A study in South Korea by Autism Speaks found as many as 2/3 of the kids who had autism were not diagnosed or receiving autism-related services which leads to the statistic of 1 in 38.
So with all that, you have a good chance of knowing someone on the autism spectrum if you know very many children at all.
And Rainman is not typical of most people with autism. In fact, of the 4 people with autism that I know have autism, none are very much like Rainman. As far as that goes, those 4 all have very different stories and symptoms. You would not group them together; one is female the other 3 are male; one is 30+, another is 15, and the other 2 are in elementary school or preschool; one was normal and then regressed, 2 others either had it at birth or before they were old enough to regress (I am not sure about the 4th).
By the way, none of them are "dumb" or "slow" or whatever derogatory term you think of. Their brains are wired differently and so they don't receive signals the way that we (neuro-typicals) do. We have satellite TV, and with a storm it can be very difficult to get a good enough signal to watch a show. The dialogue may not come through, the picture can be very pixelated and it can be impossible to understand what is going on in the show. I suspect that living with autism could be somewhat comparable, except not only is the input messed up but also being able to express thoughts or what ever is as difficult (or more difficult) than understanding the input.
The earlier the parent (or someone else) notices the signs the better because early treatment is a huge deal. The earlier the treatment, the better the outcome will be.
According to autismspeaks.org the red flags for a child needing evaluation are:
- No big smiles or other warm, joyful expressions by six months or thereafter
- No back-and-forth sharing of sounds, smiles or other facial expressions by nine months
- No babbling by 12 months
- No back-and-forth gestures such as pointing, showing, reaching or waving by 12 months
- No words by 16 months
- No meaningful, two-word phrases (not including imitating or repeating) by 24 months
- Any loss of speech, babbling or social skills at any age
Please, if you know a child who has these red flags go to http://www.autismspeaks.org/what-autism/learn-signs to learn more.
Statistics come from http://www.autismspeaks.org/science/science-news/autism-prevalence-rises-1-88