett syndrome, Fragile X syndrome and Tuberous sclerosis.