How is it possible for anyone to have a valid very high IQ, since IQ is a percentile test and there's no way to know how many very high IQ people were never properly tested?
Very high IQ is fundamentally flawed because IQ is resistant to precise measurement.
Social systems, like kindergarten, can tell if someone is smart or average or very not smart. Society generally only tests those at the outer boundaries to verify and classify them into groups.
You're right, someone who seems average (or totally unknown to social systems), but is actually super genius, might not get tested. It's the "genius in the rice paddy" argument and yes, those exist. Srinivasa Ramanujan was one.
That doesn't mean IQ is fundamentally flawed. You don't need to know if someone is 99th percentile or 99.0004 percentile to know they are probably the smartest in the average room of a hundred people and that's what we use IQ to determine.
Sorry to be nitpicky but this stuff is interesting to me. I would think someone in the 99th percentile would have a 99% chance of being smarter than another random person. In a room full of 100, to be smarter than all 99 other people would be .99^99 or only around a 37% chance of being the smartest in the room. I may be off on this, as I am definitely not in the 99th percentile.
Very high IQ is fundamentally flawed because IQ is resistant to precise measurement.