Autism spectrum disorder has no single known cause. Given the complexity of the disorder, and the fact that symptoms and severity vary, there are probably many causes. Both genetics and environment may play a role.
- Genetic problems. Several different genes appear to be involved in autism spectrum disorder. For some children, autism spectrum disorder can be associated with a genetic disorder, such as Rett syndrome or fragile X syndrome. For others, genetic changes may make a child more susceptible to autism spectrum disorder or create environmental risk factors. Still other genes may affect brain development or the way that brain cells communicate, or they may determine the severity of symptoms. Some genetic problems seem to be inherited, while others happen spontaneously.
- Environmental factors. Researchers are currently exploring whether such factors as viral infections, complications during pregnancy or air pollutants play a role in triggering autism spectrum disorder.Source
- Source: Mayo Clinic
A Practical Guide to Autism by 
