Imagine living in a world that doesn't completely reach you, and to which you can't always respond. For many people who have autism, this great divide exists between their world and the reality outside of them.
Autism is one of a group of developmental disorders called autism spectrum disorders. Autism affects about 1 out of every 2,000 people. Children with autism usually display signs before the age of 3.
Autism is 2 to 4 times more likely in boys than girls, and people with autism may be mentally challenged to some degree.
Many factors may be connected to autism, but no conclusive causes have been found.
There is some evidence that some people with autism have a chemical imbalance in their brain, specifically high levels of serotonin, though there is no explanation of why this would cause autism.
Genetics may also be a factor. In studies of identical twins, if one has autism, there's about an 85% chance the other twin will have it as well. For siblings of a person with autism, chances of having the condition are higher than for the general population.