Autism is the most common of the Pervasive Developmental Disorders, affecting an estimated 2 to 6 per 1,000 individuals (Centers for Disease Control and Prevention, 2001). This means that as many as 1.5 million Americans today are believed to have some form of autism.
The commonly quoted rate of autism is 1-per-166 children in the United States. I believe this statistic was derived from the CDC's 1998 Brick Township study. This study reported 4 cases of autism per 1000 children and 6.7 cases on the ASD spectrum per 1000 children. 6/1000 gives a rate of 1-per-166, but the more accurate measure of 6.7/1000 gives a rate of 1-per-150. Both measures are reported on the web. The 1-per-166 rate has been promoted by the Autism ALARM. This project was funded through a cooperative agreement between the American Academy of Pediatrics, the National Center on Birth Defects and Developmental Disabilities, and the Centers for Disease Control and Prevention.
Based on statistics from the U.S. Department of Education and other governmental agencies, autism is growing at a rate of 10-17 percent per year. At these rates, the ASA estimates that the prevalence of autism could reach 4 million Americans in the next decade.
The overall incidence of autism is consistent around the globe,
but is about four times more prevalent in boys than girls. Autism rates for boys are often miscalculated on web sites. Table 3 of the Brick Township study reports rates of 9.7/1000 for boys and 3.7/1000 for girls. This can be reduced to 1-per-103 for boys and 1-per-207 for girls.
Autism knows no racial, ethnic, or social boundaries, and family income, lifestyle, and educational levels do not affect the chance of autism's occurrence.
Autism Society of America
"What is Autism?"
A Report from the Surgeon General