The Facts
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 a developmental disorder that affects about five out of every 10,000 people. Children with autism usually display signs before the age of three. Autism is two to four times more likely in boys than girls, and most people with autism are mentally challenged to some degree.
Causes
Many factors may be connected to autism, but no conclusive causes have been found.
There is some evidence that 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 are a factor in a small percentage of people with autism. 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.