I had a very high IQ as a child, something that was either low above-average or high average in my teens, and something that's alternately described as low average or high borderline as an adult. I attribute this to:
1. Gaining early some talents that impress people in young children, but not having those areas progress as much as most people would. (Getting them almost fully-formed early on, rather than slowly getting them over years, in other words. This is how things generally work for musical prodigies, too -- they are good "for a child" early in life, but don't progress as much so by adulthood they're not necessarily any good at all.)
2. Gaining other talents later on that may have been more important (to my brain anyway, for whatever reason) than the stuff that makes people good at most areas of IQ tests, and having those things crowd the other things out in terms of what I was learning (and therefore losing or losing access to a few things).
In every single one of the tests my scores were extremely uneven. (For instance, in my last Wechsler test, which is the only one I have access to the subtest information on anymore, my subtests ranged from, I think, 4 (severely impaired) to 12 or 13 (high average). Which is like having different parts of an IQ range from 69 or lower to somewhere in the 110-119 range. My verbal and performance scores were nearly the same (performance slightly higher, I think), but that was both from averaging extremely different scores among the subtests.)
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams