Scientists don't know exactly what causes autism spectrum disorder (ASD).
Autism was first described in the 1940s, but very little was known about it until the last few decades. Even today, there is a great deal that we don't know about autism.
Because the disorder is so complex and no two people with autism are exactly alike, there are probably many causes for autism. It is also likely that there is not a single cause for autism, but rather that it results from a combination of causes.
Scientists are studying some of the following as possible causes of or contributors to ASD.