Autism is one of the four major developmental disabilities (in the USA). Prevalence rates of autism differ from population to population. However, autism is probably more common world wide than once thought, as more cases are being diagnosed due to increasing awareness.

Stephen M. Edelson, Ph.D. of the Center for the Study of Autism, Salem, Oregon writes, "For many years, the most cited statistic is that autism occurs in 4.5 out of 10,000 live births. This was based on large-scale surveys conducted in the United States and England. More recently, estimates on the prevalence of autism have ranged been as high as 1/4% to 1/2% of the population. These estimates typically include those with autism, Asperger syndrome, and PDD.

Autism is three* times more likely to affect males than females. This gender difference is not unique to autism since many developmental disabilities have a greater male to female ratio."

* Some studies show that the ratio of autistic males to females is close to 4:1.