> To get the point, consider that if someone has mental needs, there will be 100% correlation between performance and IQ tests. But the performance doesn’t correlate as well at higher levels, though, unaware of the effect of the nonlinearity, the psychologists will think it does.(The statistical spin, as a marketing argument, is that a person with an IQ of 70 cannot prove theorems, which is obvious for a measure of unintelligence — but they fail to reveal how many IQs of 150 are doing menial jobs. So “vey low IQ” may provide information, while “very high IQ” may convey nothing better than random — it is not even a necessary condition.).
At the very least, even the author seems to be saying that IQ is a useful metric for understanding the world on the low end.