The impression one gets from briefly skimming anything on IQ is that two people with the same IQ are (supposed to be) about equally smart. And within the normal range of IQs, for a roughly homogenous population, this is basically true.
But for those at the bottom of the IQ range, this is not true–because there are two ways to be dumb.
This is, obviously, an oversimplification. There are probably 7.2 billion ways to be dumb. But bear with me; I think the oversimplification is justified for the sake of conversation.
Some people have very low IQs because something bad happened to them–they were dropped on their heads as babies, ate a chunk of lead, or have an extra copy of chromosome 21. In these sorts of cases, had the accident not occurred, the individual would have been smarter. Their genetic potential, as it were, is not realized, and–with the exception of accidents affecting genetics–if they had kids, their kids would not share their low IQ, but reflect their parents’ genetic IQ.
The second kind of low IQ happens just because a person happened to have parents who weren’t very bright. There is a great range in people’s natures, IQs included, and some folk are on the low end. Just as Gnon made some people with 140 IQs, so Gnon made some people with 60. There is nothing wrong with these people, per se. They tend to be perfectly functional, at least in a society of other people like themselves–they just don’t score very well on IQ tests and tend not to get degrees in math.
People who have suffered some form of traumatic brain injury tend not to be very functional. They lose all kinds of functional brain processing stuff, not just the ability to predict which way the arrow is going to be rotated next. By contrast, people who are just not genetically blessed with smarts have built entire cultures.
To clarify, and use an extreme example, let’s think back to our most ancient ancestors. Since we can’t talk to them, let’s look at our cousins, the other great apes.
Koko the Gorilla can use about 1,000 signs and understands about 2,000 human words. Kanzi, a bonobo who can play Pacman, make stone tools, and cook his own dinner, understands about 3,000 words and 348 symbols. Both Kanzi and Koko are probably on the highly intelligent end for their species, because you don’t hear all that much about all of their classmates whom scientists have also been trying to teach to talk.
According to different websites on child development, a 2 year old human typically uses about 150-300 words; a 3 year old uses about 900-1000 words; a four year old uses 4,000-6,000 words.
As a general rule of thumb, human children, like great apes, understand more than they can say. So let’s say that Koko and Kanzi are about as smart as a human somewhere between 2 and 4 years old.
Now consider what would happen if you let your 3 year old (or someone else’s three year old if you don’t have one of your own) lose in the jungle: they’d die. Quickly. A three year old sucks at making omelets, their flint-knapping leaves much to be desired, and most of them aren’t even very good at Pacman. They’re also really bad at climbing trees and still need help opening their bananas, and they don’t really understand why they should avoid lions.
Bonobos and gorillas, by contrast, survive just fine in the jungle, so long as humans don’t shoot them.
A grown human with the intellect of a 4 yr old would not be functional, in human society or gorilla society. A gorilla with the language abilities of a 4 yr old human is a very smart and completely functional-in-gorilla-society gorilla. A gorilla is not dumb; a gorilla is exactly as smart as it is supposed to be.
Since the age when our ancestors diverged from the other primates, our species has been marked by increasing brain capacity and, presumably, intelligence. We’ve gone from doing one-to-one correspondence on our fingers to calculus and diff eq. Which means that a great many human societies have fallen somewhere in between–societies where the average person could count to ten; societies where the average person could do basic arithmetic; societies where the average person can do some abstract thought, but not a ton. Each society has its own level of organization and particular environment, requiring different mental toolkits.
But within our own society, we are not identical; we are not clones. So some of us are smart and some are dumb. But a person who is naturally dumb is generally more functional than someone who suffered some accident; there are different kinds of low IQ.