(waning, I got up 4 hours early today.)
So as I was saying, withing the “normal” (average) range of IQ, most people who have the same number probably have about the same level of competency. But for outliers at the bottom end, it makes a difference how their low IQ came about, whether through natural genetic variation, or an unfortunate accident. A person’s level of impairment has a lot to do with their deviation from the IQ they should have been and the society at large–thus, a person who is naturally at 70IQ and comes from a society where they are average–where everyone is about 70–is perfectly functional, whereas a person who was supposed to have a 100 IQ but got dropped on their head and suffered brain damage is not going to be very functional.
An IQ below 80 is somewhere between borderline and severely impaired in the US; about 8 or 9% of people score below 80. About 2.5% score as severely impaired, below 70. People with IQs under 70 can’t be executed, and can only reach, at max, the intellectual level of a 12 yr old.* People below 60 have severe impairments and may never be able to live alone.
*Still putting them well ahead of the smartest apes–to be honest, people blathering on about how apes and dolphins are “as smart as humans” kind of get on my nerves.
And yet, many successful societies have survived without any ability to read or numbers beyond 3. These folks tend to score badly on IQ tests, but they survive just fine in their own societies, and I assume they are very happy with their lives the way they are. When people say that people in society Foo have average IQs around Bar, this is not the same as saying they are severely disabled–it’s a different kind of low IQ.
One thing I wonder about: let’s say someone was supposed to have an IQ of 140, due to genetics, but an accident interfered–say, they got dropped on their head–and they lost 40 IQ points. If they’d started at 100, they’d drop down to 60, which is pretty darn impaired. But they’ve only dropped to 100.
I assume that, even though they would test as “normal” on an IQ test, the loss of 40 IQ points would result in severe life impairment of some sort. I don’t know if the DSM/other ways of diagnosing people with disabilities could pick up on such people at all, or maybe they’d end up with some random diagnosis due to however the condition manifests. I also wonder how such a person would compare to someone who is just naturally low-IQ–would they have a better chance of thriving in a society that’s not as IQ-demanding? Or would they be just as dysfunctional there? Or would they have enough wits left about them to make up for the damage they’d suffered, and do fine in life?
One thing I suspect: growing up would be hell for them. Their siblings (assuming their 140 IQ parents manage to have any other kids,) would pursue advanced degrees and become doctors, lawyers, finance bros, or professors. Their parents probably are professors. Their parents would be pushing them to take advanced maths in elementary school (“Your brother could work with negative numbers when he was 4!) when they struggle with fractions, and they’d feel like shit for consistently being dumber than everyone around them. Their parents would shove them into remedial classes, because “average” looks suspiciously like “dumb a rock” when you’re really smart, and then force them to go to college whether they belong there or not.
Of course, this is what our society tries to do to everyone, under the assumption that enough… pre-k? tutoring? organic food? Sesame Street?… can turn anyone into a math professor.