Autism typically appears during the first three years of life. Some children show signs from birth. Others seem to develop normally at first, only to slip suddenly into symptoms when they are 18 to 36 months old. Autism is four times more common in boys than in girls. It knows no racial, ethnic or social boundaries. Family income, lifestyle or educational levels do not affect a child's chance of having autism.

Some of the different types of autism include:

• Autistic disorder. This is what most people think of when they hear the word "autism." It refers to problems with social interactions, communication and imaginative play in children younger than 3 years.
• Asperger's syndrome. These children don't have a problem with language—in fact, they tend to score in the average or above-average range on intelligence tests. But they have the same social problems and limited scope of interests as children with autistic disorder.
• Pervasive developmental disorder or PDD—also known as atypical autism. This is a kind of catchall category for children who have some autistic problems but who don't fit into other categories.
• Rett's disorder. Known to occur only in girls, Rett's children begin to develop normally. Then they begin to lose their communication and social skills. Beginning at the age of 1 to 4 years, repetitive hand movements replace purposeful use of the hands.
• Childhood disintegrative disorder. These children develop normally for at least two years, and then lose some or most of their communication and social skills.