No one really figured it out until I was in my mid 30's. In school I'd always been regarded as odd or eccentric because of my poor coordination, bad clothing, and abysmal social skills, but that didn't get much attention because I had no problem with academic achievement.
When I was in counseling for my third episode of major depression about 11 years ago, my psychologist spotted it right away, and when she learned about my unusual language development and early reading ability it was obvious to her that I am on the spectrum. Hyperlexia almost always goes hand in hand with ASDs.
I think it's obvious to those who know what to look for, but not for most people, because I have learned to adapt and put on a good show over time.
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Intelligence is a constant. The population is growing.