Comment by tptacek

This is basically false; you might make it true if you cut somewhere other than the middle, but famously IQ has non-uniform reliability (it's heteroskedastic? is that the way to say it?). Pop science and nerd culture have fixated on it as a global ranking of people by cognitive capability, but it's not that at all; past a threshold, the actual numbers (and test-test reliability) get really noisy.

This should make intuitive sense, because IQ was designed as a diagnostic, one in a battery of diagnostics, for people with cognitive dysfunction. It's a useful tool to deploy when you have a patient who, for instance, can't seem to progress in reading class or whatever. It's broadly misapplied in studies like this (but then, this study has deeper faults than that).