On the whole, autism is characterized by difficulty in social interactions and communication. But the diagnosis of autism covers people with a huge range of behaviors and symptoms, which are often arranged along the autism spectrum.
These different forms of autism appear to be triggered by different, interacting causes. It's not genetics or environment, but both — a combination of inherited genes, random genetic mutations, and environmental factors like a mother's immune system and nutrition during pregnancy. In most cases, scientists still aren't sure exactly how these factors actually lead to the disorder.
Further, there are many other factors that are still poorly understood and need to be more fully investigated. Before proceeding, though, it's necessary to mention one thing that has been ruled out: vaccines. Years of research have turned up absolutely no link between vaccines and autism.The article contains a number of useful links to research findings.