Rate of autism used to be very rare. When I was growing up I never heard of anyone with autism. Rates were of the order of one in ten thousand. Over the last twenty to thirty years, rates have greatly increased to one in a thousand, one in a hundred. Latest numbers show about one in sixty-eight as of a few years ago. Perhaps even as much as one in forty-five. The reason for the increase in rates is partly due to better diagnosis. Some people who might have been diagnosed with intellectual disability now would get a more accurate diagnosis of autism. Most of the increase seems to be real, that in fact there are just much greater numbers of people with autism today than there used to be. In terms of the rate in boys versus girls, autism occurs about four times as commonly in boys as in girls.
-Dr. James Adams, Ph.D